# Is the delay to correlate the signal from one dish to another on a VLA radio telescope longer for red shifted objects?

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When observing 2 objects side by side close to the horizon with a very large array, one receding fast & red shifted, is the delay to correlate the signal from the closest dishes to the further ones longer for the red shifted object?

… is the delay to correlate the signal from the closest dishes to the further ones longer for the red shifted object?

Good question!

The short answer is no. The path difference between the closest and farthest dish in the array comes from local geometry, a big triangle in the air right above the VLA dishes. Here the speed of light is essentially constant (except for local temperature and humidity gradients in the air above the array, or maybe wind sheer) and so wouldn't vary between the to signals very much for reasons related to the sources' Doppler shifts.

The interference path length difference is divided by the local speed of light (in the air) to get the time difference needed for best correlation of those two dishes, and it doesn't make any difference what the source of the signal is or the speed that it was moving.

Doppler shifted photons don't look or act any different than non-shifted photons, there's no way to tell the difference in an experiment, unless you have some information about the original frequency before the shift.

The only teeny tiny difference might be if the air has a different index of refraction at the shifted frequency than the unshifted frequency. That's certainly taken into account during normal analysis, and I don't think it rises to the level and intent of your question.

## A real-time software backend for the GMRT

The new era of software signal processing has a large impact on radio astronomy instrumentation. Our design and implementation of a 32 antennae, 33 MHz, dual polarization, fully real-time software backend for the GMRT, using only off-the-shelf components, is an example of this. We have built a correlator and a beamformer, using PCI-based ADC cards and a Linux cluster of 48 nodes with dual gigabit inter-node connectivity for real-time data transfer requirements. The highly optimized compute pipeline uses cache efficient, multi-threaded parallel code, with the aid of vectorized processing. This backend allows flexibility in final time and frequency resolutions, and the ability to implement algorithms for radio frequency interference rejection. Our approach has allowed relatively rapid development of a fairly sophisticated and flexible backend receiver system for the GMRT, which will greatly enhance the productivity of the telescope. In this paper we describe some of the first lights using this software processing pipeline. We believe this is the first instance of such a real-time observatory backend for an intermediate sized array like the GMRT.

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## Introduction

Clusters of galaxies are the largest gravitationally bound systems in the Universe, with typical masses of about 10 15 Msun, and volumes of about 100 Mpc 3 . Most of the gravitating matter in any cluster is in the form of dark matter (∼80 %). Some of the luminous matter is in galaxies (∼3–5 %), the rest is in diffuse hot gas (∼15–17 %), detected in X-ray through its thermal bremsstrahlung emission. This thermal plasma, consisting of particles of energies of several keV, is commonly referred to as Intracluster Medium (ICM). Most of the detailed knowledge of galaxy clusters has been obtained in recent years from the study of ICM through X-ray Astronomy.

Clusters are formed by hierarchical structure formation processes. In this scenario, smaller units (galaxies, groups and small clusters) formed first and merged under gravitational pull to larger and larger units in the course of time. Cluster mergers are the mechanism by which clusters are assembled. Denser regions form a filamentary structure in the Universe, and clusters are formed within filaments, often at their intersection, by a combination of large and small mergers. Major cluster mergers are among the most energetic events in the Universe since the Big Bang (Sarazin 2002). During mergers, shocks are driven into the ICM, with the subsequent injection of turbulence. The merger activity, which has characterized much of the history of the Universe, appears to be continuing at the present time and explains the relative abundance of substructure and temperature gradients detected in clusters of galaxies by optical and X-ray observations.

Eventually, clusters reach a relaxed state, characterized by a giant galaxy at the center, and enhanced X-ray surface brightness peak in the core. The hot gas in the center has a high density, which implies short radiative cooling times, typically one or two order of magnitudes smaller than the Hubble time. Therefore energy losses due to X-ray emission are dramatic and produce a temperature drop towards the center. It was formerly suggested that in these conditions, the ICM plasma in the cluster core should cool and condense, giving rise to a steady, pressure-driven inward flow caused by the compression of the hot surrounding gas. Relaxed clusters were then classified as “cooling flow” clusters (Fabian 1994), with predicted gas mass deposition rates in the cluster center up to 1000 Msun year −1 . This model was the subject of much debate, when XMM-Newton spectral results failed to confirm the lines and features expected as a product of a steady state cooling flow (Peterson et al. 2001 Peterson and Fabian 2006). X-ray and optical studies showed that the gas cooling rates were overestimated by an order of magnitude or more (McNamara et al. 2006). The classical “cooling-flow” model has finally been replaced by the paradigm of “cool-core”. Observationally cool-cores are characterized by a strong peak in the surface brightness, a significant drop in the temperature and a peak in the metal distribution (e.g. De Grandi and Molendi 2001). The cooling time is often much shorter than 1 Gyr, therefore some source of heating is necessary to balance the radiation losses. At present, it is widely accepted that the source of heating in cool-core clusters is the AGN activity of the brightest cluster galaxy at the center (see McNamara and Nulsen 2007 Böhringer and Werner 2010 for recent reviews).

Galaxy clusters are also characterized by emission in the radio band. Obvious radio sources are the individual galaxies, whose emission has been observed in recent decades with sensitive radio telescopes. It often extends well beyond the galaxy optical boundaries, out to hundreds of kiloparsec, and hence it is expected that the radio emitting regions interact with the ICM. This interaction is indeed observed in tailed radio galaxies, and radio sources filling X-ray cavities at the center of cool-core clusters (McNamara and Nulsen 2007 Feretti and Giovannini 2008).

More puzzling are diffuse extended radio sources, which cannot be obviously ascribed to individual galaxies, but are instead associated with the ICM. This radio emission represents a striking feature of clusters, since it demonstrates that the thermal ICM plasma is mixed with non-thermal components. Such components are large-scale magnetic fields with a population of relativistic electrons in the cluster volume. Further demonstration of the existence of magnetic fields in the ICM is obtained by studies of the Faraday rotation of polarized radio galaxies, lying in the background or embedded within the magnetized intracluster medium.

The number density of relativistic particles is of the order of 10 −10 cm −3 , while their Lorentz factors are γ≫1000. Magnetic field intensities are around ∼0.1–1 μG, and the energy density of the relativistic plasma is globally ≲1 % than that of the thermal gas. Non-thermal components are nevertheless important for a comprehensive physical description of the intracluster medium in galaxy clusters, and play a major role in the evolution of large-scale structures in the Universe. They can have dynamic and thermodynamic effects: magnetic fields affect the heat conduction in the ICM and the gas dynamics, relativistic particles are sources of additional pressure and undergo acceleration processes that can modify the details of the ICM heating process. Realistic cosmological simulations of galaxy cluster formation also include non-thermal components, thus a deep knowledge of the properties of these components and of their interplay with the thermal gas are needed to properly constrain the large-scale structure formation scenario. The discovery of diffuse cluster radio emission has represented an important step in the understanding of the physical processes in clusters of galaxies. Diffuse synchrotron sources are sensitive to the turbulence and shock structures of large-scale environments and provide essential complements to studies at other wavebands. Studies in the radio domain will fill essential gaps in both cluster astrophysics and in the growth of structure in the Universe, especially where the signatures of shocks and turbulence, or even the thermal plasma itself, may be otherwise undetectable.

The aim of this review is to present the observational results obtained in recent years in the radio domain related to the diffuse radio sources, in order to give an overview of the state of the art of the current knowledge of non-thermal cluster components. We will show how the radio properties can be linked to the X-ray properties and to the cluster evolutionary state, and will discuss the physical implications.

Cluster non-thermal emission of inverse-Compton (IC) origin is expected in the hard X-ray domain, due to scattering of the cosmic microwave background photons by the synchrotron relativistic electrons (Sarazin 1999). To present days, the detection of a non-thermal hard tail in the X-ray spectrum of clusters of galaxies has been reported in the literature (e.g. Fusco-Femiano et al. 1999, 2004, 2005, 2007, 2011 Rephaeli and Gruber 2002 Wik et al. 2009, 2011), and it is still debated in some cases. The presence of a high frequency non-thermal emission is clearly confirmed in the Ophiuchus cluster (Murgia et al. 2010b Eckert 2011). The discussion of this topic is beyond the scope of this review, which is focused on the results in the radio band.

The organization of this paper is as follows: In Sect. 2 the general properties of diffuse cluster radio sources and their classification are presented in Sect. 3 the problems related to the detection of diffuse emission are outlined Sects. 4, 5, 6 give the observational properties of radio halos, relics and mini-halo, respectively. In Sect. 7 the current results about magnetic fields are presented. Section 8 presents the evidence for radio emission and magnetic fields beyond clusters of galaxies. Section 9 briefly summarizes the current models, and Sect. 10 depicts the future prospects.

The intrinsic parameters quoted in this paper are computed with ΛCDM cosmology with H 0=71 km s −1 Mpc −1 , Ω m=0.27, and Ω Λ=0.73. Values taken from the literature have been scaled to this cosmology.

## And computers and Trigonometry and Time

• Where is the star in the sky (the astronomers have a convention based upon the earth's revolution about the sun. Unfortunately the this is not constant, so actual sky maps refer to epochs - changed about every 50 years. Our beam widths are narrow enough that earth referenced positions stars must be updated "frequently".
• The earth turns relative to the stars in a relatively constant manner every "24 hours". Actually the "24 hours" refers to an idealized sun position at "noon". Since the earth's orbit is not a circle, astronomers speak of "mean solar day".
• But the sun appears to move in the sky relative to the stars, so the astronomers speak of siderial time - a sidereal day - 1 sidereal day = 23.9344696 hours (or: 23 hours, 56 minutes, 4.091 seconds).
• So - finding and continuing to point a star is not so easy.
• But there is more - using the altitude-azimuth (altazimuth) telescope mounts common (much lower cost) in astronomy now,
• You must constantly change both the telescope's altitude and the telescope's azimuth. The telescope is not a sidereal clock on a polar mount like the good old (expensive) daze.
• AND the stars in the view of a sensor (film, CCD, feedhorn) rotate in the field. In optical telescopes, you must rotate the film or other light sensor as time goes by, or the stars not at dead center will appear to rotate in a circle about the center of pointing - much as the Big Dipper appears to slowly rotate to people staring at it.
• And the polarity of the light or radio waves also changes with the rotation.

• For starters we will assume that our ATA array is in carefully arranged, carefullly surveyed fixed places on a flat plain at Hat Creek, north of the equator, west of London. (many corrections to be incorporated later)
• Our particular target star is offset from the tracking point, one of several being examined concurrently by the various astronomers in the current field of view (lets say in a circle 10 moon widths diameter)
• And of course we have a good regular clock adjustable to sidereal time and to run the radio equipment.
• The "antennas" have been carefully leveled, the poles are streight up, the azimuth and elevation corrections carefully made, the antennas are uniform in construction, .
• The communications to the antenna drive servos and from the antenna receivers is fully functional (OK, a few antenna are listed out of service with problems, but that is the way of the world) .
• The above is enough to get us started on our wild ride into practical phased array radio astronomy. :-))

1. The same logic could delay both polarizations - (I forgot about polarization when making the diagram)
2. Between the indicated ADC and Dual Ported Memory is a 200 mHz FIR digital filter
(Goeff actually said something was "down converted" to 200 mHz - but that confused me. So I changed the words and meaning to the above.) Unfortunately, the above diagram seems unrelated to this approach - http://casper.berkeley.edu/papers/beamforming.pdf so maybe I'm barking up the wrong tree. (I plan a trip to ATA in a week to try to correct my understanding -)
The trip was fun - but did not increase my understanding - now studying digital down converters and digital radios - http://www.mathworks.com/applications/dsp_comm/demos.html?file=/products/demos/shipping/filterdesign/ddcfilterchaindemo.html#1
I guess I should have taken up digital filtering !! The CIC filter - http://beamdocs.fnal.gov/DocDB/0015/001529/001/The%20CIC%20filter.doc
Does the quadrature output from a quadrature type mixer (or balanced detecter?) add information (about the phase of the local oscillator?) and is it useful?
3. After the Dual Ported Memory is another filter that enables finer resolution phase shifting than offered by the 800 mHz sampling (1/8 wave at 8 gHz - maybe 1/6 wave at 11 gHz)
4. Final channel gain (and some other?) corrections to the data stream are made after the Dual Ported Memory before being shipped out to the next device.
5. There are modifications to permit present FPGAs to handle the high speed tasks. Future FPGAs are expected to be even faster.

There are indeed four independent dual channel (for polarization data) per plug-in chassis (above).

I pressed for more details, and maybe drawings, but Geoff suggested that I really go to the site to get more details.

-----------------------------------
I hope I got some of the above correct - Geoff was between meetings, and very kind to talk with this drop-in and I was slightly late for the seminar I had come to see.

comments on the above section by Matt Dexter - Sept 2008

Regarding http://ed-thelen.org/ATA/HatCreekATA-StrugglingOn.html#BerkeleyOct22 Our current FPGAs could not fit the interpolating filters to get to delay steps finer than the 838.8608MHz sample clock resolution. Just not enough gates. 1/838.8608MHz is as fine of delay step as is supported at the moment. At this time only the FPGAs programmed for the Transient Search, aka Fly's Eye, experiment have fine channel gain correction on the 8bit I + 8bit Q voltage samples just after the digital down conversion. The FX correlator has gain correction on the F engine's output 1024 spectral channels which are 4bit I + 4bit Q voltage samples.

Point the radio telescope, dish or phased array, and use the rotation of the earth to bring the desired object through the fixed beam, recording the data (amplitude, frequency, phase, . ) versus time, and map that portion of the sky using the recorded variable(s) and time. This method is less stressful on the pointing mechanism (if any). The earth rotates at about 15 degrees/hour relative to the sun, or about 0.25 degrees per minute. (Slightly less if observing stellar objects (sidereal rate). The approximate beam width for ATA at 11MHz is wavelength/diameter or 0.025/700 meters or 3.6^-5 radians or 0.002 degrees. With the earth rotating at 0.004 degrees/sec, a point source would be in the beam width about 2 seconds. This may be great for mapping strong sources, but not great for other sources.

To allow a wide pointing angle (approaching 90 degrees), we should allow for delay times of plus/minus 350 meters. This is about 14,000 wavelengths a 11 MHz. To reduce noise and errors caused by delay units, probably we should allow at least 10 delay increments per wave length. The delay counters should be plus/minus almost 320,000 counts. A count of 2^20 can represent over 1,000,000 counts.

## 2. Data

### 2.1. IceCube Events

IceCube detects high-energy neutrino events of two types: cascades and tracks. The former are seen as showers that develop within the detector volume the energy of the primary neutrino is determined relatively well, but the arrival direction is uncertain. For the latter, the situation is the opposite: relatively narrow tracks pass through the detector. Hence, the angular resolution is normally of the order of 1°, but some part of the energy of the secondary particles is left outside the instrumental volume, and the primary particle energy is determined with large uncertainties. In the present study, we concentrate on the track events because of their better angular resolution. We are interested in neutrinos with estimated energies E 200 TeV because it is the value above which, assuming two flux components, the hard-spectrum component starts to dominate. This can be seen, for instance, by comparison of the best-fit spectra obtained by IceCube from the analysis of starting events (more sensitive at lower energies) and of Northern Hemisphere muon tracks (more sensitive at higher energies), as reported by Aartsen et al. (2019). Remarkably, this value, E = 200 TeV, is also the threshold value for some published IceCube Northern Hemisphere muon track data sets (Aartsen et al. 2016b, 2017c), which provides an additional technical motivation for this cut. Therefore, we fix the condition E ≥ 200 TeV for all tests discussed below. A study of the validity of our conclusions for less energetic neutrinos is beyond the scope of the present paper.

The largest published IceCube data set of high-energy track events is given by extremely high energy (EHE) alerts and alert-like (EHEA) events. This data set includes events that passed the selection criteria (Aartsen et al. 2017d) for the EHE-type alerts issued by IceCube between 2016 July and 2019 May. The list of events before 2017 September, including early events that arrived before the launch of the alert system but satisfied the same criteria, is published online 5 (IceCube Collaboration 2018). The details of similar events observed after 2017 September are available through Gamma-ray Coordinates Network 6 (GCN) and Astrophysical Multimessenger Observatory Network 7 (AMON) notices 8 see also the IceCube Catalogue of Astrophysical Neutrino Candidates. 9 For one event, we use the detailed information from Aartsen et al. (2018a). By construction, the EHEA events have a good angular resolution (the 90% containment area on the celestial sphere Ω90 < 10 deg 2 ) and high estimated energies (certainly above 200 TeV). There are 33 events in this EHEA sample.

In order to use the largest available sample of the highest-energy neutrino events of similar quality, we supplement the EHEA sample with 23 more events satisfying the following criteria: (i) track morphology, (ii) E > 200 TeV, and (iii) Ω90 < 10 deg 2 . These events were selected from all of the other publicly available IceCube event lists. They include high-energy starting event (HESE) alerts and alert-like events (HESEA), "GOLD" and "BRONZE" alerts from the IceCube Collaboration (2018) and GCN/AMON, HESE lists from Aartsen et al. (2014a, 2015, 2017a), and Northern Hemisphere muon track (MUONT) event lists from Aartsen et al. (2016b, 2017a). For a few HESE alerts, the estimated energy of the neutrino has not been published we then used the deposited charge (number of photoelectrons) divided by 100 as a proxy for the energy in TeV see Aartsen et al. (2014a). Following Padovani et al. (2016), we drop one MUONT event that was retracted. Note that some MUONT events appear in the EHEA list as well we use the information from a more recent EHEA catalog for them.

For the IceCube events, coordinate-wise intervals with 90% statistical coverage are reported in the published data we use. In addition, there exist unpublished systematic errors in the determination of the arrival directions, related in particular, but not exclusively, to the lack of knowledge of ice properties. These errors depend not only on the arrival direction but also on the part of the installation where the neutrinos land and are, therefore, hard to model. With the exception of a few events—see, e.g., Kankare et al. (2019)—only statistical errors are provided for the published IceCube arrival directions, whereas for good-resolution events, the contribution of these systematic errors can be important. The absolute IceCube pointing error was estimated by Aartsen et al. (2014b) as 02 however, the same paper states explicitly that smaller or larger errors may correspond to the events selected in particular neutrino analyses. Further, a contribution to the systematic error comes from the choice of the reconstruction procedure and may be estimated by comparison between the arrival directions of one and the same event obtained with different analyses. We found seven events whose arrival directions were published in both the EHEA and MUONT analyses (see the references above) the mean difference between the arrival directions in these two reconstruction was ≈025. Having no systematic errors published, we use as a guide the published IceCube upper limit of 10 (Aartsen et al. 2013b) on the systematic uncertainty of the arrival directions of high-energy muon tracks and further refine this value by means of the procedure defined in Section 3.

Therefore, our sample of IceCube high-energy neutrinos includes 56 events with E > 200 TeV, known arrival directions, 90% confidence level (CL) statistical uncertainty ellipses on the celestial sphere, and arrival times. These events are listed in Table 1 and shown in Figure 1. Note that a significant part of the events is not astrophysical: even at high energies, the atmospheric background is essential. For instance, the expected fraction of nonastrophysical events in the EHEA sample, assuming an E −2 astrophysical spectrum, is 32% (Aartsen et al. 2017d) for a softer assumed spectrum or other event classes, the background contribution is even larger. We also note that up until now, neither Baikal-GVD nor ANTARES have published detailed information on track events above 200 TeV.

Table 1. IceCube High-energy Neutrino Events Used in Our Analysis

Date Category E R.A. R.A. Error Decl. Decl. Error References
(TeV) (deg) (deg) (deg) (deg)
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
2009 Aug 13 MUONT 480 29.51 +0.40 −0.38 1.23 +0.18 −0.22 Aartsen et al. (2016a)
2009 Nov 6 MUONT 250 298.21 +0.53 −0.57 11.74 +0.32 −0.38 Aartsen et al. (2016a)
2010 Jun 23 MUONT 260 141.25 +0.46 −0.45 47.80 +0.56 −0.48 Aartsen et al. (2016a)
2010 Sep 25 MUONT 460 266.29 +0.58 −0.62 13.40 +0.52 −0.45 Aartsen et al. (2016a)
2010 Oct 9 EHEA 660 331.09 +0.56 −0.72 11.10 +0.48 −0.58 Aartsen et al. (2016a)

Note. Set of all 56 IceCube events selected according to our criteria see Section 2.1 for details and category notations.

Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.

### 2.2. VLBI Observations of AGNs

For our analysis, we used 8 GHz VLBI observations compiled in the Astrogeo 10 database, comprising the visibility data and images acquired from geodetic VLBI observations (Petrov et al. 2009 Piner et al. 2012 Pushkarev & Kovalev 2012) and the Very Long Baseline Array (VLBA) calibrator surveys (VCSs Beasley et al. 2002 Fomalont et al. 2003 Petrov et al. 2005, 2006, 2008 Kovalev et al. 2007 Gordon et al. 2016 Petrov 2017), together with other 8 GHz global VLBI, VLBA, European VLBI Network (EVN), and Australian Long Baseline Array (LBA) observations (Petrov 2011, 2012, 2013 Petrov et al. 2011a, 2011b, 2019 Schinzel et al. 2015 Shu et al. 2017). Their positions are determined and presented within the VLBI-based Radio Fundamental Catalogue 11 (RFC). We note that a special effort was made by the VCS program observations to compile a complete subsample of AGNs limited by the flux density integrated over VLBI images mJy at 8 GHz, and a similar effort was made with LBA observations. This complete sample consists of 3388 objects. The resulting sky coverage is shown with gray dots in Figure 1.

Note that the image database and the catalog also contain the data for other wavelengths (2.3, 5, 15, and 22 GHz) and go down to lower flux density levels at 8 GHz. Altogether, the VLBI catalog contains the measurements for more than 16,000 AGNs. However, the only deep statistically complete sample is the aforementioned one. Most of the other wavelengths lack data below −30° decl. The 15 GHz band is complete thanks to the MOJAVE project (Lister et al. 2019), but only down to Jy. Generally, samples at different bands might be biased, e.g., toward γ-ray-selected AGNs (e.g., Schinzel et al. 2015 Lister et al. 2018), AGNs seen through the galactic plane (Petrov et al. 2011a Petrov 2012), or optically bright AGNs (Petrov 2011, 2013). The 22 GHz sample might be biased toward the most compact AGNs selected to serve for the high-frequency realization of the celestial reference frame (Charlot et al. 2010). That is why, to achieve the most robust results, we use only the 8 GHz sample in our statistical studies.

In our analysis, we use the flux density integrated over VLBI images of AGNs and call it the "VLBI flux density." For most of the Doppler-boosted AGNs that comprise our sample, it is dominated by emission of the apparent parsec-scale jet base see our detailed discussion in Section 4.1. For the objects imaged by VLBI at more than one epoch, the average of all measurements is used in the analysis. The number of observations per source ranges from one to more than 150, with a median of five. The average we use throughout this paper is the geometric mean (or, equivalently, the arithmetic mean of logarithms) because the range of flux densities can cover several orders of magnitude, and relative differences are important.

### 2.3. RATAN-600 AGN Monitoring

Since the late 1980s, the Russian RATAN-600 radio telescope (Korolkov & Pariiskii 1979) of the Special Astrophysical Observatory has been monitoring, at 1–22 GHz, a sample of AGNs selected based on their VLBI flux density. The details of these observations, the data analysis, the observing sample, and the results can be found in Kovalev (1997) and Kovalev et al. (1999, 2000, 2002). The measurements of a target at a given observing epoch occur simultaneously at 1, 2, 5, 8, 11, and 22 GHz. For the analysis in this paper, we drop the lowest two frequencies, since they are often affected by radio frequency interference, which became stronger during the years used in this paper: 2009–2019, inclusive.

The RATAN observing sample was originally selected on the basis of the correlated VLBI flux density measurements by Preston et al. (1985) and was later supplemented with new objects found by the VCS. Thus, the sample contains AGNs with strong parsec-scale radio jets and has good completeness characteristics down to Jy. Due to the ring shape and the transit observing mode of the telescope, the best-monitored part of the sample, with 3–4 epochs yr –1 , is restricted to a decl. range from −30° to +43°. This range covers almost all of the IceCube high-energy track events in our sample. The full RATAN-600 data set we use in our analysis has 1099 sources observed at least five times, 758 of which were observed at least 10 times.

There is a rich multifrequency data set produced by the F-GAMMA AGN broadband spectrum monitoring program (Fuhrmann et al. 2016). Unfortunately, the published data only cover the period until 2015 (Angelakis et al. 2019). This is not long enough for our analysis, since many neutrino events were collected after 2015. We have not used these data in the paper.

Radio halos are diffuse radio sources of low surface brightness (∼1–0.1 μJy arcsec −2 at 1.4 GHz) permeating the central volume of a cluster. They are typically extended with sizes of ≳1 Mpc, regular in morphology, and are unpolarized down to a few percent level, probably because of internal or beam depolarization. The prototype of this class, Coma C at the center of the Coma cluster (Fig. 2) has been studied in detail by many authors (see e.g. Willson 1970 Giovannini et al. 1993 Kronberg et al. 2007 Brown and Rudnick 2011).

Thanks to the improvements in observations and data reduction procedures, radio halos with a smaller size and irregular morphology have also been detected in rich galaxy clusters. Their properties in most cases are similar to those of giant radio halos. Figure 3 presents images of well known radio halos of different sizes. A spectacular case is represented by the double merging system, A399 and A401, which both contain a radio halo and can be considered the only case so far of a double radio halo system (Murgia et al. 2010a, Fig. 4).

Images of the clusters A 2163, A 665 and A 2218, hosting radio halos: radio emission is represented by contours, which are overlaid onto the optical image. The maps are all scaled to the same linear scale

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Image reproduced with permission from Rioja et al. (2017), copyright by AAS

Image reproduced with permission from Rioja et al. (2015), copyright by AAS

## National Radio Astronomy Observatory Research Projects by NRAO Site

C harlottesville students Christopher (Kester') Allen, David Copeland and Edward Gray (left to right) examine David Malin's color optical image of Rho Ophiuchi and the region of Antares. David produced color radio images of the same region from data obtained at the NRAO 12m radiotelescope.

There were three students in the 1995 NSF Research Experience for Undergraduates (REU) program at NRAO-Charlottesville. Highlights of the program included a series of introductory level lectures on aspects of astronomy, particularly radio astronomy, spread over a few weeks. These lectures were intended to aquaint the students with the research which various staff members carry out.

Many of the students in the NRAO-Green Bank program visited Charlottesville for a tour of the Central Development Laboratory , and of the University of Virginia's facility for the fabrication of the Semiconductor-Insulator-Semiconductor detectors used in millimeter wave receivers, the Semiconductor Device Laboratory.

. The students had an informal get-together with astronomers from the University of Virginia at lunch, and from NRAO in the evening, followed by a visit with graduate students from the University's Astronomy Department.

Immediately afterward, the Charlottesville students visited Green Bank to see the NRAO telescopes located there, to meet members of the Green Bank staff, and to attend the annual picnic.

The students gave a series of 15 minute talks on their projects during a lunch symposium in Charlottesville before they began leaving for the summer.

Later in the summer, the students returned to Green Bank for a session of observations on the 43m telescope. This was a regularly scheduled program which Al Wootten, the Charlottesville REU coordinator, runs on the 43m to monitor the status of water masers in regions where low luminosity stars are forming. Two new water masers were discovered during the session. The students also toured the Green Bank Telescope, now about 180 feet high, the elevation axis assembled and the box structure which supports the surface being assembled on the ground.

Charlottesville students Christopher (Kester') Allen and Edward Gray (left and right) stand on the apex of the elevation axis tower of the Green Bank Telescope during their tour of the site.

## A Search for Transient Pulsar Signals

David also processed data to form images taken at NRAO's 12 meter telescope. The data was taken using a new type of observing. Normally observing is done by moving the telescope into a specific position, observing for a period of time, then moving the telescope and observing at another position for a while. This is very inefficient because most of the time is spent waiting for the telescope to stop shaking. A new observing mode called OTF or On The Fly observing is being developed at NRAO. This mode moves the telescope constantly and records the position of the telescope with very fine precision. No time is wasted waiting for the telescope to stop moving since it never does. This mode is subject to a sort of "motion blurring" but such blurring can be removed mathematically and maps can be constructed that are as good as images made with the conventional observing mode. David was working on a star forming region in the constellation Ophiuchus. This C18O J=1-0 map of the Ophiuchus Dark Cloud shows a velocity coded image, wich blue highlighting lower (blueshifted) velocity portions of the cloud, red higher (redshifted) velocity portions of the cloud, and green showing material lying near the clouds average velocity. The kinematically disturbed region to the upper right is known as the Ophiuchus A core, and here the most massive stars have formed.

## Water Masers and Cold Gas (Ammonia) in the L1448 Star Formation Region.

On much larger scales, thermal emission from ammonia molecules has been mapped using the VLA by Wootten and Mangum. Allen investigated the ammonia images for evidence of thermal gas in proximity to L1448C. The intensity maps show a pronounced ridge surrounded by several knots of bright gas around the location of the masers. Disk-like, the ridge is symmetrical around the maser positions, and has a "tied- off" pinched appearance in the section closest to the maser. This feature is oriented approximately 65 degrees E of N, and is approximately 20'' or 5000 AU across since this is perpendicular to the observed angle that a line between the blue- and red-shifted masers forms, this suggests that the ridge is related to a possible accretion disk for the hypothetical protostar within. Strong ammonia emission was also observed from the IRS3 protostars to the north.

## Socorro, New Mexico (NRAO Array Operations Center)

Students conducting their research at the NRAO Array Operations Center (AOC) in New Mexico included John Barthelmes, James Brauher, Christopher Carpenter, Amy Hronek, Audress Johnson, Allison Nugent, and Ngan Ying Lui. The program at the AOC is under the direction of Dr. Claire Chandler and Dr. Bryan Butler.

Claire wrote a report on the summer student program at the AOC:

This year the VLA held its regular Synthesis Imaging Summer School in Socorro. Since many of the lecturers who we usually invite to give talks to our summer students were already speaking for the Summer School, we asked the summer students to arrive in time for the School so that they could attend. After this, and several lectures about radio astronomy and interferometry presented by Bryan and myself, many of them showed a good understanding of the technique. We also gave talks on general topics in astronomy, and invited other members of the scientific staff to do the same. The astronomy talks went down well with the students.

I took the summer students on a field trip to visit the observatory at Kitt Peak, including the instruments of NOAO, the NRAO 12-m , and the VLBA antenna. We also visited the Mirror Lab at Steward Observatory. This trip was a big hit, and the students requested more like it in the future. The students from NOAO visited us in Socorro on their way to the National Solar Observatory at Sac Peak , and I gave them a tour of the VLA.

Our students were given a couple of hours of their own VLA time, which was used for an OH maser search toward supernova remnants, a project supervised by Dale Frail.

Our students also gave guided tours of the VLA every weekend, and at the end of their time here, each student gave a short lunch talk (about 15 minutes) about their summer project.

## Green Bank, West Virginia (NRAO 43m and 100m Telescopes)

Students conducting their research at the NRAO Green Bank Site in West Virginia included Katrina Koski, Daniel McCoy, D. J. Pisano, Douglas Williams and Thomas Wilson. The program at Green Bank is under the direction of Dr. Ron Maddalena.

## Sky Survey at X-Band and with the Janksy Antenna.

Katrina worked on two projects this summer, analyzing the raw X band (3.6 cm) Green Bank Earth Station (GBES) Survey data, and helping with the experiments to test the Jansky Antenna. The GBES Surveys are intended to be full northern sky surveys at X and Ku (2 cm) bands. The surveys will monitor the sky, searching for short term variable sources, and monitor all the bright variable radio soruces. These surveys will be repeated at two week intervals. During the summer of 1995, the modifications were being made to the tracking station, which did not allow full Ku band data to be collected. Katrina wrote parts of a C program to edit the X band data, remove solar system objects and convert from Kelvins to Janskys. She used AIPS to grid the data into images, then identify radio sources. Some of the gridded images can be viewed. For example the galactic center region , covers right ascension range 16 to 20 hours, declination -30 to 33 degrees.

2002 Update: Koski obtained a BS in Physics and MS in Astronomy and works for an optical interferometry company in Socorro NM.

## Tucson, Arizona (NRAO 12m and VLBA Telescopes)

S tudents conducting their research at the NRAO Tucson Site in Arizona included Frank Kolor, Larissa Bowles and James Wren. The program in Tucson is under the direction of Jeff Hagen. As the NRAO offices are across the street from KPNO/NOAO offices, the REU group shares in the activities of the NOAO REU program there.

Ms. Bowles used the Astronomical Image Processing System (AIPS) to process the data. The major tasks involved combining the 70 maps of the region into a single map, detecting and deleting bad data, arranging all of the observed points onto a regular grid and collecting the results into a data cube that could be further analyzed. For example, by picking out emission in different velocity ranges, she could isolate emission from different parts of the region. The results of this work will be presented as a poster at the next AAS Meeting.

Ms. Bowles wrote a beginners handbook for anyone using AIPS to do OTF mapping for the first time. This handbook will be useful to anyone with no or limited prior knowledge of AIPS. This handbook will be incorporated in more extensive NRAO documentation.

## Astronomy and Astrophysics for the 1970s: Volume 1: Report of the Astronomy Survey Committee (1972)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

CHAPTER FIVE The High-Priority Program The first II Sections of this Chapter describe in detail the programs and facilities recommended as being of highest priority. Many more suggestions, which may be justified as of great urgency now or in the future, will be found in the individual panel reports of Volume 2. We describe here, in brief, scientific justifications and content of the programs we now recommend. The first four we view as of the very highest urgency and priority. The next seven are also essential to the health and balance of the total astronomical enterprise. The costs over a decade are ap- proximately S600 million for the first four and 51200 million for the entire program. The rate of growth, as has been mentioned previously, is not large, and the manpower available or at present being trained is sufficient. In the final Section of this Chapter, we discuss a program of further new starts that we would have recommended if we bad had only scientific goals in mind with no financial restrictions. VERY LARGE ARRAY The Committee recommends construction of a very large radio telescope array with the ability to observe the u.niverse to great depth with un- precedented clarity. Such an instrument can break through existing observational barriers on a broad front and reveal important new lines of enquiry. Radio telescopes have demonstrated their value by their involvement in 76

Tltt Hl#tÂ·Prlorlry Provâ¢m 77 an extraordinary number of disa>veries in astronomy. These iJK"Iude the quasars, objec1.s of unbelievable energy production and visibility at great distance: the pulsars: the universal blackbody radiation : and the detection of the vast en.s embk of complex intemellu mole<:uks. 10ese discoveries owe mueh to the union ofenJineerlng and electronies, ,.bicb has produced large radio telescopes capable ofdeteetlna lncrodibly faint signals. Indeed, all the radio-sianal en

detected in our radioÂ·astronomical history is linle more than the energy released by the silent impact of a few snowflakes on the ground. Our te!Hropes can today detect easily the radiations of quasars to what . believe to be the edge of the observabk unin:rse. It is not surpming that there has been a flood of remukabk discoveries. However, te<:hnlques that produce great signal Knsilivily could noc as readily give us an ability to see clearly. In fac1, the limit on our ability to KC bas been the d illkulty In distinguishing from one another the numerous objec1s that we can now detect In the sky: a blurred radio picture of the sky has been normal. Great effon has been invested in finding ways to. the sky clearly. Following the development of a new instrumental concept for high resolving po'""r In Australia and England, KVeral obK<Vatories in the United States have developed to a highly successful state a technique that can provide the resolving power so long sought after. This is the method called "apenure synthesis." The basic technique of apenure synthesis involves the combining of signals received at two individual telescopes. retaining all the electrical characteristics of the sianals, including the slana) phase information. Suc.h a pair of telescopes can resolve two point sou roes as . u as can a single large telescope whose diameter l' equal to the separation of the two anÂ· tennas. ObKrvations with a radio interferometer in which the separation of the antennas is increased from zero to some large dimension, perhaps miles, can produce as detailed a picture of the object as that produced by a single prohibiti>Â·ely expensive telescope of the same large dimension. A large number of geographical orientations of the line between the two antennas must be used for the method to succeed. Very high resolving pov."ers can be achieved by this approach at relatively low cost. Indeed, KVeral obK<Vatories have uKd this technique to achieve hiah-quality radio pictures of the sky with resolutions only ten times less than that achieved by optical tekscopes. The method is, howeYer, slow, and satisfactory progress requires simultaneous UK of many antennas. Many astrOnomical probkms require a radio resolving power that approaches that of groundÂ·based npticaltelescoJ!e- - 1 sec of arc. The National Radio Astronomy Observatory has carried out extensi. and detailed studies of aperture synthesis systems to achieve this goal. The

78 ASTRONOMY AND ASTRDPIIYS ICS FOR Til E 1910"o result is a design that can achieve highÂ·quality radio pictures of the required resolution at a rate of about two pictures of new regions per day. This ingenious design achie"'es this speed and r601ution with a minimum cost by u1ilizing 27 antennas of 85-ft aperture, dâ¢ploycd in a arelltlly Calculated pattern 0""C-T an a

a 26 miles in diameter. The rotation Of the earth 0Â·er severa1 hours causes the geometric separation of the antennas as seen from the sky to be altered to produt"e the

uired antenna orientations and separatjons. The antennas are conrrolled. and the inÂ· formation from them processed. by a central large computtr system. This antenna systcn1 is called the Ver)' Large Array tvLM and will be by far the largest ttnd most advanced radio.as tronomical instrument ever conÂ· structcd. It will produce the equivalent of a radio "eye" 20 miles in din. meter. It is estimated that five years will be required to construct it at a COSI or $62 million. Although s uch a giant step in capability will certainly produce major discoveries and surprises that cannm now be predicted, there is an exÂ· tensive ensemble of new results that can be foreseen. Particularly revealing will be the dttaUtd pictures of radio galaxies and quasars. pictures that will show the distribution of high-.:nergy particles and magnetic 6elds. allowing us to trace tht t'Oiution of these vast radiating region.s as they art created by thâ¢ violent uplosh-e evenu in these objects. There will be highÂ· resolution radio pictures of normal galaxies to compaft' with tbe radio galaxies and with our theories of the radio emission or nonnaJ gaJuies and of the objects in them. Thâ¢ ' LA will be a major new tool for cosmology by virtue of its ability to distinguish large numbers or point sources one from another. A key cosmological problem is to plot a number-flux relation to very faint limiting nu.xes, so one is sure to be including sources that are distant enough to distinguish different cosmological models. The VLA can count such source$ because of its narrow beam and large collecting area. Howeler, a more subtle. problem is to eliminate from the count the numerous, but uninteresting, near-by sources that ore intrinsically faint. At present , we :a not sure how numerous such sources are. The Vt.A can rc determine this by observing all sources at a known distance, s uch as in a cluster of galaxies. The narrow bea.m will be decisive in distinguishing indivÂ·idua1 sources in such crowded regions. There is some hope that spectral or other characteristks can be used to distinguish between intrinsically bright and faint sourus: the multifreÂ· quâ¢ncy and polariution capabilities of the vu will be impocunt in this regard. Furthermore, if sourus can be found which haâ¢â¢ a definite distriÂ· but ion of linear sizes. the high angular resolution of the YLA may be able to dâ¢termine the angular sizes of such objects at large distances and thereÂ·

ars, x-ray s tars. and infrared galaxies. Most of these discoveries resulted from the expansion of astronomy into new regions of the electromagnetic spectrum. but obsenâ¢ations a t visual wavelengths have remained central in astronomy because they provide basic information about di.stancc, mass. temperature, pressure. and chemical composition. Funhcrmore. through comparisons with well-esublishod theories. optical astronomy i.s the basic tool for studying stellar C''olution and nucleosynÂ· or thesis. the: ages stars and clusters. the distances and stellar content of g:ala.xies. and the scale of the unhÂ·erse. Moreover. optical astronomy has provided data that challenge established theories. For eaamplc. r=nt photographic advances have re'ealed puzzling phenomena in highly distorted galuies. For optical astronomy to fulfill all these roles. we must have te.l acopes co collect the photons and detectors to record them. Progress in astronomy

The HlghÂ·Prlorlry Program 81 has depended heavily on our ability to build larger telescopes and more efficient detectors. Introduction of refracting telescopes more than three centuries ago led gradually to a SOO-fold improvement in angular resÂ· olution and permitted objects to be seen that are 10.000 times fainter than those that could be seen with the eye alone. These refracto"' "'ere adequate for finding new planets and charting the stellar unive"'e in the nearer parts of our Milky Way. but the astronomer was still left with only the memory of his pe=nal visual perception. Photography. beginning about a century ago. brought modern as- tronomy into being. Not only could each astronomer now share his vision with the world. but. equally important. he cou ld extend it to objects a hundred times fainter. due to the ability of photographic emulsions to store light during long exposures. Photography unveiled the extragalactic universe. but the full appreciation of its size and grandeur depended on the parallel development of large reflecting telescopes through a progression culminating in the 200-in. rellecting telescope on Palomar Mountain, with its ability to study objects 10 million times fainter than can be seen with the unaided human eye. This great instrument. after nearly 25 yea"' of use. still serves as the spearhead of world astronomy. It is worth noting that the 200-in. telescope was funded and designed during the presidency of Calvin Coolidge. before the space age and even before the first nuclear accelerators or radio telescopes. Some of the smaller telescopes still in active use in the country are nearly 100 years old. Since there has been only modest improvement in the efficiency of photographic emulsions during the last 50 years. the building of everÂ· larger telescopes was aimed almost entirely toward collecting more light. The cost of conventional telescopes increases nearly with the cube of the aperture, making this an expensive. although necessary. pursuit. ConÂ· sequently, astronomers began to investigate techniques that would detect photons more effectively than the photographic plate. which nt best can record I out of every 100 photons collected by the telescope. The inÂ· troduction of photomultiplie"' with quantum efliciencies up to 25 percent was a major improvement. but they were limited to view a single resolution element of an image at a time. Detectors were needed that would combine the high sensitivity of the photocathode with the ability of the photograph to record all parts ofa large two-dimensional picture at the same time. The first objective has been accomplished in the last few yea"' by developments that include (I) image intensifiers in which photoelectrons , from a cathode excite a phosphor screen that is then photographed. (2) eleetronographlc cameras in which the photoelectrons strike a photo- graphic emulsion directly. and (3) integrating television cameras in which the photoelectrons are stored in a target that con be read out with

82 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's an electron beam. These techniques have in tum pointed to ultimate systems that will count individual photoelectrons focused onto a two- dimensional array of sensitive elements. In some of these systems. as the data are obtained, they can be read into a computer for immediate processing so that the astronomer can watch the image build and optimize the exposure. 11>e impact of these developments on astronomy has been enormous. In many situations they render present telescopes up to 25 times more effective than before. This is equivalent to scaling each existing 40Â·in. telescope into a 200-in. and the 200-in. into a 1000-in. If a 1000-in. telescope cou ld be built. it would cost S2 bill ion: the replacement cost of the 200-in. is now near $25 million. The equivalent cost of such a fivefold transformation, assuming it could be done in the old way by actually rebuilding existing telescopes, would be a t least SS billion, whereas the cost of equipping all major American telescopes with such devices will be much less than I percent of this. These factors amply account for the unanimity of astronomers in giving high priority to the development of these electrooptical detectors and their installation on large telescopes. Additional improvements can come from the more efficient use of telescope time through various controls for automatic setting and guiding and television cameras for finding and tracking objects too faint (or too red) for the eye alone. At present. work on invisible objects requires the time-consuming procedure of offsetting the telescope from objects that can be seen. The major effecl of the new detectors will not be to observe the same objecls in shorter time but rather to study much fainter objects and to use higher spectral resolution. This will permit critical investigations not thought possible 10 years ago, such as analyzing individual stars in nearby galaxies for element abundances. studying the absorption lines in the faintest quasars. and measuring red shifts of the most distant galaxies. However, even with these impressive advances in detectors and controls, we still need more large telescopes. Some of our major reflectors are near growing urban areas whose lights make the sky too bright for work on the fainter objects. and even the Palomar telescopes are already threatened. While we make all possible elfons to improve the efficiency of present telescopes. we must also build new ones at safe dark sites where there is good seeing. The cost of a ,Â·ery large single-mirror instrument is so high that we recommend experiments with the concept of an optical telescope array. In order to achieve a large coUecting area at a moderate cost. initial efforts should be directed toward developing a multiple-mirror telescope with either an array of mirrors on a common mount or a system of separate telescopes feeding the same detector. If prototype tests prove The HI,M'rlcr/1)1 Pto,.m 8J these concepts feasible. an operating telescope of high optical quality equivalent In area to a ISO. or in. dlould be buill, follcr. d by !he daip and con.wvction or â¢ much lafJCf system in !he 4()(). to 60Q.in. dass. if uperience ,.lh the smaller one iodieates that the next step will succeed. Ho,.ever. If the multiple-mirror telescope don not fulfill exÂ· pectations, another conventional reflector of the 200Â·in. class should be built as soon as possible. While the multiple system is being designed and tested. we must proceed with the construction of at least one standard telcscope 90 in. or larger. at a dark site. In order to begin to compensate for those inÂ· struments that no longer can be used on the faintest objects because of lhe lights from eâ¢pandlng cities. Funding of at least SIO million will be needed for the development of the new elcctrooptkal detectors and installation of the bc>l â¢ystems on all major U.S. telescopes. There are at least nine eâ¢i>tina telescopes large enough to use one or more of lhese detectors profitably. three more under construction. and three proposed. Outfitting these telescopes with telÂ· evision cameras and automatic controls for serting and guiding as ,.ell as with small computers for immediate data reduction ,.11 cost anolher SS million. An operatina multimirror telescope equivalent to a ISO. to 200-in. single mirror is estimated to cost about S.S million. Further funding up to S25 million should then be provided to build the largest possible telescope within that budget-ither a multiple-mirror one with an elfective aperture of 400 to 600 in. if the concept proves to be feasible or a conÂ· ventional 200-in. telescope. An additional SS million is for the urgently needed intcrn>ediatcÂ·sizcd telescope at a dnrk site. The well -rounded program in optical astronomy requires (I) advanced sensors and controls-S IS million. (2) test of array concept- SS million. (3) a 100-in.Â·class telescope-55 million. (4) construction of a large optical array or another 200-in.Â·dass telescope-S2S million. Operatina costs for the new optical facUlties ,.ill reach S3..S million per year by the end of !he decade. INFRARED ASTRONOMY Although Herschel detected infrared radiation from the sun "ilh a thermometer more than 170 years ago. it is only in the past decade that infrared observations have become important to the mainstream of uuonomkal research. Only recently have solid-state and lowÂ·ttmperaturo technotoaies developed to the point where available infrared detectors are 84 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's sensitive enough to study objects other than the sun in any detail. Low- temperature techniques are especially important, because the earth's atmosphere and the telescope are strong sources of background radiation in the infrared and are thus seen by the detector. Infrared detectors must be cooled. often to temperatures as low as 2 K. Ideally, the entire telescope should also be cooled and then lifted into space to avoid contamination by atmospheric radiation. Going high in the atmoshere or into space would also extend the available range of wavelengths, because water vapor makes the atmosphere opaque in large portions of the infrared region of the spectrum. Unlike ultraviolet or x-ray astronomy, which can be conÂ· ducted only from space. some infrared astronomy can be carried out through the atmosphere by large ground-based telescopes. At other wavelengths. the absorption by water vapor. if not the background radiation. can be overcome by observing from an aircraft or balloon above the tropopause. The infrared has great potential for astronomical research. This part of the spectrum begins at the long-wavelength end of the visible spectrum. at about ll'fll. and stretches over a range of more than ten octaves to about I mm. where it overlaps the short-wavelength end of the radio region of the spectrum. Within this range lies the characteristic blackbody radiation of the moon and planets, cool stars. and prestellar clouds. as well as the background radiation of the expanding universe. The infrared is useful for observing any object with a temperature between 3 and JOOO K. The infrared is the realm of molecular spectroscopy. the range wherein lie the vibrational-rotational bands and lines of many cosmically im- portant molecules. Theoretical studies of the interstellar medium also indicate that many of the important heating and cooling mechanisms involve infrared radiations from atoms and ions. But as always. it is the unexpected and surprising that is the most in- teresting. Photometric studies aimed initially at improving temperature and luminosity determinations for cool stars led to the discovery of excess infrared radiation from circumstellar dust shells. A ground-based sky survey found some enormously luminous "infrared stars" that are barely detectable with optical telescopes. Exploratory observations of peculiar galaxies and qua.sars in the near infrared soon led to the realization that some of these objects emit more energy in the infrared than in all other wavelength regions combined. an unexpected and still unexplained result. Rocket observations of the cosmic background radiation. initiated mainly as a check on what had already been learned in the radio region of the spectrum, found a much greater flux than had been expected. and the resolution of the discrepancy may have profound implications for cosmology. ThelllghÂ·Priorlty Program 85 The new technology and the new exciting problems uncovered attract a large number of astronomers. particularly young experimenters. into the field . We recommend ex.pansion of support for this vigorous activity in all areas. including development programs for more sensitive detectors. exploration of new high-altitude dry sites for infra telescopes. and exploitation of multiplex spectroscopic techniques. as well as increased funding of ongoing ground -based. airborne. and rocket programs. So much has been done with so little money (les.s than S2 million per year) that a large payoff is almost sure to follow from n doubling of this effort. As port of this expansion, we recommend an imnuxliate start on a program of surveying the sky for objects bright in the far infrared. This is extremely important for understanding the nature of exploding galaxies and may uncover new and unexpected phenomena. The first step. a balloon survey down to a relatively bright limit. can be done immediately for less than 5200.000. We also foresee the future need for a telescope with a large collecting area and high angular resolution in the far infrared. Such an instrument must of necessity operate in the stratosphere. and we recommend that a design study be initiated soon to determine the most suitable and econo- mic platform. The growth of infrared astronomy is creating large demands on existing telescopes. most of which are neither at the best sites nor optimally designed for infrared work. We therefore recommend as one item in the i.ncreased infrared program. construction of moderate-sized infrared tele- scopes. particularly in the southern hemisphere. We also recommend con- struction of a large (3 to 4 m) infrared telescope (at a cost ofSS million) at the best available high-altitude site in the northern hcmi.sphcre. Such a combined program of ground Â·based, airborne. and rocket in- frared astronomy is sure to lead to many exciting discoveries in this new and expanding field . The total budget is estimated to be S25 million. HIG H-ENERGY ASTRONOMICAL PROGRA M During the first half of the last decade. the total "observing time" in x-ray astronomy had accumulated only to about one hour. through many rocket flights. During that hour it had become apparent that the â¢ Â· ray sky is extraordinarily rich in new phenomena, and that vast and vital aspects of many optical and radio objects had not been appreciated from observa- tions in those wavelengths. The Crab nebula is not only one of the brightest objects in the x-ray sky. 86 ASTRONOMY AND ASTROPHYSICS FOR THE 1970Ƈ but it is also extraordinarily complex. A s1eady xÂ·Â·ray glow is emitted by electrons spiraling in tho magnetic fidds of tho nobula. Pulm! x rays aro emitted from the pulsar created in the spectacular supernova explosion of A.. D. 1054. one of only two radio pulsars known to emit x rays. 11:te x-ray spectNm exÂ·tends up into the gamma-ray ftgion. &c><pius XÂ· l. the bright.,t xÂ·ray object most of the lime. is auociated whh a blue starlike object with strong optical emission lines. X rays are emitted from a hot plasma in the vicinity of the blue object whose nature n:mains a mystery. It appears likely thai many of the celestial x-ray sources in our galaxy are generally similar to Sco X-1. Occasionally, a new x-ray source appears in the sky, is more: intense than Sco X-I for a few months. then declines until it is no longer detect- able. We do not have good enough position measurements of these sources to attempt to identify them with optical objects. One of the first major discoveries of the Uhuru xÂ·ray satellite has been " new class of xÂ·ray sources that undergo regular (pulsarlike) and irregular fluctuations on a rime scale between 0. 1 and 10 sec. No optical identifications are yet available. Many unusual galaxies are XÂ·ray sources. These include strong radio galaxios (M87J. quasars (3C273). Seyfen galaxies. and ordinary galaxios (Jhe Magellanie Clouds shaaÂ· a c:omplex x-ray structure Tromendous amounts or energy are rele.ased in the ltÂ·ray reeion in some or these souru.. po5ing serious challenges to our understanding of high-a ergy I.Sirophysic:s. Underlying all th""' sources is a diffu>e xÂ·ray glow that appears to be featureless. Many astronomers believe that the background x rays were created far away and long ago in the early cosmological history of our universe. This brief and incomplete list of important discoveries in xÂ·ray atÂ· tronomy is reminiscent of the early exciting years of radio a.scronomy. A wide range of new phenomena had been found. but understanding of these phenomena was minimal. The search for understanding required much larger instruments. new techniques. bener detectors. better spectral coverage or the sources. polarization measurements. and the ability to repeac observations for variability. a common featu of Â·Â·compact" objects. A similar pattern of devdopment is needed in a:Â·ray astronomy. Much J.argerÂ·an:a dctecton than have been flown are required in order to find and study faint sources. For the lower-<nergy x rays. focusing optical techniques, involving gruingÂ·incidence instruments. should be Down. The>e will allow detailed pic:turos with high angular rosolution to be obtained. Thoy will also act as photon collectors. concentrating â¢ Â·ray photons from weak sources on Bragg crystal spccnomcten and on The High-Priority Program 81 polarimeters so that the detailed spectral properties of the sources can be measured. Because tbe detectors used with focusing optics can be made very small. the unwanted detector background counting rate can be greatly reduced, facilitating measurements of extended sources and of the apparently isotropic xÂ·ray background. With this major instrumentation. very large numbers of x-ray sources should be discovered. Many new examples of the various classes of x-ray sources in our galaxy should be found. so that the full range of properties of these sources can be studied. Positional determinations of these sources should be greatly improved. thus allowing large numbers of them to be identified with optical objects. With the resulting ability to study the sources in many different wavelength ranges, our theoretical understandÂ· ing of the character and structure of the sources should improve rapidly. Of great importance will be the ability to point at xÂ·ray sources steadily for hours at a time. Not only will this allow a major improvement in the statistics of tbe spectral measurements. but it will also permit studies of the time variations of the total xÂ·ray emission and of individual spectral features. One of the principal striking characteristics of the galactic â¢Â·ray sources that have so far been found has been the temporal variability of tbe x-ray Dux. ranging from rapid Ouctuations to longÂ·term changes. This characteristic is more frequently found in x-ray sources than in optical and radio sources. The major instrumentation should also have extreme importance for studies of extragalactic x-ray sources. It should permit detection of inÂ· dividual sources in nearby galaxies and of emission from active galaxies and quasars to very great depths in space. More definitive measurements of hot plasma concentrated in clusters of galaxies will be possible. allowing a determination of whether sufficient masses of such plasma exist in the clusters to bind the galaxies gravitationally. Much more definitive measurements of the spectrum and isotropy (or lack of isotropy) of the background x rays will improve our understanding of the cosmology and early history of our universe. The National Aeronautics and Space Administration ti<ASAl has recogniud the richness and promise of this field of research by requesting congressional authorization for two large rotating High Energy AsÂ· tronomical Observatories ( KEAO 's). These are to be large spacecraft in orbit about the earth, slowly rotating so that the instruments scan across tbe sky. These will be survey spacecraft. with a large collecting area inÂ· tended to discover new faint xÂ·ray sources. to measure their positions accurately. and to measure spectral properties. Combined with the xÂ·ray instrumentation would be gamma-ray and cosmic-ray instruments. The spacecraft will play an essential role in the future of astronomy. XÂ· ray astronomy will increasingly become a partner to ortical and radio 88 ASTRONOMY AND ASTROPHYSICS FOR TH E 1970Ƈ astronomy as more J[-ray sources are identified and their propc:nies a correlated with thost in other wavelength band$. h is possible that some typt:s of x-ray source may ne-er be optically identified. in -. hich case â¢--e â¢ Â·ill be entlrt-ly dependent on H Â£AO techniques to s rudy them. NAS pla nning also calls for two pointable HEAO 's. 1nc:sc will be el-cn A more imponant to the future or x-ray astronomy chan the rotating HEAO 's. They will permit short-timeÂ· scale Ouctualions in intensity to be followed continuously and to be correlated whh s imultaneous optical. radio, and perhaps infrared observations from the ground. They will take ttdvanttlge of focusing x-ray optics to concentrate the xÂ·ray photons onto small detectors. where background problems can be reduced and angular structural information and positions can be obtained with high accuracy. 11 ls important that NASA also seek authorbÂ·.alion for the pointable 1-H!.AO 's as soon as possible. in order that there not be too g.rcat a time delay berween the discovery of new x-ray objects by the first rotating H EAO and Ihe del ailed study of them by the fi"'t pointable II EAO. A measure of the importance attached to x-ray astronomy by astronomers is that they have scheduled large blocks of lime on major optical instruments to exploit the discoveries and positional measu

ments of new x-ray sources by the UhuN x-ray satellite. This rdl<tt

their expectation that a number of optical identifications will be possible of the newlydiscoVtted x-ray sour=. If this is the case. the HEAO program will make lar

demands on optical astrOnomy and probably also on infrared astronomy. There should be an expansion in major optical facilities to satisfY the requirements of xÂ·ray astronomy. Extragalactic objects in which a major portion of the energy emi.ssJon i.s in the infra.red are also proving to be xÂ·ray objects: it is possible that a similar correlation may exist among some classes of galacttc xÂ·ray objecu. Thu.s an expansion in infrared facilities may also be required for support or X Â·ray astronomy. The highÂ·encrgy astronomical program given extremely high priority by the Committee includes the four HE-AO 's in the NASA planning program. two rotating and two pointed. together with an associated expansion in oplical and infrared facilities to provide the ground support required for lhe development of x-ray astronomy. The esdmated rost of the four HEAO missions is SJ80 million. In adÂ· dition. at least one intermediateÂ·sized optical celescope to support the program should be constructed at a eos1 of SS million. MILLIMETER-WAVE ANTENNA One of the dramatic discoveries of the recent past was the detccdon in tbe clouds of interstellar space of an astonishing variety of molecular spc:ctes.

The High-Priority Program 89 The- e findings contradicted our expectations that the formation of such s molecules was a rare event and that their destruction was rapid bec.ause of the flood of ultraviolet light in the galaxy. The species found range from the sma11. diatomic molecules. such as CO. CS. and CN, to such complex substances as cyanoacetylene. methyl alcohol. formaldehyde. and formam ide. containing as many as six atoms. Carbon monoxide is present in an abundance some thousand times greater than other molecules. probably reflecting lts resis

ance to dissociation by ultraviolet light. The molecules of greatest abundance are those found in our laboratories to form the basic constituents of biochemical systems. For instance. formaldehyde is a precursor of both amino acids and sugars in experiments simulating conditions on t he primitive earth. Thus the molecules observed seem to indicate that the chemistry of life on earth is closely paratteled in interstellar space. The diatomic molecules are almost always best observed at relatively short radio wavelengths of a few millimeters. They form the basic building blocks for the larger molecules, and the physical interpretation of their spectra is much simpler than for the larger molecules. The larger molecules have great significance. however. since they often possess a rich spectrum. both at cent-imeter and millimeter wa'elengths, and form a particularly powerful tool for probing the physical conditions in the interstellar medium. High resolution is necessary to define the distribution of the molecules from which the-modes oflheir formation and destruction can be studied. High sensitivity is necessary to discOÂ·er large molecules. which may have low abundances, and other low-abundance substances such as molecules containing rare isotopes. High resolution and high sensitivity require a very large stecrablc tele- scope with a very precise reflecting surface. Such a telescope has many other important uses. particularly for the study of variations of quasar spectra and intensities and planetary emissions. Such a telescope is not easy to build, because it must n1aintain its geometry to accuracies of tenths of millimeters under the influence of changing gravity forces, wind. and thermal stresses. A great deal of research has been carried out at the National Radio Astronomy Ob- servatory on such precise and stable telescopes. A new approach to telescope design, called the "homology telescope"' has b<-.:n developed. which appears capable of attaining the desired performance. Indeed. some of the principles of this approach have been applied successfully in the new 100-m radio telescope of t he Ma.x Planck lnstitut fiir Radio-- astronomie in Gennany. The very large radio telescope recommended for observations a t milli- meter wavelengths would very likely be a fully steerable parabolic reflector with an aperture of 215ft. performing satisfactorily at wavelengths of 3

90 ASTRONOMY AND ASTROPHYSICS FOR THE 1970Ƈ mm and longer. The cost of this instrument is not as well determined as that of the vu but is estimated to be SIO million. The ronstruction of this telescope will provide a major capability in a particularly promising area of astronomical research and will capitalize on our receiver technology. momentum, and d<sign capabilities in a field developed in the United States and in which the rountry is pre-eminent. AIRCRAFT, BALLOONS, AND ROCKETS An essential part of space research is carried out using small vehicles- aircraft, balloons, and rockets. They are relatively inexpensive and ideally suited for programs of observation with specialized instrumentation where a few minutes or hours of data-taking will accomplish the research obÂ· jective. They have also been essential for testing astronomical in- strumentation for use in space. These vehicles have proved invaluable in the past their utility in the future is assured by t.h e steadily increasing requirements for their use. At a time of severe fiscal ronstraints, the reduction of the number and variety of large astronomical missions in space can, in part, be balanced by the initiation of much less costly programs utilizing small vehicles. These may be able to carry out some of the research contemplated in the abandoned missions, thus maintaining a degree of flexibility and vitality in the affected field of research. The scientifically sensible rourse of action is to increase funding for aircraft, balloons, and rockets when fewer major satellite experiments are planned. If satellite programs are increased, an accompanying increase in rocket research, with smaller but innovative goals, will lead to optimum satellite design and therefore be of high value. Until recently, x-ray astronomy depended entirely upon rocket research. The x-ray sources were discovered by rockets. and quite ac- curate positions were measured for some of them with ingenious rocket instrumentation. Rocket measurements made duri. g a lunar eclipse of the n Crab nebula revealed that the x rays were not a point source. At the present time, rockets are proving essential to the further study of some xÂ· ray phenomena discovered by the UhuTV x-ray satellite. Unexpectedly rapid x-ray fluctuations of the Cyg X-1 source were discovered utilizing the satellite, but since the satellite rotates, it is not suitable for following the fluctuations. Rockets are capable of pointing at a source like this for several minutes at a time, and missions can now be Instrumented to provide the data essential to a better understanding of Cyg X-1. Since this source appears to be but one of several classes of strange x-ray objects, it is clear that there will be a pressing need for more x-ray astronomy

The HigltÂ·Prlorfty Program 91 rockets for the next several years- and most certainly through the era of the Kigh Energy As1ronomical Observatories. Ultraviolet astronomy also began with rockets, first for studies of the sun and then for studies of the stars. Differences were found between theoretically calculated ultraviolet stellar spectra and the rocket obÂ· servations. Rapid rates of mass loss from hot supergiant stars were discovered by spectroscopic observation in the ultraviolet. Perhaps one of the most important of the ultraviolet astronomical discoveries was that of molecular hydrogen in interstellar space. Today the bulk of the ultraviolet astronomical observations are carried out with an Orbiting Astronomical Observatory. but the instrumentation in this vehicle is relatively infiexible, even though it returns a great amount of data. It is necessary to supÂ· plement and enrich these data with selective rocket measurements using a wider range of instrumentation. The loss of OAOÂ·B has been a severe setback for ultraviolet astronomy. The authorized program will conclude with the launching of OAOÂ·C in fiscal year 1973. For many years, the program of ultraviolet astronomy from spacecraft is likely to be modest even if new satellites such as the proposed SASÂ· D are authorized. In these circumstances, it will be all the more important that a supplementary program of rocket observations in the ultraviolet be provided to maintain vigor in this field of research. The instruments carried in these rockets may provide some of the measurements that would have been made by OAOÂ· B. They also will provide an opportunity to exploit the discoveries made by OAOÂ·A and OAO.C and will provide an important survey of certain classes of ultraviolet phenomena. There will undoubtedly be many celestial objects found in these ultraviolet studies that will turn out to pose important scientific puzzles. many of which can be further studied and elucidated by resea.rch using rockets. Infrared astronomy now relies heavily upon aircraft and balloons. While a few infrared windows can be exploited from the ground, most of the wavelength region, and especially the far infrared, requires an obÂ· serving platform above the bulk of the atmospheric water vapor. Observations from balloons and aircraft have given important new spectroscOpic information in the infrared about the sun and planetary atmospheres. Observations from aircraft have detected high Ouxes of radiation in the infrared from the cores of active galaxies and quasars. Large numbers of strong infrared sources near the center of the galaxy have been discovered during surveys made from aircraft and balloons. NASA is providing an aircraft platform for a 36-in. infrared telescope, which should produce important new results. The Committee recomÂ· mends that a first, crude, long-wavelength infrared sky survey be carried

92 ASTRONOMY AND ASTROPIIYSJCS I' OR Till! 1910 's out from balloons in the near furure. In the longerÂ·range future. a deep- sky SUf'ey in the infrared will probably require satellite techniques, but these will require a prior ro<ket de..elopment program. Hen

. in&aml astronomy will be a major user of aircraft. balloons, and rockets in the next few )'tars. Solar rescareh has been heavily dependent on ro<kets as well as on satellites in the Orbiting Solar Observatory series. These have produ

detailed ultraviolet spectra and x-ray pictures. They have been Oown on command at times of solar Oares. There is a continuins need to sup- plemenc che sacellite coverage or che sun with special, Oexible. quickÂ· response rocket instrumentation. Thus essentially all che major are.as of space astronomy have an exÂ· panding need f'or small researeh vehicles: aircraft, balloons, or rockets. The expenditure on these research vehicles for astronomical research presenlly amounts 10$12 million to SIJ m illion per year. The Committee slrongly recommends that the expend iture for chis type or researeh be doubled os rapidly as possible. cenainly within the next ch years. SOLAR PROGRAM The Opening up of the extreme ultraviolet and Xâ¢ray region of the solar spectrum by ro<ket and satellite observations has proÂ¥ided many imÂ· ponant new advances in solar research in the lase decade. l.n this region or the spectrum occur the dominant emissions from the solar corona, where mechanical energy, generated in the solar outer convection zone, is deposited both in the form of steady heating and in violenl even IS such as solar nares. Apan from tcoching us more about eoronal heating and the origin of flares and cosmic rays. euv and x-ray observations of the sun, as the brlghcest astronomical object, also play a role in leading the way to the understanding of similar observations elsewhere in the universe. The Orbiting Solar Observatory tOSOI program was started in the beginning or the last decade. The oso 's provide â¢ platform ror studying both raptd events and slow variations of radiation over time intervals up to one year. There has been steady improvement in the capabilicies or these sacellites. Early oso 's bad vinually no spatial resolution and carried only small payloads. Rapid tecllnologi<:al development will ma.k e it possible for the eighth oso . to be Oown in 1973, to cany instrumeniS that attxin a spacial resolution or - 1 sec or are. comparable with chat obcained with the better around-based telescopes. This proaram or continuous development and gradual improÂ¥ement has J The HighÂ·Priority Program 93 made the oso program among the most successful and productive of all astronomical satellite programs. We recommend the continuation of this program beyond the present oso series. through oSOÂ·L, . ., , and Â·N (at a cost of S30 million each), to be Oown during the next solar maximum (1977-1931 ). These oso 'swill probably provide for the first time a spatial resolution equal to or better than that of the very best observations ob- tained from the ground or balloons. This improved spatial resolution is of utmost importance, since we know from ground-based observations that the energy transfer to the chromosphere, to flares and cosmic rays, and perhaps to the corona, occurs on scales probably less than or equal to 1 sec of arc. oso. L., Â·M, and Â·N will fly during the next period of maximum solar activity, with a spatial resolution 10 to SO times better than was possible in the last period. They will carry instruments capable of analyzing the properties of flares and active regions in the spectral region from 3000 A down to the very energetic x rays below 0.1 A. It is entirely reasonable to expect that these observations will result in a significant increase of our understanding of the layers of the sun above the photosphere, of solar activity, and of solar Hares. We envisage this continued oso program, together with the expanded solar rocket program discussed in the space astronomy recommendation, as the bac.kbone of the solar space program. It is of the greatest imÂ· portance, however, that improved observations from space go hand in hand with the improvement and extension of observations from the ground. The solar photosphere, best observed in visible and near infrared radiaHon, reveaJs most of the sources of the energy input in the chromosphere. and eorona in the form of granulation, magnetic structures, and mechanical motions. Coronagraphs, eclipse experiments, anticipated observations of far infrared recombination lines, and radio observations provide relatively inexpensive ways to observe other aspects of the sun's upper atmosphere. We therefore recommend the continuous updating of existing groundÂ·based and aircraft facilities and the construction of small specialized telescopes for the visible and infrared spectral regions (at a cost of approximately Sl.O million per year). This updating includes improved image detection, storage, and analysis, as well as improvement of image quality by telescope refinement and site selection. For the study of the interaction of solarÂ·Oare plasma with the magnetic field and plasma of the outer solar corona, we suggest the construction of a relatively inexpensive multifrequency metric and decametric radioheliograph with moderate (I -5 min of arc) spatial resolution (at a cost of approximately$1.5 million). The cost of the program over the next decade will be S90 million for oso.L. Â·M. and .Nand SIO.Omillion for ground-based facilities.

94 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'o THEORETICAL ASTROPHYSICS AND COMPUTING REQUIREMENTS Physical theory has always played a crucial role in astronomy-from the period when Newton's theory of gravitation provided the uplanation of planetary orbits to the present time. when nuclear reattion theory promises to el<plain the synthesis of chemical elements in supernova explosions. Any balanced program for progress in astronomy wiU necessarily contain a vital, if relatively inexpensive, program of theoretical research. Much theoretical astrophysics today is concerned with model building. In this type of activity, physical principles substantiated in the laboratory, including those of quantum theory, nuclear physics, and plasma physics, are used to construct a mathematical model of an observable astronomical object, such as a star, a galaxy, or even the whole universe. The relevant equations are. usually complex and nonHnear and must be solved on a computer. The resulting models are then compared with observations to fix parameters of the model, such as the mass of the star or the random velocities of stars in a galaxy, and to show how the model should be im- proved to attain agreement with observations. Model building is essen- tially the only way known to convert the stream of photons entering a telescope into a physical picture of what is going on. The theoretical astrophysicist thus stands astride physics and asÂ· tronomy. Oose contact with physkists is essential if current developments there are to be properly included in the model. Constant interaction with observen is essential if theoretical work is to be aimed in the most productive directions for interpreting nature and if observational work is to be focused on the most theoretically significant questions. In the recent past there has been increasing exploration of dynamic states. The theory of stellar evolution can be largely constructed from a sequence of static stellar models, but in the final stage of a star's life-in some ways the most interesting one-events occur very rapidly, with gravitational collapse and outgoing shock waves playing a vital role. To reconstruct these phenomena, it is vital to simulate the dynamics in a computer. Dynamical modeling is playing an ever-increasing role, from stellar explosions to interstellar shock waves to the spiral structure of galuies. Such modeling is orders of magnitude more time-consuming than static modeling, so fasttr computers with larger memories are required. A prime example of the success of this apptoKh is the modeling of a supernova el<plosion, in which the progress of a shock wave is followed in detail, and a netWOrk of about 100 nuclear reactions is followed at each time step. The result is a prediction of the abundances of the chemical elements. which seems to agree remarkably well with observation.

The High-1+/orlty Ptogram 95 A related activity is theoretical work in dynamical astronomy-the application of Newton's equations of motion (with small relativistic corrections) to the positions of planets and satellites of the solar system. Here the problem is to compute the orbits using interactions between all bodies to extract precise values for the parameters of the system, including the masses of the bodies involved. Recently, such work has demonstrated its vitality by providing extremely ae<:urate motions of the earth for use in reduction of optical observations of pulsars. Without these precise positions (about J0Â·8 of the distance to the sun), it would have been imÂ· possible to utilize the precise optical timing measurements, which require correction for light-travel time within the solar system. It would thus have been impossible to infer the existence of abrupt changes in the period of the Crab pulsar, which have been interpreted as due to starquakes in the crust of a neutron star. Such is the unity of astronomy, of the old and the new. We believe that increasing the effort in the universities, where there is strong interaction of theoretical astrophysicists with both observers and physicists, is the best way to optimize results in theoretical research. We suggest particular emphasis on relativistic astrophysics, stellar evolution (particularly early and late phases), derivation of physical data needed to construct precise stellar models (including opacity sources, nuclearÂ·energy generation rates, convection theory, and equations of state), and theoretical interstellar physics and chemistry (including the solid-state theory of grains, molecular and atomic cross sections and transition probabilities, the theory of masers, and the plasma physics of interstellar gas and magnetic fields). Interaction between relatively isolated theoretical groups should be increased wherever possible, for example, between groups working on stellar interiors, stellar atmospheres, and observational stellar spec.. troscopy, between plasma theorists and astrophysicists working on stellar and interstellar plasma processes, and between chemists and astronomers working on molecular astronomy. Support should be increased for both theoretical and experimental study of atomic and nuclear collis-ion cross sections and transition probabilities, taking care to locate this work in several independent groups to increase the effectiveness of cross checking. By and large, this can be accomplished by supporting physicists in universities where there is an active astrophysics group that can be helpful in establishing priorities for experimentation and calculation. We recommend that in the specific areas of beam-foil spectroscopy and low-energy nuclear cross sections, the U.S. Atomic Energy Commission lAEC) consider support of groups utilizing existing facilities for this work. Funds are needed for individual university investigators to increase

96 ASTRONOMY AND ASTROPHYSICS FOR TH E 1970's their efforts using suclt university computers as are available. The fund.s available for computation generally need to be increased. Theoretieal astrophysicists and dynamical astronomers are moving into an era when the maximum speed and storage capacity available will be needed to solve dynamical problems, but many university and national center computers are not equal to this task selected ones should be upgraded. In addition, state-of-the-art computers in mission-oriented agencies such as the AEC and I<ASA would be extremely useful if means for using them part-time can be worked out. The additional funds needed for first-rate activity in this area are not trivial-perhaps SS million per year. The theoretical etfort at the national observatories needs to be fostered. Research output would be optimized by increasing the availability of theoreticians at the national centers. To succeed, it is essential to find highly quali6ed versatile individuals as visitors or on the staff. Such a goal involves enhancing the computer facilities, as required, to make the observatory attractive both to resident and visiting theorists. Joint activities between physics and astronomy programs in universities should be encouraged. Because of the close relationship of theoretical ast.rophysies to both physics and observational astronomy. productivity is served by every possible mode of cooperation. including. in some cases. merged departments, joint academic programs. and shared facilities. It is most important that astronomy PhD students receive as thorough training as possible in physics. and to this end. special seminars should be designed. A National Institute of Theoretical Astrophysics has been suggested. to provide a focus for theoretical research, to promote interchange between astrophysicists from different suhfields and between astrophysicists and other scientists, and to provide a stimulating atmosphere for postdoctoral fellows before they accept permanent appointments. A proposal by the Panel on Theoretical Astronomy would fund an institute at an annual rate of approximately $750.000 for a fixed period of seven years. The institute would have some six permanent statf members. with an outstanding scientist as director. and would be located in an anractive place close to a researclt university and close to a group of observational astronomers. There would be particular emphasis on p015tdoctoral and visiting ap- pointments. and in keeping with the need to keep administrative and other expenses low. the support statf and computarional facilities would be strictly limited . The Committee concurs with the panel in the thrust of its recom- mendation for an institute. Nevertheless. it believes that for both pragmatic and historical reasons. the main strength of theoretical astrophysics is likely to remain in the universities. There It can have the The High-Priority Program 97 greatest impaÂ£1 on the educational process and on young men from a wide diversity of backgrounds and fields of interest. The institute. if it is set up. should strengthen. not compete with. university groups. Emphasis on interaction bÂ«ween groups. on funding of young people. and on a moderate budget. whi<:h will suffice if the staff and computer facilities are limited, is consistent with this goal. We recommend. to this end, that if the institute postdoctoral fellowship program is established. it be used also for purposes not immediately related to a"endance at the institute. including travel funds for visits to other institutions and the cost of computing at home institutions or other facilities. While there are advantages in such a permanent institute. we recom- mend that, as a first step. consideration be given to smaller funding for a summer institute. Such an Institute would have no permanent staff beyond the director and would occupy rented space at one of a number of possible sites that may prove attractive. No computation facilities would be provided the entire funds beyond rental and minimal administrative expen.ses would be expended on travel and subsistence for a few senior and a larger number of junior people. We believe that the final plans for a possible permanent institute would be beneficially affected by one or two yearsâ¢ experience with such a summer institute. Both the Theoretical Astrophysics Panel and the Commi"ee wrestled at length with a problem that theoretical astrophysicists, along with others in all areas of theory, now face in their needs for a very large computer. Our conclusion may be viewed as suggesting something for evel')-one. We are probably in a state of transition from a stage in which large generalÂ· purpose university centers were optimum to a stage when the needs of many different research groups will share much larger computers through sophisticated data-communication links. We understand that quantum chemists have considered a national center with high -power computers. comprehensive software library. and staff of computer-oriented theoretical chemists. able to do large-scale service-type calculations for others. The needs of the Global Atmospheric Research Program suggest that an international network of large computers would be desirable. It will ultimately be necessary for scientists to assess these requirements and discuss the problems of a national computing system. making maximum use of facilities already in place, or needed, for calculations in industry. the space program, weather forecasting. and reactor design, among othen. The needs of astronomy should be considered when such an over- all national computing system is discussed . Theoretical astrophysics is a growing field rhat aruacts young astronomers and physicists with a broad range of interests. The speed of modern computers makes it possible to construct models of atoms. stars. 98 ASTRONOMY AND ASTROPHYSICS FOR T HE 1970's and galaxies and to study the dynamics of the solar system or the universe. The tools of the theoretician, excq>t for the large computers. are inex- pensive. The pa!!C1'n for the bes! range of computing racili!ies. national and local. muse still be â¢'Orked out. We recommend an inCTeased program of abouc SJ million a year. For the theoretician. travel. co make new contacts and co anend summer institutes. performs a spcc:ial function. Interdisciplinary research is particularly elfec!h'O and nor erpensive. Theoreticians ean work at small institutions. often at colleges or u_ ivcrs1lies without large facilities. n OPTICAL SPACE ASTRONOMY-LEADING TO THE LARGE SPACE TELESCOPE Some of the- most farÂ·reaching additions to our kn Â·ledge of the universe occurred during the first half of this century with the development of asuonomkal speetroscopy and its utilization with large telescopes. During thls time, spec!roscopic analysis of planetary atmospheres. the sun, the stan. and the intersce11ar medium brought about clarifications in our understanding or these objects. Of equal significance was the speeÂ· troscopic StUdy or extC1'nal galaxies, leading to the discovery Of the in- CTCUO of Spec!r()SCOpic red shift with distance and the realiurion that we live in an exptnding universe. Throughout this development. ucronomers have been acutely conscious of the fact that their analyses ..'eft inromplete and tentative. since much of the information that they would have liked to have obtained was in the inaccessible ultraviolet r-ange of wavelengths. The mlssing spectroscopic information oonsists of two classes: one is the spectral lines in the ultraviolet due to elements and stages or ionization of elements that do not have lines in the visible region of the spectrum: the other is the general shape of the spectrum ln the ultraviolet and the relation of this to the distribution of emitted energy In the visible and Infrared wavelength regions. Ultraviolet observations c.an be made only above the atmosphere. During the last IS years, the technological barriers against such ob- servations have progressively been broken. Rockel obsC1'Vations of the sun and the stars have resulted in a numbÂ« of important discoveries conÂ· cerning the ultraviolet spec!rum of the brightest objects risible in space. At the same time. the discovery of quasars. some of them with large Spec!r()Scopic red shifts. has pi'OYided a means .. â¢hereby the ulttariol<t emission frun a limited doss of objects can be studied frun the ground. because the light originally emitted in the ultraviolet has been red-shifted into the risible region of the spec!rum. Tilt H/tlt-I'Worlty Pro,.m 99 llÂ«auâ¢ objÂ«ts emitting ultraviold lipt are also likely 10 emil visible li&lll. il has not been expected thai completely n. classes of objÂ«ts would be di-ed. NevertMlas. there ha. been a number of important discoveries made concunlng IM properties in IM ultraviolet of some of the objÂ«ts that had previously been studied in tM viJiblt: I. 'The ultraviolet resonance lints in oa-tain earlyÂ·lype nellar aianiS have shown that manor is 01reaming out from lheoe stan with velocities of the order of 1000 km per sec. wilh total mass loss raltS of the order of toÂ·' solar mass per year. 2. 'The extinction of ultraviolet light by the interstellar medium has turned out to be dllferent from that predicted on the basis of observations made In the visual region. There is a prominent absorption feature ncar 2200 J. and a gradual increase in the extinction toward shorter wavelenat)ls. These results are leading to extensive m-sions of our ideas concemina tM character of interstellar grains. and the prest!KC of considerable variations of these features In difl'erent pans of the inÂ· terstellar medium l ndleates that individual stan can mndify tMlr inÂ· terstdlar environments. 3. MOOiplaxles have been found to emit more radiation in tM shorter ultraviolet wavelengths than would have been eapected on the basis of tMir apparent c:olor temperatures in the visible rqion. 4, Loree hydroeen clouds have been found â¢urTOUndlng tM recent bright c:ome1S Tago-Sato-Kosaka and Bennett. Such laree clouds appear to c:onstitute a fourth ma>o< structural component of the c:omet. S. A broad absorption feature at ). 2550 has been discovered in the spectrum of Man, possibly due to ozone. The Orbiting Astronomical Observatory program is becomlna a true national facility for astronomers. On the firs OAO. about ten groups of astronomers have been observing approximately 100 objects. 'The OAOÂ·C is upected to have a c:onsiderably greater obsenlna capability. and c:onnquently il should be of great service to the astronomical c:ommunhy throup IM pcstÂ·obsene< program. 'The Orbiting Astroncmical Observatory proeram has. unfortunately, been marked by tragedy. Tbe first and third launcbcs were failures. the fintthroup troubles â¢ith tbe bancry. and the third through a failure in the launch vehicle. Afier the launch of oâ¢OÂ·C. tMr< are no further authorlz.cd proerams in space ultraviolet astronomy. At the present time, no satellite capable of carrying on intermediate spectral and spatial observations in the ultraviolet is funded. 'The ultimate objective of the ultraviolet astronomy program should be 100 ASTRONOMY AND ASTRO PHYS ICS I'OR TilE 1910'â¢ the: development of a National Space Observatory containing a large diffrattion Â·limited telescope capable of operating in the nearÂ·infrared and visual rcaions as well as in the ultraviolet: . The exciting role that such a large space lclcscopc(LST)could play in astronomy during the decades to rome is disaJsscd in 1he final Section of this Chapter. The nominal apcrlure lhal has been utilized in stud.. of the UT is 120 in. Such an instrument roukl anack problems that arc of 'he: most fundamental astronomical significance and that are unlikely ever 10 be solved using ground based instruments. Perhaps of even greater importance than its ultraviolet capability would be the high angular resolulion of s uch a telescope. Turbulence in the atmosphere limits the angulor resolution obtainable with large telescopes to the equivalenl ol' that obtainable with a 12Â·in.Â·aperture telescope, although the lightÂ·galhering power of a larger instrument is superior. In the visible region, the L would have an ST angular resolulion better by a factor of 10. whic.h means that one resolution element observed with a groundÂ·ba.â¢ ed telescope could be divided into 100 resoturion elements with the uT. The angular resolu1ion in the ultraviolet would be still better by a factor ncar 2. One result of this high a ngular resolu1ion should be the capability of observing stars and stellar-appearing objo:ts at nearly ten times the di>tancc at which such objects can now be studied with the 200-in. telesropc. Tt.e LST should lead to a much improved understanding of the most fundamental problems in cosmology. as well as of the broad range of astronomical problems pmenlly being in,Â·estigated by groundÂ·based astronomers. A great deal of technological dcvclapment will be required before such an LST can be launched. It will be desirable to test the new 1cchnology, not only Chrough rocket instrumentation for ultraviolet studies but also through the construction and flight of intermediate instruments. For example. a diffractionÂ·limited space telescope of about(>() in. would have a tremendously useful versatility and capability beginning to approach that of the csÂ·r itself. It is now technically feasible t o build s uch an instrument, and it would be useful to incorporate into its design che results of new technological developments intended foc the LST. Yet no high Â·quality large telescope is in the current planning stagt:. The Committee recommends very strongly that a vigorous program be maintÂ· ined in uhraviolet astronomy. This program should be directed a toward the ultimate use of an 1ST'. One or more intennedi.ate instruments, designed to test the technology of the ur and to return large amounts of data of immense value to the astronomical community. should be launched. If there is to be an extended delay between the launch of OAO.C and the first of these intermediate in.struments. then it is most desirable that an interim ultraviolet telescope be launched. perhaps a replacement for the OAOÂ· B or a smaller instrument in a Small Ascronon'y Satellite. The HlghÂ·Prlorlty Progrom 101 The program for ulrraviolet astronomy that "'e have outlined is a large one. leading, as it eventually should. toward a large spaee telescope as a majoc prosram for the next two decades of astroncmy. Within il there is enoogh Ouibility to provide ample trade-off pclOSibilities t>enoÂ·een smallÂ· scale acdvities and larger instruments. If we cannot alford the largest diffractionÂ·limited instrUment soon. then a much more vigorous rocket and lntermediateÂ·size ultraviolet and infrared telescope program is needed to avoid losing all opportunities in this aru. If, as appears likely. the 120Â·in. must be delayed to the midÂ·l980's. the 6Q.in. diffractionÂ· limited tcleseope is an important prototype. giving both valuable exÂ· perience and important scientific results. The cost of continuing the ultraviolet satellite program throughout the next decade at a pproximately the current level of expenditure (SJS million per ycor) Is SJSO million. LARGE CENTIMETERÂ·WAVE PARABOLOID large stcerable paraboloids have been the basic instrument of radio astronomy. Within minutes. a modern radio dish can be converted from one frequency band to anOiher. and its mode of operation can change from polarimetry to spcctroseopy at the Dick of a switch. Even major changes: in receiving equipment. suth as the installation of masers aod other refrigerated amplifiers or the installation of radar transmitters. take only a few hoors. This versatility has paid rich scientific dividends. especially in the study oftime variation of radio sourees. In spectrographic studies of the interstellar medium. and in studying the polariurion of radio sources. Large steerable paraboloids have been esstntial elements in the recent developments of ''ery-long-baseline interferometry CVLBU. in which the study of radio-source struccure to angular resolutions of better than 0.001 sec of arc has been possible. They have geodetic applications. Each larger instrument has, in its first few years of operation. produced new discoveries. Even a modest increase i_n size gives a surprising ad- vant:lge. bt(ause the etrective sensitivity, for observation in a given period of time. varies as the fourth power of the diameter. An add itional ad Â· vantaae is the freedom, with a Oexible instrument. to pursue occasional speculative programs. The recent explosive growth of diseovery of new molecules in the intcntellar medium provides an exÂ«.llent example, IU a new subbranch of astronomy- the chemistry of spac&-has staned to grow. The choice of instrument size. and of its wavelength capability (determined by the precision of its eonstruction). has been carefully considered. An instrument whose diameter is approximately 440 ft v.'Ould represent a significant nep beyond any existing or planned steerablc 102 ASTRONOMY AND ASTROPHYSICS FOR THe 1970's paraboloid. and it appean that a dish that performs well at 2 em and is usable with somewhat reduced efficiency to 1-cm wavelength is well within present ongineering pract . Tho largost comparablo antonna. tho 100-m telescope of Cormany's Max Planck lnstitut. is actually only an SS.m telescope at wavolengths shorter than 6 em. Thus the projtcted instrument has three times grtater observing capability at all wavtlengths, and at wavelengths of 6 em and smalkr o-Â·er six times grtater observing capability. An especially attractivo feature of the new paraboloid is its com- plementary role with our proposed millimeter-wave telescope. The simple basic molecules such as CO, CN. and CS have spectra that lie in the millimeterÂ·wave region, while the larger. quasi-organic compounds such as methyl alcohol, formaldehyde, cyanoacetylene. and formic acid have spectral lines in the band from 2 to 30 em. Many of the larger molecules, and ammonia. possess lines that could be observed with either system, although tho grcater angular rtsolving powor of the 440-fi telescope would give it an advantage for certain problems. The large centimotor-wave paraboloid would certainly servo as the hub of many VLII observing programs, and its large area would inertase ononnously the classes of objtct accessible to study. In conjunction with the other large paraboloids of the world. stntcbing from Australia to the Soviet Union. the present observations of the closer, bri&ht objtcts would be extended to quasan and radio galaxies that are far more distant and faint. The radar capability of the new instrument would also be impressive. With the exception of Pluto. all the planets and the larger moons of Jupiter and Saturn would be within range of its 6-cm radar, while the greatly enhanced signal-to-noise ratio would enable the radar astronomers to study the surfaces of Venus and Mars in great detail, enhancing the effectiveness of space missions to those planets. The estimated cost of such an installation. including the telcsonpe. land acquisition, site development, controls, computers, radiometers, and radar, would be approximately SJS million. Some economies could be effected by s haring common support facilities with other instruments such as the very large array or the large millimeter-wave tele=pe. Operating costs would be S3.5 million per year following its completion. ASTROMETRY The establishment of a system of star positions based on an absolute inertial system is essential, and the system of proper motions should be detennined with respect to such an inertial frame. 1M HiiJtÂ·l+forlty . m 103 The mean propu modons of faint stars are of fundamental importance to the study of unusual stars found in the galutic halo. Many interesting objects in the halo are between I and 5 lqx from the plactk plane. and ..en with the rapid spac:e m(l(ions of elttrmle halo nars, thdr angular proper m(l(ions are small-approximately 0.25 sec of arc: pu year. The modons must be determined ..th high indmdual accuracy. This requires that the inertial frame be determined to an oa:uracy of at least 0.005 sec of arc per year Ideally, the accuracy should be several times hlgber. Ooe type of fundamental data that astronomers mu$1 have is the distan<e to the object studied. Interesting objects are at great distances, which can be calibrated in successive steps if nearby objects of similar characteristics have accurate distance measurements. The m0$1 funÂ· damental method uses accurate trigonometric parallax-the anaular displacement of a star caused by the earth's motion about the sun. These parallues are the backbone of the stellar distance scale. They are oeeded for faint stars near the sun and for bri&ht stars at ereater dittanc:es. An insufficient number of trigonometric parallaxes in the southern hemisÂ· phere will reduce the beoefiu of the laraer facilities built there by the United States and Europe.a n countries. Stars morina parallel in space appear to converac. becauJe of perÂ· spective effects this method provides individual distances for nearby star clusters. Ou$ter parallaxes should be extended to the southern hemiÂ· sphere and to fainter clu5ters in the northern hemisphere. For other distant types of stars, we mU51 take advantage of the accumulated drift provided by the moe ion oft he sun through space, which cauJeS the 5tars to drift bacbâ¢ard at angular speeds proportional to their parallax. Such group or secular parallaxes are often the only possible distance measure for the moSII.nteresting stars of high luminosity. They depend directly on the accuracy of the fundamental system of proper motions. Theories of stellar interiors would have a sounder basis If a sufficient number of parallaxes and masKs of nearby stars and clusters could be provided. These should include interestlna and important objects like rapid variables. hiably luminous B Slars. plaoeta. y ncbulu, hoi sub- r dwarfs, bright white dwarfs, and cool red d

nerate stars. The establishment of the actual luminosity-temperature diaaram for stars like the sun and fainter is CSJeDtial for the determination of the distances to the &lobular ciU5ters and the luminosities of the RR Lyrae stars. For these important determinations, a combination of trigono- metric. clu5ter. secular parallaxes. and all (

sible methods must be used. Recently. the possibility has appeared of detecting companions of low mass by the nonlinearity of the motion of a nearby star throut

h space. Several companions have been announced that have masses like that of

104 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'o Jupiter-or even lower values. These astrometric binaries have been studied . ntially in very few institutions. take a long time to give results, and yet will provide us witb our only diRe! proof of the existence of other planetary systems until radio communication from some of these may eventually be rurived. The changes of period detected in pulsan are fundamental to the theory of neutron stan. Yet the lint observations of these changes . comÂ· promised by uncertainties in such suppusedly well-known subjects as tbe orbits of the planets around tbe sun and the masses of t.he planets. The motion oft he earth around the center of gravity or the earth-moon S)Ƈtem is detectable in the accurate observations of the radio pulsars. Jmpro'lcd planetary orbits are necessary to take full advantage or this technique. Similarly, the very-long-baseline-interferometry technique requires ac- curate geodesy anq accurate timekeeping. The improvement and e,xte,nsion of astrOmetric measurements neeÂ· essary to Interpret the problems mentioned above rests ultimately on ob- serva1ions by small astrometric instruments. We 1herefore recommend const.ruction of two automatic transit circles. three photographic zenith tubes, three astrolabes. and three automatic measuring engines, as weU as modernitation or several existing long-focus telescopes. the equipment to be located geographically so as to provide systematic observations in both the northern and southern hemispheres. The precision attained by these rundamentalastrometri<: instrumenu bas hardly been affected by modem electronic technology (u. pt for tbe timekeeping funetlon). However, the modem technology or automatic measu_ement is in fact successful. and r we recommend it. together with some or the classical smaller telescopes mentioned above . part or our fundamental program. The estimated cost of these small instruments is 56.4 million. BEYOND THE RECOMMENDATIONS After concluding a detailed study of the state or our science and making our recommendations within the framework of recent available funding, we feel that it is important to discuss. in certain areas, what additional programs our science requires to meet fully the seientilic challenges tbat -..'e face. We have therefore re-examined the manpo--er raourees that will be available in the decade and tbe technologk:ally feasible and desirable projects studied by tbe panels. What areas have ""'omitted, disearded, or redueed in siu mostly becau.e or financial constraints? How much have we failed to recommend or the urgent needs pressed by our technical panels?

The H/xhÂ·l'rlorlty Pro,.m lOS Larg<' Space Telescope Without any doubt. the largest and most exciting area is the ronnruction and launch of a large space telescope u..sn . for highÂ·rcsolution nudics in the normal and ultraviolet spectral regions. possibly with manned resupply and maintenance (e.gâ¢â¢ by the space shuttle). This development can be underuken in a vigorous way only at budget levels for astronomy and physics that represent c:onsiderable growth over the nut decade. The LST concept is based on two major exploitation.s of the orbital environment. First, the mirror-from 60 to 120 in. in diameter. depending on available fundo-will c:over completely the wavelength interval from 1000 A (the eutoH' imposed by interstellar attenuation) to 10.000 A(or 1 pm). with considerable utility out to 1 mm. thereby covering the entire ultraviolet and Infrared range not accessible from the ground. as well as the optical window. The large collecting area and high angular resolution over this entire range would provide unmatched versatility. But a more important dimension of the LST is the precis,ton of its image in the ultraviolet and optical ranges. On the ground, the d<let<rious effects of atmospheric seeing smear the image to one or m6re seconds of an: even at an excellent she. This means that the obse"er is in eft"tc1 comparing the image ohhe tarect object with tbat of tbe night sky (including ba<kground galactic light. zodiacal light. and airglow) in a comparable solid angle. lf a 120-in. t<lcs<ope can be designed to achieve diffrocrion limitation at SOOO A. an image as small as 0.04 sec of arc in diamet<r would result. If an image ofO. l sec of arc can be achieved in practitt, the nightÂ·sky radiation. which tends to obscure the imago of a faint object, is effectively redu<cd by a ractor of 1()0-a five-magnitude gain in sensitivity over ground-based instrumcnt,sofcomparable aperture. There is an additiona.l gain from the fact that the tel<scope operates above the alrglow layer and, of course. dots not sulfer from atmospheric attenuation. 11 should be possibl< to observe to apparent magnitude 29 in several hours of int<gration. Th< implications of such a capability for all branches of astronomy are great. The Committee feels that the LST has extraordinary potential for a wid< variety of astronomical uses and believes that it should be a major goal in any Vo'ellÂ·pla.n ned program of groundÂ· and spaccÂ·bascd as1ronomy. The Committee recognizes that th< large <ost involved can be acÂ· commodated only "'Â·itbin a vigorously growing prosram. Therefore. it has adopted the view that, wlhio the main program. the emphasis on the LST is at a moderate level of som< SJS million per year. enough to fund tcchnoloaioal development of smaller.apenure telescopes aod an LST in the following de<ad<. A much more expensive program is required if the LST is to become a

106 ASTRONOMY AND ASTROPHYSICS FOR THE 1970Ƈ reality in the 1980-1985 period. This Committee sees the l.ST as a natural program goal to follow the High Energy Astronomical Observatories I HEAOI mission. To achieve this will require budgets for diffractionÂ· limited missions that grow from a level of t.h e order of S20 million per year in 1970 to the order of $200 million per year in 1980, with launch scheduled for the early 1980's. Total cost of the program leading to the final fabricat.ion of a 120-in. telescope will be of the order of Sl billion over 10 years. A program of this magnitude requires the highest quality scientific leadership a.n d the most advanced space engineering available. The highest quality scientific leadership in this field can be found in the academic community. and the highest degree of space engineering talent exists in the centers of the National Aeronautics and Space AdÂ· ministration. Therefore, the best chance for success lies in a merging of academic talent with that in the NASA centers. We suggest that NASA select one or more centers to carry out the engineering phases of the program and that the National Academy of Sciences encourage the formation of a new corporate entity representing universities with strong programs in space astronomy. The latter should be limited to less than eight members in the interests of efficiency. This corporation would be responsible for establishing a National Ultraviolet Space Observatory I NUSOI - a working scientific laboratory under conÂ· tract to NASA and the National Science Foundation. The Director of the NUSO should be a scientist of top rank in space astronomy. The NUSO would be responsible for the planning and utilization of a series of satellite ultraviolet observatories, including the LST , and for administeri.ng them on behalf of the entire scientific community, as is done for the ground-based national observatories. To achieve this mission, the NUSO would work closely with the responsible NASA centers. Effective control of the engineering task ofthe Nuso would be exercised by NASA effective control of the scientific direction would rest in the Director and in the Board to which he would report. OpticalÂ· and Radio-Astronomy Instruments Certain major facilities in optical and radio astronomy were omitted from our program, for reasons of economy. Optical astronomers could make effective use of two more telescopes in the 200-in. class, with modern electronic auxiliaries. The pressures generated by space and radio astronomy have so overcrowded the few large instruments that even the two ISO-in. telescopes under construction fail to match the present needs. In addition to our recommended optical program, it would be desirable to ThtlllghÂ·Prlority Program 107 double the effeclive collecling area of existing large telescopes. To acÂ· complish this, at least two additional 200-in. telescopes or two equivalentÂ· cost larger arrays or possibly one even larger array would have to be built in addition to those that we have recommended. Such a program would cost SS() million (whh site development) plus the modem instrumentation described earlier in this chapter under Optical Astronomy-Electronic Teehnology and UghtÂ·Gathering Power. or the radio telescope systems planned. studied. and repeatedly recommended. one major ite,m is omitted from our list of new starts. It is the only large. university-based plan that goes bac.k to the Whitford report-the completion of the Owens Valley aperture-synthesis inter- ferometer of 130-ft radio telescopes. The original plan required five adÂ· ditlonal antennas. tracks. receiver. and computer. The high quality of the mechanical design makes the present 130-ft good at 2 em and possibly usable at I em. An aperture-synthesis array working at high frequencies, usable for molecular and atomic lines, can be constructed for SlS million. Its beam, at 2 em, will give 2 to 4 sec of arc resolution its collecting area and sensitivity is about half that of the VLA. One advantage of the relat.ivdy small number of IJO.ft antennas is the Oexlbility, ability to change rapidly, and reduced cost of the receivers needed to permit aperture synthesis, at high resolution, in moleeular and atomic emission and absorption lines. In addil.ion, the interferometer could be used for extragalactic astronomy at higher frequencies, providing data on the time- f&riable radio sources ,.th Oat or rising speetra. VtryÂ·Long-Baseline Interferometry A tremendous breakthrough in our ability to perceive fine details in radio sources has come in the last four years as the result of the development of very-long-baseline Interferometry <VLBil. By using highly stable atomic clocks. highÂ·speed magnetic recording. and modern computing techniques. antennas distributed over the entire world can now be used as elements or a single radio telescope. If we were to extrapolate from our present pioneering observations of the brightest sources and construct a vision of future developments. we could confidently sketch a technically feasible system that could construct complete maps of the details of quasars and interstellar masers. The present network of large antennas gives a sketchy view because there is a lack of intermediate spacings and north-south baselines. The situation could be remedied by the development of a mobile vLao terminal, consisting of two dishes, one large and one small, plus the necessary atomic clocks and recording apparatus. The large dish would be designed to permit rapid assembly and 108 ASTR ONOMY AND ASTROPHYSICS FOR THE 1970 Ƈ disassembly. so that it could be transported 10 new locations. The small antenna would constantly monitor one of the stronger sources, to provide constant updating of the station clock. S.Venl terminals would be needed, cutainly 01 least two on each continent, although the best disposition would have t o be determined by a cueful study. The resulting network, if operated at 1 wavelen81h (which recent observation of H20 masers at 1.35 em have shown to be feasible) could give us a complete pictun: of the radio structure of quasars. with 0.0001 sec of arc resolution. If our ideas of the distances or quasan are correct. we could see structures appro:Umately I light-year in siu and could follow the development of dynamic events from year to year, seeing the details of these enormously energetic events. There are other. more speculative areas that one can also foresee-the study of the coronas of other scars, the observation of their sunspots and flares, the study of supernova shell developments in other galaxies, and the analysis of the mysterious nuclei of Seyfert galulcs. In add ilion to the Yl.BI program at radio wavelengths, we foresee the development of interferometer techniques at both infrared and opti<al waveltngtbs. Bec-ause the angular resolving power or an intttferometer vari inversely with the waÂ·ele.ngth, one can anticipate remarkable di.sccweries by such systems. rivaling the recent radio vu 1 demonstration of motions appan:ntly faster than light in a quasar explosion . The ultimate instrument would be a 10-pm YLBI having a global baseline ooâ¢ kml. Such a devWe would have a n:solution of JOÂ·' sec of are, pennitdng one to peer deep into a quasar, perhaps to see explosive events on the surface of a superma.sslve star, which, some say. powers a quasar. The surface features of exotic: stan that sporadic-ally shoot dust and molecules into Interstellar space could also be studied. The choice of 10Â· .urn wavelength IJ dictated partly by the fact that atmospheric phase shifts are small there. permitting the use of large apertures, and partly by the fact that quasars and n:d giants-key objects in relativistic astrophysics and molecular astronomy--radiate a major f-raction of their energy there. The 10- m VLBI might use a superheterodyne system, which mixes the incoming infrared signal with a stabilized CO, laser to produce a microwave signal that can he recorded at each telescope. The bandwidth of available tape recorders (100Hz) should be sufficient to detect at least the brighter sources. A f nner of this device is nov.â¢ under C-'Onstruc:don, usina Hne-ofÂ· sight transmission of a w t.Hz bandwidth microwave signal to a common point to form an interference pattern. Following tests of the system with a 0. 1-km baseline (loÂ·> sec of an:), it wiD be expanded to 10 km (10 .. sec of arc). It will be sensitive enough to study nearby Seyfert galuies and bright The lligh-Prlorlry Progrom 109 galactic objects. but a version sensitive enough to study quasars (where the resolution will be I light-year) will require larger telescopes and better detectors. Of course, most astronomical objects emit more powerfully with visible light so that there also is need for devices that can work in that spectral range. Fundamental studies of angular sizes are possible with both the intensity interferometer, which conelates the intensities in the two signal$, and the Michelson interferometer. wbich brings togethor the raw signals to fonn fringes. A large-intensity interforometer could be built imÂ· mediately with a l Â·km baseline to givo tO"' sec of arc resolution, but perfection of the Michelson system requires dovelopment of an optical delay line and techniques of fringe detection. The Optical Facilities Panel believes th3l both the delay line and fringe detection should be studied immediately with funding up to $200.000. Beyond these preliminary investigations, worthy goals of a teo-year program include a sensitive 10-km infrared interferometer, and perhaps a IO'Â·km infrared VLBt, and for visible wavelengths a J. or 2-km intensity interferometer and a Michelson interferometer with a similar baseline. The sensitive IO.km infrared interferometer is estimated to cost SIO million over the decado. and the large intensity interferometer S4 million. Further studies are needed before the cost of the infrared vut or the Michelson interferometer can be estimated. Infrared Astronomy The growth of infrared technology resulted in discovery of quite unexÂ· pected objects that radiated most of thoir energy in the infrared. The energy maximum atiSOO K is at 2,um and is observable from the ground. A survey with a 62Â·in. light collector discovered 20,000 cool stellar and prestellar objects. Observations in the far infrared are needed to study objects near 500 K. most of whose radiation falls in regions of high atÂ· mospheric absorption to study objects at 50 K, observations above the atmosphere are needed. The Infrared Panel put highest priority on a large stratospheric telescope, about 120-in. in diameter, in a large, high-Oying aircraft or possibly supported by balloons, gliders, or kites. We recomÂ· mended funds only for study of the most economical modo of operating suc.h a large infrared telescope. but the scienti6c goals of the large stratospheric telescope are extremely important. No realistic financial estimate can yet be made both the study and e"J)Crience with the NA SA CÂ· 141 airplane (with a 36-in. telescope) will determine the best course of ac1ion. The infrared groups are small at many universities. in both astronomy and physics. The changes in technology, the availability of new 110 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'a detedOI'$, and the revelations of new types of objects make this an un- predictable but challenging field. lntenlisclplinary grants to physics and asuophyslcs departments will enlist the aid of low-temperature physicists for astrophysical applications. Solar Phyâ¢ics Solar physics has benefited enormously from the oso seri.. of solar observations. osoÂ· s are rapidly becoming more sophisticated and more reliable. However, a large diffraction-limited solar telescope (about 40-in. diameter) is needed, carrying a heavy payload (over 1000 lb) and capable of accurate pointing and 0. 1 sec of arc gu iding. This will provide high spectral resolution in the optical and near ultraviolet a nd will permit very fi ne-scale study of the rapidly fluctuating solar plasma, iu excitation temperature, velocity, and magnetic field . This is a large project, of the order of S200 million, but it is one that will both provide experience use

ful for the UT and be a nearly ultimate solar space telescope. High-resolution observations of the radio sun provide information on the energetic partido acceleration ptOC0$5, as revealed by the gyrosyn- chrotron radiation. The relativistic electrons are studied near the site of the acceleration of solar cosmic-ray baryons. This s1udy requires a high- resolution radio telescope with about S sec of are resolution, which works on a short time scale and ..sentially giv.. a radio picture. A radio spec- troheliograph in Australia has already demons1rated iu usefulness in the study of the interaction of fast particles and the hot solar plasma and has shown that Hares are triggered across the sun as disturbances run out through the corona or return to other activecentel'$ on the disk. Theoretical Astrophysics Facilities should not monopolize our attention. The present a nd planned facilities, the space astronomy program, and the importance of the fie ld for itself justify a strong case for theoretical as1rophyslcs, over the wides1 possible range of topic$-$tudy of neutron Sill'$, the quieter phases of stellar evolution, planetary dynamics. galact ic structure, supernovae, collapse nuclcosynthesls. explosions in galaxies, black holes. relativity, and cosmology. A test of the concept and viability of an Institute of Theoretical Astrophysics is an inexpensive recommendation. Also linked to theoretlcal needs is a fourth- or fifth-generation computer at a single National Computing Center. The total cost of an Institute and Computer Center ror 10 years might be S40 million. About JO percent of our recent PhD's in astronomy have their degrees in, and wish to work in, theoretical The High-Priority Program I II astrophysics or dynamical astronomy. The issue of a National Computing Center is not clear-cut. since the efficiency and costs of high-speed long- distance lines are not yet known, but the 't'ery large computer is at the heart of much theoretical model building in astrophysics. To take ad- vantage of the presently available theoretical talent among young astronomers and physicists, we also urge that an expanded postdoctoral and senior postdoctoral program be considered. The goal would be to provide a number of theoreticians with at least a summer's or, preferably, a year's visit to other universities, national, ooo, or NASA centers, by direct fellowship grants. with freedom to travel, or by small research grants covering their salaries and expenses. ## Astronomy and Astrophysics for the 1970s: Volume 1: Report of the Astronomy Survey Committee (1972) Unfortunately, this book can't be printed from the OpenBook. If you need to print pages from this book, we recommend downloading it as a PDF. Visit NAP.edu/10766 to get more information about this book, to buy it in print, or to download it as a free PDF. Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages. CHAPTER FIVE The High-Priority Program The first II Sections of this Chapter describe in detail the programs and facilities recommended as being of highest priority. Many more suggestions, which may be justified as of great urgency now or in the future, will be found in the individual panel reports of Volume 2. We describe here, in brief, scientific justifications and content of the programs we now recommend. The first four we view as of the very highest urgency and priority. The next seven are also essential to the health and balance of the total astronomical enterprise. The costs over a decade are ap- proximately S600 million for the first four and 51200 million for the entire program. The rate of growth, as has been mentioned previously, is not large, and the manpower available or at present being trained is sufficient. In the final Section of this Chapter, we discuss a program of further new starts that we would have recommended if we bad had only scientific goals in mind with no financial restrictions. VERY LARGE ARRAY The Committee recommends construction of a very large radio telescope array with the ability to observe the u.niverse to great depth with un- precedented clarity. Such an instrument can break through existing observational barriers on a broad front and reveal important new lines of enquiry. Radio telescopes have demonstrated their value by their involvement in 76 Tltt Hl#tÂ·Prlorlry Provâ¢m 77 an extraordinary number of disa>veries in astronomy. These iJK"Iude the quasars, objec1.s of unbelievable energy production and visibility at great distance: the pulsars: the universal blackbody radiation : and the detection of the vast en.s embk of complex intemellu mole<:uks. 10ese discoveries owe mueh to the union ofenJineerlng and electronies, ,.bicb has produced large radio telescopes capable ofdeteetlna lncrodibly faint signals. Indeed, all the radio-sianal en detected in our radioÂ·astronomical history is linle more than the energy released by the silent impact of a few snowflakes on the ground. Our te!Hropes can today detect easily the radiations of quasars to what . believe to be the edge of the observabk unin:rse. It is not surpming that there has been a flood of remukabk discoveries. However, te<:hnlques that produce great signal Knsilivily could noc as readily give us an ability to see clearly. In fac1, the limit on our ability to KC bas been the d illkulty In distinguishing from one another the numerous objec1s that we can now detect In the sky: a blurred radio picture of the sky has been normal. Great effon has been invested in finding ways to. the sky clearly. Following the development of a new instrumental concept for high resolving po'""r In Australia and England, KVeral obK<Vatories in the United States have developed to a highly successful state a technique that can provide the resolving power so long sought after. This is the method called "apenure synthesis." The basic technique of apenure synthesis involves the combining of signals received at two individual telescopes. retaining all the electrical characteristics of the sianals, including the slana) phase information. Suc.h a pair of telescopes can resolve two point sou roes as . u as can a single large telescope whose diameter l' equal to the separation of the two anÂ· tennas. ObKrvations with a radio interferometer in which the separation of the antennas is increased from zero to some large dimension, perhaps miles, can produce as detailed a picture of the object as that produced by a single prohibiti>Â·ely expensive telescope of the same large dimension. A large number of geographical orientations of the line between the two antennas must be used for the method to succeed. Very high resolving pov."ers can be achieved by this approach at relatively low cost. Indeed, KVeral obK<Vatories have uKd this technique to achieve hiah-quality radio pictures of the sky with resolutions only ten times less than that achieved by optical tekscopes. The method is, howeYer, slow, and satisfactory progress requires simultaneous UK of many antennas. Many astrOnomical probkms require a radio resolving power that approaches that of groundÂ·based npticaltelescoJ!e- - 1 sec of arc. The National Radio Astronomy Observatory has carried out extensi. and detailed studies of aperture synthesis systems to achieve this goal. The 78 ASTRONOMY AND ASTRDPIIYS ICS FOR Til E 1910"o result is a design that can achieve highÂ·quality radio pictures of the required resolution at a rate of about two pictures of new regions per day. This ingenious design achie"'es this speed and r601ution with a minimum cost by u1ilizing 27 antennas of 85-ft aperture, dâ¢ploycd in a arelltlly Calculated pattern 0""C-T an a a 26 miles in diameter. The rotation Of the earth 0Â·er severa1 hours causes the geometric separation of the antennas as seen from the sky to be altered to produt"e the uired antenna orientations and separatjons. The antennas are conrrolled. and the inÂ· formation from them processed. by a central large computtr system. This antenna systcn1 is called the Ver)' Large Array tvLM and will be by far the largest ttnd most advanced radio.as tronomical instrument ever conÂ· structcd. It will produce the equivalent of a radio "eye" 20 miles in din. meter. It is estimated that five years will be required to construct it at a COSI or$62 million. Although s uch a giant step in capability will certainly produce major discoveries and surprises that cannm now be predicted, there is an exÂ· tensive ensemble of new results that can be foreseen. Particularly revealing will be the dttaUtd pictures of radio galaxies and quasars. pictures that will show the distribution of high-.:nergy particles and magnetic 6elds. allowing us to trace tht t'Oiution of these vast radiating region.s as they art created by thâ¢ violent uplosh-e evenu in these objects. There will be highÂ· resolution radio pictures of normal galaxies to compaft' with tbe radio galaxies and with our theories of the radio emission or nonnaJ gaJuies and of the objects in them. Thâ¢ ' LA will be a major new tool for cosmology by virtue of its ability to distinguish large numbers or point sources one from another. A key cosmological problem is to plot a number-flux relation to very faint limiting nu.xes, so one is sure to be including sources that are distant enough to distinguish different cosmological models. The VLA can count such source$because of its narrow beam and large collecting area. Howeler, a more subtle. problem is to eliminate from the count the numerous, but uninteresting, near-by sources that ore intrinsically faint. At present , we :a not sure how numerous such sources are. The Vt.A can rc determine this by observing all sources at a known distance, s uch as in a cluster of galaxies. The narrow bea.m will be decisive in distinguishing indivÂ·idua1 sources in such crowded regions. There is some hope that spectral or other characteristks can be used to distinguish between intrinsically bright and faint sourus: the multifreÂ· quâ¢ncy and polariution capabilities of the vu will be impocunt in this regard. Furthermore, if sourus can be found which haâ¢â¢ a definite distriÂ· but ion of linear sizes. the high angular resolution of the YLA may be able to dâ¢termine the angular sizes of such objects at large distances and thereÂ· The HlglrÂ·l'rioriry Program 79 fore study the angular diameter-flux relation, which should be sensitive to cosmological effects. In summary, the VLA will be able to approach the solution to the cosmological problem by a variety of avenues. The vu will also open a new method for study ofthe stars-by providÂ· ing information on the continuum radio emission of many normal stars. Just as radio telescopes have revealed imponant new information about high-energy envelopes of the sun, particularly about the solar corona, the VLA will give us our first opportunity to observe these phenomena in other stars, opening the door to important advances in stellar and plasma physics and perhaps providing clues to unsolved mysteries of the sun itself. Galactic novae have been observed with interferometers. and the VLA will give the detailed evolution of the clouds of plasma and gas ejected violently in the nova outburst. Perhaps emission from Wolf-Rayet, P Cygni, and magnetic stars will be detectable. Prototypes of the VLA have measured the astonishing changes in the emission of xÂ·ray stars in only hours. Nevertheless. the searches for x-ray star radio emission have been panicularly frustrating. contributing little data toward the solution of the enigma of x-ray st.ars. The great imÂ· provement in sensitivity offered by the vu may well remove a barrier to the understanding of these intriguing objects. The VLA will give us for the first time a clear picture of the bean of our galaxy. where there is a complex ensemble of radio-emitting regions, concealed from optical telescopes by the dense dust clouds of the Milky Way. There is evidence that violent events in the nucleus of the galaxy have strongly influenced galatic evolution. Indeed, one object in the center may be the same type of structure that produces the quasar phenomenon. By measuring the radiation of individual radio spectral lines, such as that of atomic hydrogen at 21-cm wavelength, the VLA will be able to give pictures of the gas clouds of our galaxy in such detail that we will see the processes taking place in them the effects of heating, cooling, and supersonic collisions should all be discernible. The structure of the gas system of nearby galaxies will be sharply defined, testing theories of galactic dynamics and evolution. The vu will be able to distinguish detail in t.h e radio emission of all the planets but Pluto, enabling the temperatures of the planets at various latitudes, seasons, and times of day to be established. The radiation belts of other planets could be measured in detail, and the atmospheric structure and nature of the planetary surface, be it rock. soil, or waterÂ· containing material, could be studied. The VLA, and some other radio-astronomy facilities. will require a new site. It is possible that the large steerable dish or millimeter-wave dish could be located in the same area. Site development economies are 80 ASTRONOMY AND ASTROPHYSICS FOR T HE 1970'o possible in radio astronomy, since the major common requirement for all these innruncnu is a large area. free from industrial and radar elex1:rical interference and direct aircraft routes. They all require highly develoPÂ«! technical suppon fOT retth-ers, computers, data analysis. and control. A dry, high -altitude site is preferable for the millimeter-waâ¢Â·e dish, although nCM so important for the 01ber devices. Whh 1he program for the vu. which -.u rome into operation only near the c:nd of this decade. we recomme-nd expansion of research suppon and funding of' moderateÂ·si:zed instruments at university or consot1ium- operated r3dio observatories at a rate of S2.5 million per year. This will permit smaller groups to probe new areas of technology: new concepts in antenna and receiver design, ultraÂ·highÂ·frequency detectors. small millimeter-wave antennas and interferometers, centimeter-wave interÂ· fcromcters and receivers, adaptable to the new atomic nnd molecular lines disc.'O'tred, and vcry-fong-bascline interferometric terminals and arrays. A balanced program in radio astronomy requires a variety of less expenshÂ·e racHides and innovative, Oexible research projecrs, in addition to the large national facilhy described. The oosts over ttn years for university facilities would be S2S million, and S62 million for the VLA _ About S6 million per year (10 percent of the capital cÂ»st) will be required to operate t.he vu . The full operating costs " 'ill not oecur until the last half of the deeade. OPTICAL ASTRONOMY-ELECTRONIC TECHNOLOGY AND LIGHT- GATHERING POWER We have witnessed a decade of remarkable discoveries in astronomy, including qua ars, x-ray s tars. and infrared galaxies. Most of these discoveries resulted from the expansion of astronomy into new regions of the electromagnetic spectrum. but obsenâ¢ations a t visual wavelengths have remained central in astronomy because they provide basic information about di.stancc, mass. temperature, pressure. and chemical composition. Funhcrmore. through comparisons with well-esublishod theories. optical astronomy i.s the basic tool for studying stellar C''olution and nucleosynÂ· or thesis. the: ages stars and clusters. the distances and stellar content of g:ala.xies. and the scale of the unhÂ·erse. Moreover. optical astronomy has provided data that challenge established theories. For eaamplc. r=nt photographic advances have re'ealed puzzling phenomena in highly distorted galuies. For optical astronomy to fulfill all these roles. we must have te.l acopes co collect the photons and detectors to record them. Progress in astronomy The HlghÂ·Prlorlry Program 81 has depended heavily on our ability to build larger telescopes and more efficient detectors. Introduction of refracting telescopes more than three centuries ago led gradually to a SOO-fold improvement in angular resÂ· olution and permitted objects to be seen that are 10.000 times fainter than those that could be seen with the eye alone. These refracto"' "'ere adequate for finding new planets and charting the stellar unive"'e in the nearer parts of our Milky Way. but the astronomer was still left with only the memory of his pe=nal visual perception. Photography. beginning about a century ago. brought modern as- tronomy into being. Not only could each astronomer now share his vision with the world. but. equally important. he cou ld extend it to objects a hundred times fainter. due to the ability of photographic emulsions to store light during long exposures. Photography unveiled the extragalactic universe. but the full appreciation of its size and grandeur depended on the parallel development of large reflecting telescopes through a progression culminating in the 200-in. rellecting telescope on Palomar Mountain, with its ability to study objects 10 million times fainter than can be seen with the unaided human eye. This great instrument. after nearly 25 yea"' of use. still serves as the spearhead of world astronomy. It is worth noting that the 200-in. telescope was funded and designed during the presidency of Calvin Coolidge. before the space age and even before the first nuclear accelerators or radio telescopes. Some of the smaller telescopes still in active use in the country are nearly 100 years old. Since there has been only modest improvement in the efficiency of photographic emulsions during the last 50 years. the building of everÂ· larger telescopes was aimed almost entirely toward collecting more light. The cost of conventional telescopes increases nearly with the cube of the aperture, making this an expensive. although necessary. pursuit. ConÂ· sequently, astronomers began to investigate techniques that would detect photons more effectively than the photographic plate. which nt best can record I out of every 100 photons collected by the telescope. The inÂ· troduction of photomultiplie"' with quantum efliciencies up to 25 percent was a major improvement. but they were limited to view a single resolution element of an image at a time. Detectors were needed that would combine the high sensitivity of the photocathode with the ability of the photograph to record all parts ofa large two-dimensional picture at the same time. The first objective has been accomplished in the last few yea"' by developments that include (I) image intensifiers in which photoelectrons , from a cathode excite a phosphor screen that is then photographed. (2) eleetronographlc cameras in which the photoelectrons strike a photo- graphic emulsion directly. and (3) integrating television cameras in which the photoelectrons are stored in a target that con be read out with 82 ASTRONOMY AND ASTROPHYSICS FOR THE 1970's an electron beam. These techniques have in tum pointed to ultimate systems that will count individual photoelectrons focused onto a two- dimensional array of sensitive elements. In some of these systems. as the data are obtained, they can be read into a computer for immediate processing so that the astronomer can watch the image build and optimize the exposure. 11>e impact of these developments on astronomy has been enormous. In many situations they render present telescopes up to 25 times more effective than before. This is equivalent to scaling each existing 40Â·in. telescope into a 200-in. and the 200-in. into a 1000-in. If a 1000-in. telescope cou ld be built. it would cost S2 bill ion: the replacement cost of the 200-in. is now near$25 million. The equivalent cost of such a fivefold transformation, assuming it could be done in the old way by actually rebuilding existing telescopes, would be a t least SS billion, whereas the cost of equipping all major American telescopes with such devices will be much less than I percent of this. These factors amply account for the unanimity of astronomers in giving high priority to the development of these electrooptical detectors and their installation on large telescopes. Additional improvements can come from the more efficient use of telescope time through various controls for automatic setting and guiding and television cameras for finding and tracking objects too faint (or too red) for the eye alone. At present. work on invisible objects requires the time-consuming procedure of offsetting the telescope from objects that can be seen. The major effecl of the new detectors will not be to observe the same objecls in shorter time but rather to study much fainter objects and to use higher spectral resolution. This will permit critical investigations not thought possible 10 years ago, such as analyzing individual stars in nearby galaxies for element abundances. studying the absorption lines in the faintest quasars. and measuring red shifts of the most distant galaxies. However, even with these impressive advances in detectors and controls, we still need more large telescopes. Some of our major reflectors are near growing urban areas whose lights make the sky too bright for work on the fainter objects. and even the Palomar telescopes are already threatened. While we make all possible elfons to improve the efficiency of present telescopes. we must also build new ones at safe dark sites where there is good seeing. The cost of a ,Â·ery large single-mirror instrument is so high that we recommend experiments with the concept of an optical telescope array. In order to achieve a large coUecting area at a moderate cost. initial efforts should be directed toward developing a multiple-mirror telescope with either an array of mirrors on a common mount or a system of separate telescopes feeding the same detector. If prototype tests prove

The HI,M'rlcr/1)1 Pto,.m 8J these concepts feasible. an operating telescope of high optical quality equivalent In area to a ISO. or

in. dlould be buill, follcr. d by !he daip and con.wvction or â¢ much lafJCf system in !he 4()(). to 60Q.in. dass. if uperience ,.lh the smaller one iodieates that the next step will succeed. Ho,.ever. If the multiple-mirror telescope don not fulfill exÂ· pectations, another conventional reflector of the 200Â·in. class should be built as soon as possible. While the multiple system is being designed and tested. we must proceed with the construction of at least one standard telcscope 90 in. or larger. at a dark site. In order to begin to compensate for those inÂ· struments that no longer can be used on the faintest objects because of lhe lights from eâ¢pandlng cities. Funding of at least SIO million will be needed for the development of the new elcctrooptkal detectors and installation of the bc>l â¢ystems on all major U.S. telescopes. There are at least nine eâ¢i>tina telescopes large enough to use one or more of lhese detectors profitably. three more under construction. and three proposed. Outfitting these telescopes with telÂ· evision cameras and automatic controls for serting and guiding as ,.ell as with small computers for immediate data reduction ,.11 cost anolher SS million. An operatina multimirror telescope equivalent to a ISO. to 200-in. single mirror is estimated to cost about S.S million. Further funding up to S25 million should then be provided to build the largest possible telescope within that budget-ither a multiple-mirror one with an elfective aperture of 400 to 600 in. if the concept proves to be feasible or a conÂ· ventional 200-in. telescope. An additional SS million is for the urgently needed intcrn>ediatcÂ·sizcd telescope at a dnrk site. The well -rounded program in optical astronomy requires (I) advanced sensors and controls-S IS million. (2) test of array concept- SS million. (3) a 100-in.Â·class telescope-55 million. (4) construction of a large optical array or another 200-in.Â·dass telescope-S2S million. Operatina costs for the new optical facUlties ,.ill reach S3..S million per year by the end of !he decade. INFRARED ASTRONOMY Although Herschel detected infrared radiation from the sun "ilh a thermometer more than 170 years ago. it is only in the past decade that infrared observations have become important to the mainstream of uuonomkal research. Only recently have solid-state and lowÂ·ttmperaturo technotoaies developed to the point where available infrared detectors are

ThelllghÂ·Priorlty Program 85 The new technology and the new exciting problems uncovered attract a large number of astronomers. particularly young experimenters. into the field . We recommend ex.pansion of support for this vigorous activity in all areas. including development programs for more sensitive detectors. exploration of new high-altitude dry sites for infra

telescopes. and exploitation of multiplex spectroscopic techniques. as well as increased funding of ongoing ground -based. airborne. and rocket programs. So much has been done with so little money (les.s than S2 million per year) that a large payoff is almost sure to follow from n doubling of this effort. As port of this expansion, we recommend an imnuxliate start on a program of surveying the sky for objects bright in the far infrared. This is extremely important for understanding the nature of exploding galaxies and may uncover new and unexpected phenomena. The first step. a balloon survey down to a relatively bright limit. can be done immediately for less than 5200.000. We also foresee the future need for a telescope with a large collecting area and high angular resolution in the far infrared. Such an instrument must of necessity operate in the stratosphere. and we recommend that a design study be initiated soon to determine the most suitable and econo- mic platform. The growth of infrared astronomy is creating large demands on existing telescopes. most of which are neither at the best sites nor optimally designed for infrared work. We therefore recommend as one item in the i.ncreased infrared program. construction of moderate-sized infrared tele- scopes. particularly in the southern hemisphere. We also recommend con- struction of a large (3 to 4 m) infrared telescope (at a cost ofSS million) at the best available high-altitude site in the northern hcmi.sphcre. Such a combined program of ground Â·based, airborne. and rocket in- frared astronomy is sure to lead to many exciting discoveries in this new and expanding field . The total budget is estimated to be S25 million. HIG H-ENERGY ASTRONOMICAL PROGRA M During the first half of the last decade. the total "observing time" in x-ray astronomy had accumulated only to about one hour. through many rocket flights. During that hour it had become apparent that the â¢ Â· ray sky is extraordinarily rich in new phenomena, and that vast and vital aspects of many optical and radio objects had not been appreciated from observa- tions in those wavelengths. The Crab nebula is not only one of the brightest objects in the x-ray sky.

86 ASTRONOMY AND ASTROPHYSICS FOR THE 1970Ƈ but it is also extraordinarily complex. A s1eady xÂ·Â·ray glow is emitted by electrons spiraling in tho magnetic fidds of tho nobula. Pulm! x rays aro emitted from the pulsar created in the spectacular supernova explosion of A.. D. 1054. one of only two radio pulsars known to emit x rays. 11:te x-ray spectNm exÂ·tends up into the gamma-ray ftgion. &c><pius XÂ· l. the bright.,t xÂ·ray object most of the lime. is auociated whh a blue starlike object with strong optical emission lines. X rays are emitted from a hot plasma in the vicinity of the blue object whose nature n:mains a mystery. It appears likely thai many of the celestial x-ray sources in our galaxy are generally similar to Sco X-1. Occasionally, a new x-ray source appears in the sky, is more: intense than Sco X-I for a few months. then declines until it is no longer detect- able. We do not have good enough position measurements of these sources to attempt to identify them with optical objects. One of the first major discoveries of the Uhuru xÂ·ray satellite has been " new class of xÂ·ray sources that undergo regular (pulsarlike) and irregular fluctuations on a rime scale between 0. 1 and 10 sec. No optical identifications are yet available. Many unusual galaxies are XÂ·ray sources. These include strong radio galaxios (M87J. quasars (3C273). Seyfen galaxies. and ordinary galaxios (Jhe Magellanie Clouds shaaÂ· a c:omplex x-ray structure

Tromendous amounts or energy are rele.ased in the ltÂ·ray reeion in some or these souru.. po5ing serious challenges to our understanding of high-a

ergy I.Sirophysic:s. Underlying all th""' sources is a diffu>e xÂ·ray glow that appears to be featureless. Many astronomers believe that the background x rays were created far away and long ago in the early cosmological history of our universe. This brief and incomplete list of important discoveries in xÂ·ray atÂ· tronomy is reminiscent of the early exciting years of radio a.scronomy. A wide range of new phenomena had been found. but understanding of these phenomena was minimal. The search for understanding required much larger instruments. new techniques. bener detectors. better spectral coverage or the sources. polarization measurements. and the ability to repeac observations for variability. a common featu

of Â·Â·compact" objects. A similar pattern of devdopment is needed in a:Â·ray astronomy. Much J.argerÂ·an:a dctecton than have been flown are required in order to find and study faint sources. For the lower-<nergy x rays. focusing optical techniques, involving gruingÂ·incidence instruments. should be Down. The>e will allow detailed pic:turos with high angular rosolution to be obtained. Thoy will also act as photon collectors. concentrating â¢ Â·ray photons from weak sources on Bragg crystal spccnomcten and on

The High-Priority Program 81 polarimeters so that the detailed spectral properties of the sources can be measured. Because tbe detectors used with focusing optics can be made very small. the unwanted detector background counting rate can be greatly reduced, facilitating measurements of extended sources and of the apparently isotropic xÂ·ray background. With this major instrumentation. very large numbers of x-ray sources should be discovered. Many new examples of the various classes of x-ray sources in our galaxy should be found. so that the full range of properties of these sources can be studied. Positional determinations of these sources should be greatly improved. thus allowing large numbers of them to be identified with optical objects. With the resulting ability to study the sources in many different wavelength ranges, our theoretical understandÂ· ing of the character and structure of the sources should improve rapidly. Of great importance will be the ability to point at xÂ·ray sources steadily for hours at a time. Not only will this allow a major improvement in the statistics of tbe spectral measurements. but it will also permit studies of the time variations of the total xÂ·ray emission and of individual spectral features. One of the principal striking characteristics of the galactic â¢Â·ray sources that have so far been found has been the temporal variability of tbe x-ray Dux. ranging from rapid Ouctuations to longÂ·term changes. This characteristic is more frequently found in x-ray sources than in optical and radio sources. The major instrumentation should also have extreme importance for studies of extragalactic x-ray sources. It should permit detection of inÂ· dividual sources in nearby galaxies and of emission from active galaxies and quasars to very great depths in space. More definitive measurements of hot plasma concentrated in clusters of galaxies will be possible. allowing a determination of whether sufficient masses of such plasma exist in the clusters to bind the galaxies gravitationally. Much more definitive measurements of the spectrum and isotropy (or lack of isotropy) of the background x rays will improve our understanding of the cosmology and early history of our universe. The National Aeronautics and Space Administration ti<ASAl has recogniud the richness and promise of this field of research by requesting congressional authorization for two large rotating High Energy AsÂ· tronomical Observatories ( KEAO 's). These are to be large spacecraft in orbit about the earth, slowly rotating so that the instruments scan across tbe sky. These will be survey spacecraft. with a large collecting area inÂ· tended to discover new faint xÂ·ray sources. to measure their positions accurately. and to measure spectral properties. Combined with the xÂ·ray instrumentation would be gamma-ray and cosmic-ray instruments. The spacecraft will play an essential role in the future of astronomy. XÂ· ray astronomy will increasingly become a partner to ortical and radio

88 ASTRONOMY AND ASTROPHYSICS FOR TH E 1970Ƈ astronomy as more J[-ray sources are identified and their propc:nies a

correlated with thost in other wavelength band$. h is possible that some typt:s of x-ray source may ne-er be optically identified. in -. hich case â¢--e â¢ Â·ill be entlrt-ly dependent on H Â£AO techniques to s rudy them. NAS pla nning also calls for two pointable HEAO 's. 1nc:sc will be el-cn A more imponant to the future or x-ray astronomy chan the rotating HEAO 's. They will permit short-timeÂ· scale Ouctualions in intensity to be followed continuously and to be correlated whh s imultaneous optical. radio, and perhaps infrared observations from the ground. They will take ttdvanttlge of focusing x-ray optics to concentrate the xÂ·ray photons onto small detectors. where background problems can be reduced and angular structural information and positions can be obtained with high accuracy. 11 ls important that NASA also seek authorbÂ·.alion for the pointable 1-H!.AO 's as soon as possible. in order that there not be too g.rcat a time delay berween the discovery of new x-ray objects by the first rotating H EAO and Ihe del ailed study of them by the fi"'t pointable II EAO. A measure of the importance attached to x-ray astronomy by astronomers is that they have scheduled large blocks of lime on major optical instruments to exploit the discoveries and positional measu ments of new x-ray sources by the UhuN x-ray satellite. This rdl<tt their expectation that a number of optical identifications will be possible of the newlydiscoVtted x-ray sour=. If this is the case. the HEAO program will make lar demands on optical astrOnomy and probably also on infrared astronomy. There should be an expansion in major optical facilities to satisfY the requirements of xÂ·ray astronomy. Extragalactic objects in which a major portion of the energy emi.ssJon i.s in the infra.red are also proving to be xÂ·ray objects: it is possible that a similar correlation may exist among some classes of galacttc xÂ·ray objecu. Thu.s an expansion in infrared facilities may also be required for support or X Â·ray astronomy. The highÂ·encrgy astronomical program given extremely high priority by the Committee includes the four HE-AO 's in the NASA planning program. two rotating and two pointed. together with an associated expansion in oplical and infrared facilities to provide the ground support required for lhe development of x-ray astronomy. The esdmated rost of the four HEAO missions is SJ80 million. In adÂ· dition. at least one intermediateÂ·sized optical celescope to support the program should be constructed at a eos1 of SS million. MILLIMETER-WAVE ANTENNA One of the dramatic discoveries of the recent past was the detccdon in tbe clouds of interstellar space of an astonishing variety of molecular spc:ctes. The High-Priority Program 89 The- e findings contradicted our expectations that the formation of such s molecules was a rare event and that their destruction was rapid bec.ause of the flood of ultraviolet light in the galaxy. The species found range from the sma11. diatomic molecules. such as CO. CS. and CN, to such complex substances as cyanoacetylene. methyl alcohol. formaldehyde. and formam ide. containing as many as six atoms. Carbon monoxide is present in an abundance some thousand times greater than other molecules. probably reflecting lts resis ance to dissociation by ultraviolet light. The molecules of greatest abundance are those found in our laboratories to form the basic constituents of biochemical systems. For instance. formaldehyde is a precursor of both amino acids and sugars in experiments simulating conditions on t he primitive earth. Thus the molecules observed seem to indicate that the chemistry of life on earth is closely paratteled in interstellar space. The diatomic molecules are almost always best observed at relatively short radio wavelengths of a few millimeters. They form the basic building blocks for the larger molecules, and the physical interpretation of their spectra is much simpler than for the larger molecules. The larger molecules have great significance. however. since they often possess a rich spectrum. both at cent-imeter and millimeter wa'elengths, and form a particularly powerful tool for probing the physical conditions in the interstellar medium. High resolution is necessary to define the distribution of the molecules from which the-modes oflheir formation and destruction can be studied. High sensitivity is necessary to discOÂ·er large molecules. which may have low abundances, and other low-abundance substances such as molecules containing rare isotopes. High resolution and high sensitivity require a very large stecrablc tele- scope with a very precise reflecting surface. Such a telescope has many other important uses. particularly for the study of variations of quasar spectra and intensities and planetary emissions. Such a telescope is not easy to build, because it must n1aintain its geometry to accuracies of tenths of millimeters under the influence of changing gravity forces, wind. and thermal stresses. A great deal of research has been carried out at the National Radio Astronomy Ob- servatory on such precise and stable telescopes. A new approach to telescope design, called the "homology telescope"' has b<-.:n developed. which appears capable of attaining the desired performance. Indeed. some of the principles of this approach have been applied successfully in the new 100-m radio telescope of t he Ma.x Planck lnstitut fiir Radio-- astronomie in Gennany. The very large radio telescope recommended for observations a t milli- meter wavelengths would very likely be a fully steerable parabolic reflector with an aperture of 215ft. performing satisfactorily at wavelengths of 3 90 ASTRONOMY AND ASTROPHYSICS FOR THE 1970Ƈ mm and longer. The cost of this instrument is not as well determined as that of the vu but is estimated to be SIO million. The ronstruction of this telescope will provide a major capability in a particularly promising area of astronomical research and will capitalize on our receiver technology. momentum, and d<sign capabilities in a field developed in the United States and in which the rountry is pre-eminent. AIRCRAFT, BALLOONS, AND ROCKETS An essential part of space research is carried out using small vehicles- aircraft, balloons, and rockets. They are relatively inexpensive and ideally suited for programs of observation with specialized instrumentation where a few minutes or hours of data-taking will accomplish the research obÂ· jective. They have also been essential for testing astronomical in- strumentation for use in space. These vehicles have proved invaluable in the past their utility in the future is assured by t.h e steadily increasing requirements for their use. At a time of severe fiscal ronstraints, the reduction of the number and variety of large astronomical missions in space can, in part, be balanced by the initiation of much less costly programs utilizing small vehicles. These may be able to carry out some of the research contemplated in the abandoned missions, thus maintaining a degree of flexibility and vitality in the affected field of research. The scientifically sensible rourse of action is to increase funding for aircraft, balloons, and rockets when fewer major satellite experiments are planned. If satellite programs are increased, an accompanying increase in rocket research, with smaller but innovative goals, will lead to optimum satellite design and therefore be of high value. Until recently, x-ray astronomy depended entirely upon rocket research. The x-ray sources were discovered by rockets. and quite ac- curate positions were measured for some of them with ingenious rocket instrumentation. Rocket measurements made duri. g a lunar eclipse of the n Crab nebula revealed that the x rays were not a point source. At the present time, rockets are proving essential to the further study of some xÂ· ray phenomena discovered by the UhuTV x-ray satellite. Unexpectedly rapid x-ray fluctuations of the Cyg X-1 source were discovered utilizing the satellite, but since the satellite rotates, it is not suitable for following the fluctuations. Rockets are capable of pointing at a source like this for several minutes at a time, and missions can now be Instrumented to provide the data essential to a better understanding of Cyg X-1. Since this source appears to be but one of several classes of strange x-ray objects, it is clear that there will be a pressing need for more x-ray astronomy The HigltÂ·Prlorfty Program 91 rockets for the next several years- and most certainly through the era of the Kigh Energy As1ronomical Observatories. Ultraviolet astronomy also began with rockets, first for studies of the sun and then for studies of the stars. Differences were found between theoretically calculated ultraviolet stellar spectra and the rocket obÂ· servations. Rapid rates of mass loss from hot supergiant stars were discovered by spectroscopic observation in the ultraviolet. Perhaps one of the most important of the ultraviolet astronomical discoveries was that of molecular hydrogen in interstellar space. Today the bulk of the ultraviolet astronomical observations are carried out with an Orbiting Astronomical Observatory. but the instrumentation in this vehicle is relatively infiexible, even though it returns a great amount of data. It is necessary to supÂ· plement and enrich these data with selective rocket measurements using a wider range of instrumentation. The loss of OAOÂ·B has been a severe setback for ultraviolet astronomy. The authorized program will conclude with the launching of OAOÂ·C in fiscal year 1973. For many years, the program of ultraviolet astronomy from spacecraft is likely to be modest even if new satellites such as the proposed SASÂ· D are authorized. In these circumstances, it will be all the more important that a supplementary program of rocket observations in the ultraviolet be provided to maintain vigor in this field of research. The instruments carried in these rockets may provide some of the measurements that would have been made by OAOÂ· B. They also will provide an opportunity to exploit the discoveries made by OAOÂ·A and OAO.C and will provide an important survey of certain classes of ultraviolet phenomena. There will undoubtedly be many celestial objects found in these ultraviolet studies that will turn out to pose important scientific puzzles. many of which can be further studied and elucidated by resea.rch using rockets. Infrared astronomy now relies heavily upon aircraft and balloons. While a few infrared windows can be exploited from the ground, most of the wavelength region, and especially the far infrared, requires an obÂ· serving platform above the bulk of the atmospheric water vapor. Observations from balloons and aircraft have given important new spectroscOpic information in the infrared about the sun and planetary atmospheres. Observations from aircraft have detected high Ouxes of radiation in the infrared from the cores of active galaxies and quasars. Large numbers of strong infrared sources near the center of the galaxy have been discovered during surveys made from aircraft and balloons. NASA is providing an aircraft platform for a 36-in. infrared telescope, which should produce important new results. The Committee recomÂ· mends that a first, crude, long-wavelength infrared sky survey be carried 92 ASTRONOMY AND ASTROPIIYSJCS I' OR Till! 1910 's out from balloons in the near furure. In the longerÂ·range future. a deep- sky SUf'ey in the infrared will probably require satellite techniques, but these will require a prior ro<ket de..elopment program. Hen . in&aml astronomy will be a major user of aircraft. balloons, and rockets in the next few )'tars. Solar rescareh has been heavily dependent on ro<kets as well as on satellites in the Orbiting Solar Observatory series. These have produ detailed ultraviolet spectra and x-ray pictures. They have been Oown on command at times of solar Oares. There is a continuins need to sup- plemenc che sacellite coverage or che sun with special, Oexible. quickÂ· response rocket instrumentation. Thus essentially all che major are.as of space astronomy have an exÂ· panding need f'or small researeh vehicles: aircraft, balloons, or rockets. The expenditure on these research vehicles for astronomical research presenlly amounts 10$12 million to SIJ m illion per year. The Committee slrongly recommends that the expend iture for chis type or researeh be doubled os rapidly as possible. cenainly within the next ch

The HighÂ·Priority Program 93 made the oso program among the most successful and productive of all astronomical satellite programs. We recommend the continuation of this program beyond the present oso series. through oSOÂ·L, . ., , and Â·N (at a cost of S30 million each), to be Oown during the next solar maximum (1977-1931 ). These oso 'swill probably provide for the first time a spatial resolution equal to or better than that of the very best observations ob- tained from the ground or balloons. This improved spatial resolution is of utmost importance, since we know from ground-based observations that the energy transfer to the chromosphere, to flares and cosmic rays, and perhaps to the corona, occurs on scales probably less than or equal to 1 sec of arc. oso. L., Â·M, and Â·N will fly during the next period of maximum solar activity, with a spatial resolution 10 to SO times better than was possible in the last period. They will carry instruments capable of analyzing the properties of flares and active regions in the spectral region from 3000 A down to the very energetic x rays below 0.1 A. It is entirely reasonable to expect that these observations will result in a significant increase of our understanding of the layers of the sun above the photosphere, of solar activity, and of solar Hares. We envisage this continued oso program, together with the expanded solar rocket program discussed in the space astronomy recommendation, as the bac.kbone of the solar space program. It is of the greatest imÂ· portance, however, that improved observations from space go hand in hand with the improvement and extension of observations from the ground. The solar photosphere, best observed in visible and near infrared radiaHon, reveaJs most of the sources of the energy input in the chromosphere. and eorona in the form of granulation, magnetic structures, and mechanical motions. Coronagraphs, eclipse experiments, anticipated observations of far infrared recombination lines, and radio observations provide relatively inexpensive ways to observe other aspects of the sun's upper atmosphere. We therefore recommend the continuous updating of existing groundÂ·based and aircraft facilities and the construction of small specialized telescopes for the visible and infrared spectral regions (at a cost of approximately Sl.O million per year). This updating includes improved image detection, storage, and analysis, as well as improvement of image quality by telescope refinement and site selection. For the study of the interaction of solarÂ·Oare plasma with the magnetic field and plasma of the outer solar corona, we suggest the construction of a relatively inexpensive multifrequency metric and decametric radioheliograph with moderate (I -5 min of arc) spatial resolution (at a cost of approximately $1.5 million). The cost of the program over the next decade will be S90 million for oso.L. Â·M. and .Nand SIO.Omillion for ground-based facilities. 94 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'o THEORETICAL ASTROPHYSICS AND COMPUTING REQUIREMENTS Physical theory has always played a crucial role in astronomy-from the period when Newton's theory of gravitation provided the uplanation of planetary orbits to the present time. when nuclear reattion theory promises to el<plain the synthesis of chemical elements in supernova explosions. Any balanced program for progress in astronomy wiU necessarily contain a vital, if relatively inexpensive, program of theoretical research. Much theoretical astrophysics today is concerned with model building. In this type of activity, physical principles substantiated in the laboratory, including those of quantum theory, nuclear physics, and plasma physics, are used to construct a mathematical model of an observable astronomical object, such as a star, a galaxy, or even the whole universe. The relevant equations are. usually complex and nonHnear and must be solved on a computer. The resulting models are then compared with observations to fix parameters of the model, such as the mass of the star or the random velocities of stars in a galaxy, and to show how the model should be im- proved to attain agreement with observations. Model building is essen- tially the only way known to convert the stream of photons entering a telescope into a physical picture of what is going on. The theoretical astrophysicist thus stands astride physics and asÂ· tronomy. Oose contact with physkists is essential if current developments there are to be properly included in the model. Constant interaction with observen is essential if theoretical work is to be aimed in the most productive directions for interpreting nature and if observational work is to be focused on the most theoretically significant questions. In the recent past there has been increasing exploration of dynamic states. The theory of stellar evolution can be largely constructed from a sequence of static stellar models, but in the final stage of a star's life-in some ways the most interesting one-events occur very rapidly, with gravitational collapse and outgoing shock waves playing a vital role. To reconstruct these phenomena, it is vital to simulate the dynamics in a computer. Dynamical modeling is playing an ever-increasing role, from stellar explosions to interstellar shock waves to the spiral structure of galuies. Such modeling is orders of magnitude more time-consuming than static modeling, so fasttr computers with larger memories are required. A prime example of the success of this apptoKh is the modeling of a supernova el<plosion, in which the progress of a shock wave is followed in detail, and a netWOrk of about 100 nuclear reactions is followed at each time step. The result is a prediction of the abundances of the chemical elements. which seems to agree remarkably well with observation. The High-1+/orlty Ptogram 95 A related activity is theoretical work in dynamical astronomy-the application of Newton's equations of motion (with small relativistic corrections) to the positions of planets and satellites of the solar system. Here the problem is to compute the orbits using interactions between all bodies to extract precise values for the parameters of the system, including the masses of the bodies involved. Recently, such work has demonstrated its vitality by providing extremely ae<:urate motions of the earth for use in reduction of optical observations of pulsars. Without these precise positions (about J0Â·8 of the distance to the sun), it would have been imÂ· possible to utilize the precise optical timing measurements, which require correction for light-travel time within the solar system. It would thus have been impossible to infer the existence of abrupt changes in the period of the Crab pulsar, which have been interpreted as due to starquakes in the crust of a neutron star. Such is the unity of astronomy, of the old and the new. We believe that increasing the effort in the universities, where there is strong interaction of theoretical astrophysicists with both observers and physicists, is the best way to optimize results in theoretical research. We suggest particular emphasis on relativistic astrophysics, stellar evolution (particularly early and late phases), derivation of physical data needed to construct precise stellar models (including opacity sources, nuclearÂ·energy generation rates, convection theory, and equations of state), and theoretical interstellar physics and chemistry (including the solid-state theory of grains, molecular and atomic cross sections and transition probabilities, the theory of masers, and the plasma physics of interstellar gas and magnetic fields). Interaction between relatively isolated theoretical groups should be increased wherever possible, for example, between groups working on stellar interiors, stellar atmospheres, and observational stellar spec.. troscopy, between plasma theorists and astrophysicists working on stellar and interstellar plasma processes, and between chemists and astronomers working on molecular astronomy. Support should be increased for both theoretical and experimental study of atomic and nuclear collis-ion cross sections and transition probabilities, taking care to locate this work in several independent groups to increase the effectiveness of cross checking. By and large, this can be accomplished by supporting physicists in universities where there is an active astrophysics group that can be helpful in establishing priorities for experimentation and calculation. We recommend that in the specific areas of beam-foil spectroscopy and low-energy nuclear cross sections, the U.S. Atomic Energy Commission lAEC) consider support of groups utilizing existing facilities for this work. Funds are needed for individual university investigators to increase 96 ASTRONOMY AND ASTROPHYSICS FOR TH E 1970's their efforts using suclt university computers as are available. The fund.s available for computation generally need to be increased. Theoretieal astrophysicists and dynamical astronomers are moving into an era when the maximum speed and storage capacity available will be needed to solve dynamical problems, but many university and national center computers are not equal to this task selected ones should be upgraded. In addition, state-of-the-art computers in mission-oriented agencies such as the AEC and I<ASA would be extremely useful if means for using them part-time can be worked out. The additional funds needed for first-rate activity in this area are not trivial-perhaps SS million per year. The theoretical etfort at the national observatories needs to be fostered. Research output would be optimized by increasing the availability of theoreticians at the national centers. To succeed, it is essential to find highly quali6ed versatile individuals as visitors or on the staff. Such a goal involves enhancing the computer facilities, as required, to make the observatory attractive both to resident and visiting theorists. Joint activities between physics and astronomy programs in universities should be encouraged. Because of the close relationship of theoretical ast.rophysies to both physics and observational astronomy. productivity is served by every possible mode of cooperation. including. in some cases. merged departments, joint academic programs. and shared facilities. It is most important that astronomy PhD students receive as thorough training as possible in physics. and to this end. special seminars should be designed. A National Institute of Theoretical Astrophysics has been suggested. to provide a focus for theoretical research, to promote interchange between astrophysicists from different suhfields and between astrophysicists and other scientists, and to provide a stimulating atmosphere for postdoctoral fellows before they accept permanent appointments. A proposal by the Panel on Theoretical Astronomy would fund an institute at an annual rate of approximately$750.000 for a fixed period of seven years. The institute would have some six permanent statf members. with an outstanding scientist as director. and would be located in an anractive place close to a researclt university and close to a group of observational astronomers. There would be particular emphasis on p015tdoctoral and visiting ap- pointments. and in keeping with the need to keep administrative and other expenses low. the support statf and computarional facilities would be strictly limited . The Committee concurs with the panel in the thrust of its recom- mendation for an institute. Nevertheless. it believes that for both pragmatic and historical reasons. the main strength of theoretical astrophysics is likely to remain in the universities. There It can have the

The High-Priority Program 97 greatest impaÂ£1 on the educational process and on young men from a wide diversity of backgrounds and fields of interest. The institute. if it is set up. should strengthen. not compete with. university groups. Emphasis on interaction bÂ«ween groups. on funding of young people. and on a moderate budget. whi<:h will suffice if the staff and computer facilities are limited, is consistent with this goal. We recommend. to this end, that if the institute postdoctoral fellowship program is established. it be used also for purposes not immediately related to a"endance at the institute. including travel funds for visits to other institutions and the cost of computing at home institutions or other facilities. While there are advantages in such a permanent institute. we recom- mend that, as a first step. consideration be given to smaller funding for a summer institute. Such an Institute would have no permanent staff beyond the director and would occupy rented space at one of a number of possible sites that may prove attractive. No computation facilities would be provided the entire funds beyond rental and minimal administrative expen.ses would be expended on travel and subsistence for a few senior and a larger number of junior people. We believe that the final plans for a possible permanent institute would be beneficially affected by one or two yearsâ¢ experience with such a summer institute. Both the Theoretical Astrophysics Panel and the Commi"ee wrestled at length with a problem that theoretical astrophysicists, along with others in all areas of theory, now face in their needs for a very large computer. Our conclusion may be viewed as suggesting something for evel')-one. We are probably in a state of transition from a stage in which large generalÂ· purpose university centers were optimum to a stage when the needs of many different research groups will share much larger computers through sophisticated data-communication links. We understand that quantum chemists have considered a national center with high -power computers. comprehensive software library. and staff of computer-oriented theoretical chemists. able to do large-scale service-type calculations for others. The needs of the Global Atmospheric Research Program suggest that an international network of large computers would be desirable. It will ultimately be necessary for scientists to assess these requirements and discuss the problems of a national computing system. making maximum use of facilities already in place, or needed, for calculations in industry. the space program, weather forecasting. and reactor design, among othen. The needs of astronomy should be considered when such an over- all national computing system is discussed . Theoretical astrophysics is a growing field rhat aruacts young astronomers and physicists with a broad range of interests. The speed of modern computers makes it possible to construct models of atoms. stars.

98 ASTRONOMY AND ASTROPHYSICS FOR T HE 1970's and galaxies and to study the dynamics of the solar system or the universe. The tools of the theoretician, excq>t for the large computers. are inex- pensive. The pa!!C1'n for the bes! range of computing racili!ies. national and local. muse still be â¢'Orked out. We recommend an inCTeased program of abouc SJ million a year. For the theoretician. travel. co make new contacts and co anend summer institutes. performs a spcc:ial function. Interdisciplinary research is particularly elfec!h'O and nor erpensive. Theoreticians ean work at small institutions. often at colleges or u_ ivcrs1lies without large facilities. n OPTICAL SPACE ASTRONOMY-LEADING TO THE LARGE SPACE TELESCOPE Some of the- most farÂ·reaching additions to our kn

Â·ledge of the universe occurred during the first half of this century with the development of asuonomkal speetroscopy and its utilization with large telescopes. During thls time, spec!roscopic analysis of planetary atmospheres. the sun, the stan. and the intersce11ar medium brought about clarifications in our understanding or these objects. Of equal significance was the speeÂ· troscopic StUdy or extC1'nal galaxies, leading to the discovery Of the in- CTCUO of Spec!r()SCOpic red shift with distance and the realiurion that we live in an exptnding universe. Throughout this development. ucronomers have been acutely conscious of the fact that their analyses ..'eft inromplete and tentative. since much of the information that they would have liked to have obtained was in the inaccessible ultraviolet r-ange of wavelengths. The mlssing spectroscopic information oonsists of two classes: one is the spectral lines in the ultraviolet due to elements and stages or ionization of elements that do not have lines in the visible region of the spectrum: the other is the general shape of the spectrum ln the ultraviolet and the relation of this to the distribution of emitted energy In the visible and Infrared wavelength regions. Ultraviolet observations c.an be made only above the atmosphere. During the last IS years, the technological barriers against such ob- servations have progressively been broken. Rockel obsC1'Vations of the sun and the stars have resulted in a numbÂ« of important discoveries conÂ· cerning the ultraviolet spec!rum of the brightest objects risible in space. At the same time. the discovery of quasars. some of them with large Spec!r()Scopic red shifts. has pi'OYided a means .. â¢hereby the ulttariol<t emission frun a limited doss of objects can be studied frun the ground. because the light originally emitted in the ultraviolet has been red-shifted into the risible region of the spec!rum.

Tilt H/tlt-I'Worlty Pro,.m 99 llÂ«auâ¢ objÂ«ts emitting ultraviold lipt are also likely 10 emil visible li&lll. il has not been expected thai completely n. classes of objÂ«ts would be di-ed. NevertMlas. there ha. been a number of important discoveries made concunlng IM properties in IM ultraviolet of some of the objÂ«ts that had previously been studied in tM viJiblt: I. 'The ultraviolet resonance lints in oa-tain earlyÂ·lype nellar aianiS have shown that manor is 01reaming out from lheoe stan with velocities of the order of 1000 km per sec. wilh total mass loss raltS of the order of toÂ·' solar mass per year. 2. 'The extinction of ultraviolet light by the interstellar medium has turned out to be dllferent from that predicted on the basis of observations made In the visual region. There is a prominent absorption feature ncar 2200 J. and a gradual increase in the extinction toward shorter wavelenat)ls. These results are leading to extensive m-sions of our ideas concemina tM character of interstellar grains. and the prest!KC of considerable variations of these features In difl'erent pans of the inÂ· terstellar medium l ndleates that individual stan can mndify tMlr inÂ· terstdlar environments. 3. MOOiplaxles have been found to emit more radiation in tM shorter ultraviolet wavelengths than would have been eapected on the basis of tMir apparent c:olor temperatures in the visible rqion. 4, Loree hydroeen clouds have been found â¢urTOUndlng tM recent bright c:ome1S Tago-Sato-Kosaka and Bennett. Such laree clouds appear to c:onstitute a fourth ma>o< structural component of the c:omet. S. A broad absorption feature at ). 2550 has been discovered in the spectrum of Man, possibly due to ozone. The Orbiting Astronomical Observatory program is becomlna a true national facility for astronomers. On the firs

OAO. about ten groups of astronomers have been observing approximately 100 objects. 'The OAOÂ·C is upected to have a c:onsiderably greater obsenlna capability. and c:onnquently il should be of great service to the astronomical c:ommunhy throup IM pcstÂ·obsene< program. 'The Orbiting Astroncmical Observatory proeram has. unfortunately, been marked by tragedy. Tbe first and third launcbcs were failures. the fintthroup troubles â¢ith tbe bancry. and the third through a failure in the launch vehicle. Afier the launch of oâ¢OÂ·C. tMr< are no further authorlz.cd proerams in space ultraviolet astronomy. At the present time, no satellite capable of carrying on intermediate spectral and spatial observations in the ultraviolet is funded. 'The ultimate objective of the ultraviolet astronomy program should be

100 ASTRONOMY AND ASTRO PHYS ICS I'OR TilE 1910'â¢ the: development of a National Space Observatory containing a large diffrattion Â·limited telescope capable of operating in the nearÂ·infrared and visual rcaions as well as in the ultraviolet: . The exciting role that such a large space lclcscopc(LST)could play in astronomy during the decades to rome is disaJsscd in 1he final Section of this Chapter. The nominal apcrlure lhal has been utilized in stud.. of the UT is 120 in. Such an instrument roukl anack problems that arc of 'he: most fundamental astronomical significance and that are unlikely ever 10 be solved using ground

based instruments. Perhaps of even greater importance than its ultraviolet capability would be the high angular resolulion of s uch a telescope. Turbulence in the atmosphere limits the angulor resolution obtainable with large telescopes to the equivalenl ol' that obtainable with a 12Â·in.Â·aperture telescope, although the lightÂ·galhering power of a larger instrument is superior. In the visible region, the L would have an ST angular resolulion better by a factor of 10. whic.h means that one resolution element observed with a groundÂ·ba.â¢ ed telescope could be divided into 100 resoturion elements with the uT. The angular resolu1ion in the ultraviolet would be still better by a factor ncar 2. One result of this high a ngular resolu1ion should be the capability of observing stars and stellar-appearing objo:ts at nearly ten times the di>tancc at which such objects can now be studied with the 200-in. telesropc. Tt.e LST should lead to a much improved understanding of the most fundamental problems in cosmology. as well as of the broad range of astronomical problems pmenlly being in,Â·estigated by groundÂ·based astronomers. A great deal of technological dcvclapment will be required before such an LST can be launched. It will be desirable to test the new 1cchnology, not only Chrough rocket instrumentation for ultraviolet studies but also through the construction and flight of intermediate instruments. For example. a diffractionÂ·limited space telescope of about(>() in. would have a tremendously useful versatility and capability beginning to approach that of the csÂ·r itself. It is now technically feasible t o build s uch an instrument, and it would be useful to incorporate into its design che results of new technological developments intended foc the LST. Yet no high Â·quality large telescope is in the current planning stagt:. The Committee recommends very strongly that a vigorous program be maintÂ· ined in uhraviolet astronomy. This program should be directed a toward the ultimate use of an 1ST'. One or more intennedi.ate instruments, designed to test the technology of the ur and to return large amounts of data of immense value to the astronomical community. should be launched. If there is to be an extended delay between the launch of OAO.C and the first of these intermediate in.struments. then it is most desirable that an interim ultraviolet telescope be launched. perhaps a replacement for the OAOÂ· B or a smaller instrument in a Small Ascronon'y Satellite.

102 ASTRONOMY AND ASTROPHYSICS FOR THe 1970's paraboloid. and it appean that a dish that performs well at 2 em and is usable with somewhat reduced efficiency to 1-cm wavelength is well within present ongineering pract

. Tho largost comparablo antonna. tho 100-m telescope of Cormany's Max Planck lnstitut. is actually only an SS.m telescope at wavolengths shorter than 6 em. Thus the projtcted instrument has three times grtater observing capability at all wavtlengths, and at wavelengths of 6 em and smalkr o-Â·er six times grtater observing capability. An especially attractivo feature of the new paraboloid is its com- plementary role with our proposed millimeter-wave telescope. The simple basic molecules such as CO, CN. and CS have spectra that lie in the millimeterÂ·wave region, while the larger. quasi-organic compounds such as methyl alcohol, formaldehyde, cyanoacetylene. and formic acid have spectral lines in the band from 2 to 30 em. Many of the larger molecules, and ammonia. possess lines that could be observed with either system, although tho grcater angular rtsolving powor of the 440-fi telescope would give it an advantage for certain problems. The large centimotor-wave paraboloid would certainly servo as the hub of many VLII observing programs, and its large area would inertase ononnously the classes of objtct accessible to study. In conjunction with the other large paraboloids of the world. stntcbing from Australia to the Soviet Union. the present observations of the closer, bri&ht objtcts would be extended to quasan and radio galaxies that are far more distant and faint. The radar capability of the new instrument would also be impressive. With the exception of Pluto. all the planets and the larger moons of Jupiter and Saturn would be within range of its 6-cm radar, while the greatly enhanced signal-to-noise ratio would enable the radar astronomers to study the surfaces of Venus and Mars in great detail, enhancing the effectiveness of space missions to those planets. The estimated cost of such an installation. including the telcsonpe. land acquisition, site development, controls, computers, radiometers, and radar, would be approximately SJS million. Some economies could be effected by s haring common support facilities with other instruments such as the very large array or the large millimeter-wave tele=pe. Operating costs would be S3.5 million per year following its completion. ASTROMETRY The establishment of a system of star positions based on an absolute inertial system is essential, and the system of proper motions should be detennined with respect to such an inertial frame.

1M HiiJtÂ·l+forlty . m 103 The mean propu modons of faint stars are of fundamental importance to the study of unusual stars found in the galutic halo. Many interesting objects in the halo are between I and 5 lqx from the plactk plane. and ..en with the rapid spac:e m(l(ions of elttrmle halo nars, thdr angular proper m(l(ions are small-approximately 0.25 sec of arc: pu year. The modons must be determined ..th high indmdual accuracy. This requires that the inertial frame be determined to an oa:uracy of at least 0.005 sec of arc per year Ideally, the accuracy should be several times hlgber. Ooe type of fundamental data that astronomers mu$1 have is the distan<e to the object studied. Interesting objects are at great distances, which can be calibrated in successive steps if nearby objects of similar characteristics have accurate distance measurements. The m0$1 funÂ· damental method uses accurate trigonometric parallax-the anaular displacement of a star caused by the earth's motion about the sun. These parallues are the backbone of the stellar distance scale. They are oeeded for faint stars near the sun and for bri&ht stars at ereater dittanc:es. An insufficient number of trigonometric parallaxes in the southern hemisÂ· phere will reduce the beoefiu of the laraer facilities built there by the United States and Europe.a n countries. Stars morina parallel in space appear to converac. becauJe of perÂ· spective effects this method provides individual distances for nearby star clusters. Ou$ter parallaxes should be extended to the southern hemiÂ· sphere and to fainter clu5ters in the northern hemisphere. For other distant types of stars, we mU51 take advantage of the accumulated drift provided by the moe ion oft he sun through space, which cauJeS the 5tars to drift bacbâ¢ard at angular speeds proportional to their parallax. Such group or secular parallaxes are often the only possible distance measure for the moSII.nteresting stars of high luminosity. They depend directly on the accuracy of the fundamental system of proper motions. Theories of stellar interiors would have a sounder basis If a sufficient number of parallaxes and masKs of nearby stars and clusters could be provided. These should include interestlna and important objects like rapid variables. hiably luminous B Slars. plaoeta. y ncbulu, hoi sub- r dwarfs, bright white dwarfs, and cool red d nerate stars. The establishment of the actual luminosity-temperature diaaram for stars like the sun and fainter is CSJeDtial for the determination of the distances to the &lobular ciU5ters and the luminosities of the RR Lyrae stars. For these important determinations, a combination of trigono- metric. clu5ter. secular parallaxes. and all ( sible methods must be used. Recently. the possibility has appeared of detecting companions of low mass by the nonlinearity of the motion of a nearby star throut h space. Several companions have been announced that have masses like that of 104 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'o Jupiter-or even lower values. These astrometric binaries have been studied . ntially in very few institutions. take a long time to give results, and yet will provide us witb our only diRe! proof of the existence of other planetary systems until radio communication from some of these may eventually be rurived. The changes of period detected in pulsan are fundamental to the theory of neutron stan. Yet the lint observations of these changes . comÂ· promised by uncertainties in such suppusedly well-known subjects as tbe orbits of the planets around tbe sun and the masses of t.he planets. The motion oft he earth around the center of gravity or the earth-moon S)Ƈtem is detectable in the accurate observations of the radio pulsars. Jmpro'lcd planetary orbits are necessary to take full advantage or this technique. Similarly, the very-long-baseline-interferometry technique requires ac- curate geodesy anq accurate timekeeping. The improvement and e,xte,nsion of astrOmetric measurements neeÂ· essary to Interpret the problems mentioned above rests ultimately on ob- serva1ions by small astrometric instruments. We 1herefore recommend const.ruction of two automatic transit circles. three photographic zenith tubes, three astrolabes. and three automatic measuring engines, as weU as modernitation or several existing long-focus telescopes. the equipment to be located geographically so as to provide systematic observations in both the northern and southern hemispheres. The precision attained by these rundamentalastrometri<: instrumenu bas hardly been affected by modem electronic technology (u. pt for tbe timekeeping funetlon). However, the modem technology or automatic measu_ement is in fact successful. and r we recommend it. together with some or the classical smaller telescopes mentioned above . part or our fundamental program. The estimated cost of these small instruments is 56.4 million. BEYOND THE RECOMMENDATIONS After concluding a detailed study of the state or our science and making our recommendations within the framework of recent available funding, we feel that it is important to discuss. in certain areas, what additional programs our science requires to meet fully the seientilic challenges tbat -..'e face. We have therefore re-examined the manpo--er raourees that will be available in the decade and tbe technologk:ally feasible and desirable projects studied by tbe panels. What areas have ""'omitted, disearded, or redueed in siu mostly becau.e or financial constraints? How much have we failed to recommend or the urgent needs pressed by our technical panels? The H/xhÂ·l'rlorlty Pro,.m lOS Larg<' Space Telescope Without any doubt. the largest and most exciting area is the ronnruction and launch of a large space telescope u..sn . for highÂ·rcsolution nudics in the normal and ultraviolet spectral regions. possibly with manned resupply and maintenance (e.gâ¢â¢ by the space shuttle). This development can be underuken in a vigorous way only at budget levels for astronomy and physics that represent c:onsiderable growth over the nut decade. The LST concept is based on two major exploitation.s of the orbital environment. First, the mirror-from 60 to 120 in. in diameter. depending on available fundo-will c:over completely the wavelength interval from 1000 A (the eutoH' imposed by interstellar attenuation) to 10.000 A(or 1 pm). with considerable utility out to 1 mm. thereby covering the entire ultraviolet and Infrared range not accessible from the ground. as well as the optical window. The large collecting area and high angular resolution over this entire range would provide unmatched versatility. But a more important dimension of the LST is the precis,ton of its image in the ultraviolet and optical ranges. On the ground, the d<let<rious effects of atmospheric seeing smear the image to one or m6re seconds of an: even at an excellent she. This means that the obse"er is in eft"tc1 comparing the image ohhe tarect object with tbat of tbe night sky (including ba<kground galactic light. zodiacal light. and airglow) in a comparable solid angle. lf a 120-in. t<lcs<ope can be designed to achieve diffrocrion limitation at SOOO A. an image as small as 0.04 sec of arc in diamet<r would result. If an image ofO. l sec of arc can be achieved in practitt, the nightÂ·sky radiation. which tends to obscure the imago of a faint object, is effectively redu<cd by a ractor of 1()0-a five-magnitude gain in sensitivity over ground-based instrumcnt,sofcomparable aperture. There is an additiona.l gain from the fact that the tel<scope operates above the alrglow layer and, of course. dots not sulfer from atmospheric attenuation. 11 should be possibl< to observe to apparent magnitude 29 in several hours of int<gration. Th< implications of such a capability for all branches of astronomy are great. The Committee feels that the LST has extraordinary potential for a wid< variety of astronomical uses and believes that it should be a major goal in any Vo'ellÂ·pla.n ned program of groundÂ· and spaccÂ·bascd as1ronomy. The Committee recognizes that th< large <ost involved can be acÂ· commodated only "'Â·itbin a vigorously growing prosram. Therefore. it has adopted the view that, wlhio the main program. the emphasis on the LST is at a moderate level of som< SJS million per year. enough to fund tcchnoloaioal development of smaller.apenure telescopes aod an LST in the following de<ad<. A much more expensive program is required if the LST is to become a 106 ASTRONOMY AND ASTROPHYSICS FOR THE 1970Ƈ reality in the 1980-1985 period. This Committee sees the l.ST as a natural program goal to follow the High Energy Astronomical Observatories I HEAOI mission. To achieve this will require budgets for diffractionÂ· limited missions that grow from a level of t.h e order of S20 million per year in 1970 to the order of$200 million per year in 1980, with launch scheduled for the early 1980's. Total cost of the program leading to the final fabricat.ion of a 120-in. telescope will be of the order of Sl billion over 10 years. A program of this magnitude requires the highest quality scientific leadership a.n d the most advanced space engineering available. The highest quality scientific leadership in this field can be found in the academic community. and the highest degree of space engineering talent exists in the centers of the National Aeronautics and Space AdÂ· ministration. Therefore, the best chance for success lies in a merging of academic talent with that in the NASA centers. We suggest that NASA select one or more centers to carry out the engineering phases of the program and that the National Academy of Sciences encourage the formation of a new corporate entity representing universities with strong programs in space astronomy. The latter should be limited to less than eight members in the interests of efficiency. This corporation would be responsible for establishing a National Ultraviolet Space Observatory I NUSOI - a working scientific laboratory under conÂ· tract to NASA and the National Science Foundation. The Director of the NUSO should be a scientist of top rank in space astronomy. The NUSO would be responsible for the planning and utilization of a series of satellite ultraviolet observatories, including the LST , and for administeri.ng them on behalf of the entire scientific community, as is done for the ground-based national observatories. To achieve this mission, the NUSO would work closely with the responsible NASA centers. Effective control of the engineering task ofthe Nuso would be exercised by NASA effective control of the scientific direction would rest in the Director and in the Board to which he would report. OpticalÂ· and Radio-Astronomy Instruments Certain major facilities in optical and radio astronomy were omitted from our program, for reasons of economy. Optical astronomers could make effective use of two more telescopes in the 200-in. class, with modern electronic auxiliaries. The pressures generated by space and radio astronomy have so overcrowded the few large instruments that even the two ISO-in. telescopes under construction fail to match the present needs. In addition to our recommended optical program, it would be desirable to

108 ASTR ONOMY AND ASTROPHYSICS FOR THE 1970 Ƈ disassembly. so that it could be transported 10 new locations. The small antenna would constantly monitor one of the stronger sources, to provide constant updating of the station clock. S.Venl terminals would be needed, cutainly 01 least two on each continent, although the best disposition would have t o be determined by a cueful study. The resulting network, if operated at 1

wavelen81h (which recent observation of H20 masers at 1.35 em have shown to be feasible) could give us a complete pictun: of the radio structure of quasars. with 0.0001 sec of arc resolution. If our ideas of the distances or quasan are correct. we could see structures appro:Umately I light-year in siu and could follow the development of dynamic events from year to year, seeing the details of these enormously energetic events. There are other. more speculative areas that one can also foresee-the study of the coronas of other scars, the observation of their sunspots and flares, the study of supernova shell developments in other galaxies, and the analysis of the mysterious nuclei of Seyfert galulcs. In add ilion to the Yl.BI program at radio wavelengths, we foresee the development of interferometer techniques at both infrared and opti<al waveltngtbs. Bec-ause the angular resolving power or an intttferometer vari

inversely with the waÂ·ele.ngth, one can anticipate remarkable di.sccweries by such systems. rivaling the recent radio vu 1 demonstration of motions appan:ntly faster than light in a quasar explosion . The ultimate instrument would be a 10-pm YLBI having a global baseline ooâ¢ kml. Such a devWe would have a n:solution of JOÂ·' sec of are, pennitdng one to peer deep into a quasar, perhaps to see explosive events on the surface of a superma.sslve star, which, some say. powers a quasar. The surface features of exotic: stan that sporadic-ally shoot dust and molecules into Interstellar space could also be studied. The choice of 10Â· .urn wavelength IJ dictated partly by the fact that atmospheric phase shifts are small there. permitting the use of large apertures, and partly by the fact that quasars and n:d giants-key objects in relativistic astrophysics and molecular astronomy--radiate a major f-raction of their energy there. The 10-

m VLBI might use a superheterodyne system, which mixes the incoming infrared signal with a stabilized CO, laser to produce a microwave signal that can he recorded at each telescope. The bandwidth of available tape recorders (100Hz) should be sufficient to detect at least the brighter sources. A f

nner of this device is nov.â¢ under C-'Onstruc:don, usina Hne-ofÂ· sight transmission of a w t.Hz bandwidth microwave signal to a common point to form an interference pattern. Following tests of the system with a 0. 1-km baseline (loÂ·> sec of an:), it wiD be expanded to 10 km (10 .. sec of arc). It will be sensitive enough to study nearby Seyfert galuies and bright

The lligh-Prlorlry Progrom 109 galactic objects. but a version sensitive enough to study quasars (where the resolution will be I light-year) will require larger telescopes and better detectors. Of course, most astronomical objects emit more powerfully with visible light so that there also is need for devices that can work in that spectral range. Fundamental studies of angular sizes are possible with both the intensity interferometer, which conelates the intensities in the two signal$, and the Michelson interferometer. wbich brings togethor the raw signals to fonn fringes. A large-intensity interforometer could be built imÂ· mediately with a l Â·km baseline to givo tO"' sec of arc resolution, but perfection of the Michelson system requires dovelopment of an optical delay line and techniques of fringe detection. The Optical Facilities Panel believes th3l both the delay line and fringe detection should be studied immediately with funding up to$200.000. Beyond these preliminary investigations, worthy goals of a teo-year program include a sensitive 10-km infrared interferometer, and perhaps a IO'Â·km infrared VLBt, and for visible wavelengths a J. or 2-km intensity interferometer and a Michelson interferometer with a similar baseline. The sensitive IO.km infrared interferometer is estimated to cost SIO million over the decado. and the large intensity interferometer S4 million. Further studies are needed before the cost of the infrared vut or the Michelson interferometer can be estimated. Infrared Astronomy The growth of infrared technology resulted in discovery of quite unexÂ· pected objects that radiated most of thoir energy in the infrared. The energy maximum atiSOO K is at 2,um and is observable from the ground. A survey with a 62Â·in. light collector discovered 20,000 cool stellar and prestellar objects. Observations in the far infrared are needed to study objects near 500 K. most of whose radiation falls in regions of high atÂ· mospheric absorption to study objects at 50 K, observations above the atmosphere are needed. The Infrared Panel put highest priority on a large stratospheric telescope, about 120-in. in diameter, in a large, high-Oying aircraft or possibly supported by balloons, gliders, or kites. We recomÂ· mended funds only for study of the most economical modo of operating suc.h a large infrared telescope. but the scienti6c goals of the large stratospheric telescope are extremely important. No realistic financial estimate can yet be made both the study and e"J)Crience with the NA SA CÂ· 141 airplane (with a 36-in. telescope) will determine the best course of ac1ion. The infrared groups are small at many universities. in both astronomy and physics. The changes in technology, the availability of new

110 ASTRONOMY AND ASTROPHYSICS FOR THE 1970'a detedOI'$, and the revelations of new types of objects make this an un- predictable but challenging field. lntenlisclplinary grants to physics and asuophyslcs departments will enlist the aid of low-temperature physicists for astrophysical applications. Solar Phyâ¢ics Solar physics has benefited enormously from the oso seri.. of solar observations. osoÂ· s are rapidly becoming more sophisticated and more reliable. However, a large diffraction-limited solar telescope (about 40-in. diameter) is needed, carrying a heavy payload (over 1000 lb) and capable of accurate pointing and 0. 1 sec of arc gu iding. This will provide high spectral resolution in the optical and near ultraviolet a nd will permit very fi ne-scale study of the rapidly fluctuating solar plasma, iu excitation temperature, velocity, and magnetic field . This is a large project, of the order of S200 million, but it is one that will both provide experience use ful for the UT and be a nearly ultimate solar space telescope. High-resolution observations of the radio sun provide information on the energetic partido acceleration ptOC0$5, as revealed by the gyrosyn- chrotron radiation. The relativistic electrons are studied near the site of the acceleration of solar cosmic-ray baryons. This s1udy requires a high- resolution radio telescope with about S sec of are resolution, which works on a short time scale and ..sentially giv.. a radio picture. A radio spec- troheliograph in Australia has already demons1rated iu usefulness in the study of the interaction of fast particles and the hot solar plasma and has shown that Hares are triggered across the sun as disturbances run out through the corona or return to other activecentel'$on the disk. Theoretical Astrophysics Facilities should not monopolize our attention. The present a nd planned facilities, the space astronomy program, and the importance of the fie ld for itself justify a strong case for theoretical as1rophyslcs, over the wides1 possible range of topic$-$tudy of neutron Sill'$, the quieter phases of stellar evolution, planetary dynamics. galact ic structure, supernovae, collapse nuclcosynthesls. explosions in galaxies, black holes. relativity, and cosmology. A test of the concept and viability of an Institute of Theoretical Astrophysics is an inexpensive recommendation. Also linked to theoretlcal needs is a fourth- or fifth-generation computer at a single National Computing Center. The total cost of an Institute and Computer Center ror 10 years might be S40 million. About JO percent of our recent PhD's in astronomy have their degrees in, and wish to work in, theoretical

The High-Priority Program I II astrophysics or dynamical astronomy. The issue of a National Computing Center is not clear-cut. since the efficiency and costs of high-speed long- distance lines are not yet known, but the 't'ery large computer is at the heart of much theoretical model building in astrophysics. To take ad- vantage of the presently available theoretical talent among young astronomers and physicists, we also urge that an expanded postdoctoral and senior postdoctoral program be considered. The goal would be to provide a number of theoreticians with at least a summer's or, preferably, a year's visit to other universities, national, ooo, or NASA centers, by direct fellowship grants. with freedom to travel, or by small research grants covering their salaries and expenses.