# Theoretically, what is the biggest optical telescope that may exist?

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Recently, I read yet another news about E-ELT. It will have 39.3-metre-diameter segmented primary mirror. And I was interested in the next question: Theoretically, what size of the primary mirror (single/multiple/segmented) can have a telescope on Earth for observing at optical wavelengths? And why? I mean what are physical limitations exists?

And the same question about space (not on Earth)?

Update:

1. Primary mirror should be segmented (or it variations) like on E-ELT.
2. Suppose we have a large (several square kilometers), flat surface high above sea level.
3. We have to build telescope for observing at optical wavelengths.

I know that, there is concept of OWL with 100 metre-diameter segmented primary mirror.

But what about 500 metre-diameter or 1000? Is it possible in theory?

It's complicated.

Until late-20th century, we've tried to make bigger and bigger monolithic telescopes. That worked pretty well up to the 5 meter parabolic mirror on Mount Palomar in California in the 1940s. It kind of worked, but just barely, for the 6 meter mirror on Caucasus in Russia in the 1970s. It did work, but that was a major achievement, for the twin 8.4 meter mirrors for the LBT in Arizona in the 2000s.

We've learned eventually that the way to go is not by pouring larger and larger slabs of low-expansion glass. It is generally accepted that somewhere just below 10 meters diameter is about as large as possible for monolithic mirrors.

The way to go is by choosing to make smaller mirror segments (1 meter to a few meters in diameter each) and combining those into a tiled mirror. It's somewhat harder to carve the asymmetrical parabolic (or hyperbolic, or elliptic, or spherical) reflecting curved surface in a segment like that, but it's far easier to manage thermal and cooling issues when you have to deal with smaller solid objects.

Each segment is mounted in an active mirror cell, with piezo actuators that very precisely control its position. All segments must combine into a single smooth surface with a precision better than 100 microns (much better than that in reality). So now you have a large array of massive objects, dynamically controlled via computer, each with its own vibration modes, each with its own source of mechanical noise, each with its own thermal expansion motions, all of them "dancing" up and down a few microns on piezo elements.

Is it possible to orchestrate a very large system like that? Yes. The 100 meter OWL was considered feasible technically. From the perspective of keeping the mirrors aligned, an even larger structure should be doable; the computer-controlled actuators should overcome most vibrations and shifts up to quite large distances.

Like you said, the real limits are financial. The complexity of such a system increases with the square of the diameter, and with complexity comes cost.

The entire discussion above was about "filled aperture" telescopes: given a round shape of a certain diameter, it is filled with mirror segments. For a given aperture, this design captures the largest amount of light.

But the aperture does not have to be filled. It can be mostly empty. You could have a few reflecting segments on the periphery, and the center would be mostly void. You'd have the same resolving power (you would see the same small details), it's just that the brightness of the image would decrease, because you're capturing less light total.

This is the principle of the interferometer. The twin 10 meter segmented Keck mirrors in Hawaii can work as an interferometer with a baseline of 85 meters. This is effectively equivalent to a single 85 meter aperture in terms of resolving power, but obviously not in terms of image brightness (amount of light captured).

The US Navy has an interferometer in Arizona with mirrors placed on 3 arms in a Y shape, each arm 250 meters long. That gives the instrument a baseline (equivalent aperture) of several hundreds of meters.

U of Sydney has a 640 meter baseline interferometer in the Australian desert.

Interferometers cannot be used to study very faint objects, because they can't capture enough light. But they can produce very high resolution data from bright objects - e.g. they are used to measure the diameter of stars, such as Betelgeuse.

The baseline of an interferometer can be made extremely large. For terrestrial instruments, a kilometers-wide baseline is very doable now. Larger will be doable in the future.

There are talks about building interferometers in outer space, in orbit around Earth or even bigger. That would provide a baseline at least in the thousands of kilometers. That's not doable now, but seems feasible in the future.

## World's Largest Telescope To Finally See Stars Without Artificial Spikes

The enormous, 25-meter Giant Magellan Telescope (GMT) will not only usher in a new era in . [+] ground-based astronomy, but will take the first cutting-edge images of the Universe where stars are seen exactly as they actually are: without diffraction spikes.

Giant Magellan Telescope - GMTO Corporation

When you look out at the greatest images of the Universe, there are a few sights that light up our memories and fire our imaginations. We can see the planets in our own Solar System to incredible detail, galaxies lying millions or even billions of light years away, nebulae where new stars are being birthed, and stellar remnants that give an eerie, fatalistic look into our cosmic past and our own Solar System's future. But the most common sight of all are stars, lying everywhere and in any direction we care to look, both in our own Milky Way and beyond. From ground-based telescopes to Hubble, stars almost always come with spikes on them: an image artifact due to how telescopes are constructed. As we prepare for the next generation of telescopes, however, one of them — the 25-meter Giant Magellan Telescope — stands out: it's the only one that won't have those artificial spikes.

Hickson compact group 31, as imaged by Hubble, is a spectacular "constellation", but almost as . [+] prominent are the few stars from our own galaxy visible, noted by the diffraction spikes. In only one case, that of the GMT, will those spikes be absent.

ASA, ESA, S. Gallagher (The University of Western Ontario), and J. English (University of Manitoba)

There are a lot of ways to make a telescope in principle, all you need to do is collect-and-focus light from the Universe onto a single plane. Early telescopes were built on the concept of a refractor, where the incoming light passes through a large lens, focusing it down to a single point, where it can then be projected onto an eye, a photographic plate, or (in more modern fashion) a digital imaging system. But refractors are limited, fundamentally, by how large you can physically build a lens to the necessary quality. These telescopes barely top 1 meter in diameter, at maximum. Since the quality of what you can see is determined by the diameter of your aperture, both in terms of resolution and light-gathering power, refractors fell out of fashion over 100 years ago.

Reflecting telescopes surpassed refractors long ago, as the size you can build a mirror greatly . [+] surpasses the size to which you can build a similar-quality lens.

The Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, Calif.

But a different design — the reflecting telescope — can be far more powerful. With a highly reflective surface, a properly shaped mirror can focus incoming light onto a single point, and mirrors can be created, cast and polished to much greater sizes than lenses can. The largest single-mirror reflectors can be up to a whopping 8-meters in diameter, while segmented mirror designs can go even larger. At present, the segmented Gran Telescopio Canarias, with a 10.4 meter diameter, is the largest in the world, but two (and potentially three) telescopes will break that record in the coming decade: the 25-meter Giant Magellan Telescope (GMT) and the 39-meter Extremely Large Telescope (ELT).

A comparison of the mirror sizes of various existing and proposed telescopes. When GMT comes online, . [+] it will be the world's largest, and will be the first 25 meter+ class optical telescope in history, later to be surpassed by the ELT. But all of these telescopes have mirrors, and each of the ones shown in color (foreground) are reflecting telescopes.

Wikimedia Commons user Cmglee

Both of these are reflecting telescopes with many segments, poised to image the Universe like never before. The ELT is larger, is made of more segments, is more expensive, and should be completed a few years after GMT, while the GMT is smaller, made of fewer (but larger) segments, is less expensive, and should reach all of its major milestones first. These include:

• excavations that began in February of 2018,
• concrete pouring in 2019,
• a completed enclosure against weather by 2021,
• the delivery of the telescope by 2022,
• the installation of the first primary mirrors by early 2023,
• first light by the end of 2023,
• first science in 2024,
• and a scheduled completion date by the end of 2025.

That's soon! But even with that ambitious schedule, there's one huge optical advantage that GMT has, not only over the ELT, but over all reflactors: it won't have diffraction spikes on its stars.

The star powering the Bubble Nebula, estimated at approximately 40 times the mass of the Sun. Note . [+] how the diffraction spikes, due to the telescope itself, interferes with nearby detailed observations of fainter structures.

NASA, ESA, Hubble Heritage Team

These spikes that you're used to seeing, from observatories like Hubble, come about not from the primary mirror itself, but from the fact that there needs to be another set of reflections that focus the light onto its final destination. When you focus that reflected light, however, you need some way to place-and-support a secondary mirror to refocus that light onto its final destination. There's simply no way to avoid having supports to hold that secondary mirror, and those supports will get in the way of the light. The number and the arrangement of the supports for the secondary mirror determine the number of spikes — four for Hubble, six for James Webb — you'll see on all of your images.

Comparison of diffraction spikes for various strut arrangements of a reflecting telescope. The inner . [+] circle represents the secondary mirror, while the outer circle represents the primary, with the "spike" pattern shown underneath.

Wikimedia Commons / Cmglee

All ground-based reflectors have these diffraction spikes, and so will the ELT. The gaps between the 798 mirrors, despite making up just 1% of the surface area, contribute to the magnitude of the spikes. Whenever you image something faint that unluckily happens to be near something close and bright — like a star — you have these diffraction spikes to contend with. Even by using shear imaging, which takes two almost-identical images that are only slightly mis-positioned and subtract them, you can't get rid of those spikes entirely.

The Extremely Large Telescope (ELT), with a main mirror 39 metres in diameter, will be the world’s . [+] biggest eye on the sky when it becomes operational early in the next decade. This is a detailed preliminary design, showcasing the anatomy of the entire observatory.

But with seven enormous, 8-meter diameter mirrors arranged with one central core and six symmetrically-positioned circles surrounding it, the GMT is brilliantly designed to eliminate these diffraction spikes. These six outer mirrors, the way they're arranged, allows for six very small, narrow gaps that extend from the edge of the collecting area all the way into the central mirror. There are multiple "spider arms" that hold the secondary mirror in place, but each arm is precisely positioned to run exactly in between those mirror gaps. Because the arms don't block any of the light that's used by the outer mirrors, there are no spikes at all.

The 25-meter Giant Magellan Telescope is currently under construction, and will be the greatest new . [+] ground-based observatory on Earth. The spidar arms, seen holding the secondary mirror in place, are specially designed so that their line-of-sight falls directly between the narrow gaps in the GMT mirrors.

Giant Magellan Telescope / GMTO Corporation

Instead, owing to this unique design — including the gaps between the different mirrors and the spider arms crossing the central primary mirror — there's a new set of artifacts: a set of circular beads that appear along ring-like paths (known as Airy rings) surrounding each star. These beads will appear as empty spots in the image, and are inevitable based on this design whenever you look. However, these beads are low-amplitude and are only instantaneous as the sky and the telescope rotate over the course of a night, these beads will be filled in as a long-exposure image is accumulated. After about 15 minutes, a duration that practically every image should attain, those beads will be completely filled in.

The core of the globular cluster Omega Centauri is one of the most crowded regions of old stars. GMT . [+] will be able to resolve more of them than ever before, all without diffraction spikes.

NASA/ESA and The Hubble Heritage Team (STScI/AURA)

The net result is that we'll have our first world-class telescope that will be able to see stars exactly as they are: with no diffraction spikes around them! There is a slight trade-off in the design to achieve this goal, the biggest of which is that you lose a little bit of light-gathering power. Whereas the end-to-end diameter of the GMT, as designed, is 25.4 meters, you "only" have a collecting area that corresponds to a 22.5 meter diameter. The slight loss of resolution and light-gathering power, however, is more than made up for when you consider what this telescope can do that places it apart from all others.

A selection of some of the most distant galaxies in the observable Universe, from the Hubble Ultra . [+] Deep Field. GMT will be capable of imaging all of these galaxies with ten times the resolution of Hubble.

NASA, ESA, and N. Pirzkal (European Space Agency/STScI)

It will achieve resolutions of between 6-10 milli-arc-seconds, depending on what wavelength you look at: 10 times as good as what Hubble can see, at speeds 100 times as fast. Distant galaxies will be imaged out to distances of ten billion light years, where we can measure their rotation curves, look for signatures of mergers, measure galactic outflows, look for star formation regions and ionization signatures. We can directly image Earth-like exoplanets, including Proxima b, out to somewhere between 15-30 light years distant. Jupiter-like planets will be visible out to more like 300 light years. We'll also measure the intergalactic medium and the elemental abundances of matter everywhere we look. We'll find the earliest supermassive black holes.

The more distant a quasar or supermassive black hole is, the more powerful a telescope (and camera) . [+] you need to find it. GMT will have the advantage of being able to do spectroscopy on these ultra-distant objects that it finds.

NASA and J. Bahcall (IAS) (L) NASA, A. Martel (JHU), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory), the ACS Science Team and ESA (R)

And we'll make direct, spectroscopic measurements of individual stars in crowded clusters and environments, probe the substructure of nearby galaxies, and observe close-in binary, trinary and multi-star systems. This even includes stars in the galactic center, located some 25,000 light years away. All, of course, without diffraction spikes.

This image illustrates the improvement in resolution in the central 0.5” of the Galaxy from . [+] seeing-limited to Keck + Adaptive Optics to future Extremely Large Telescopes like GMT with adaptive optics. Only with GMT will the stars appear without diffraction spikes.

A. Ghez / UCLA Galactic Center Group - W.M. Keck Observatory Laser Team

Compared to what we can presently see with the world's greatest observatories, the next generation of ground-based telescopes will open up a slew of new frontiers that will peel back the veil of mystery that enshrouds the unseen Universe. In addition to planets, stars, gas, plasma, black holes, galaxies, and nebulae, we'll be looking for objects and phenomena that we've never seen before. Until we look, we have no way of knowing exactly what wonders the Universe has waiting for us. Owing to the clever and innovative design of the Giant Magellan Telescope, however, the objects we've missed due to diffraction spikes of bright, nearby stars will suddenly be revealed. There's a whole new Universe to be observed, and this one, unique telescope will reveal what no one else can see.

For now optical telescopes located in the contiguous United States by aperture.

Name Image Effective aperture
m
Aper.
in
Mirror type Nationality / Sponsors Site Built
Large Binocular Telescope (LBT) 11.9 m (8.4 m×2) 330″×2 Multiple mirror, 2 USA, Italy, Germany Mount Graham International Obs., Arizona, USA 2004
Hobby–Eberly Telescope (HET) (11 m × 9.8 m mirror) 10 m 394″ Segmented, 91 USA, Germany McDonald Observatory, Texas, USA 1997
MMT (1 x 6.5 M1) 6.5 m 256″ Single USA F. L. Whipple Obs., Arizona, USA 2000
Hale Telescope (200 inch) 5.08 m 200″ Single USA Palomar Observatory, California, USA 1948
MMT (6×1.8 m) original optics 4.7 m
(6×1.8 m) [1]
186″ Segmented, 6 USA F. L. Whipple Obs., Arizona, USA 1979–1998
Lowell Discovery Telescope [2] 4.3 m 169″ Single USA Lowell Observatory, Happy Jack, Arizona 2012
Nicholas U. Mayall 4m [3] 4 m 158 inch Single USA Kitt Peak National Obs., Arizona, USA 1973
USAF Starfire 3.5 m [4] 3.5 m 138″ Single USA Starfire Optical Range, New Mexico, USA 1994
WIYN Telescope 3.5 m 138″ Single USA Kitt Peak National Obs., Arizona, USA 1994
Space Surveillance Telescope 3.5 m 138″ Single USA White Sands Missile Range, New Mexico, USA 2011
Astrophysical Research Consortium (ARC) 3.48 m 137″ Single USA Apache Point Obs., New Mexico, USA 1994
Shane Telescope 3.05 m 120″ Single USA Lick Observatory, California, USA 1959
NASA-LMT [5] retired 3 m 118″ Liquid USA NASA Orbital Debris Obs., New Mexico, USA 1995–2002
For telescopes below 3 meters see List of large optical telescopes

Some of the big traditional refractors (telescope with lens) in North America:

## Future Plans

Plans for future telescopes will have even more viewing potential than those of the present-day hopefully these instruments will catch a glimpse into the birth of a universe. One telescope planned for 2020 is the Giant Magellan Telescope, which will measure 80 feet in diameter and promptly take the place of the largest telescope on earth. This telescope will also be located in Chile and is expected to provide a direct view of the planets in other solar systems, a first for astronomy.

## Coming Soon: World’s Largest Optical Telescope

The world’s largest optical/infrared telescope has been given the initial go-ahead to be built. Called the European Extremely Large Telescope (E-ELT) this long-proposed new ground-based telescope will have a 40-meter main mirror and observe the universe in visible and infrared light, making direct images of exoplanets, perhaps find Earth-sized and even Earth-like worlds, and study the first galaxies that formed after the Big Bang.

“This is an excellent outcome and a great day for ESO. We can now move forward on schedule with this giant project,” said the ESO Director General, Tim de Zeeuw.

At a meeting in Garching, France this week, the ESO (European Southern Observatory) Council approved the E-ELT program, with 6 out of 10 countries giving firm approval and four gave “ad referendum” approval, meaning that they needed an official green light from their governments. With that approval, officials are hopeful the E-ELT could start operations by the early 2020’s.

The new super-large eye on the sky will be built at Cerro Armazones in northern Chile, close to ESO’s Paranal Observatory.

The cost is expected to be $1.35 billion USD (1.083-billion-euro) “World-leading projects of this kind inspire us all and are hugely effective in bringing young people into careers in science and technology,” said David Southwood, president of the Royal Astronomical Society. This type of telescope has been on the priority list for astronomy by scientists around the world. The E-ELT will gather 100 million times more light than the human eye, eight million times more than Galileo’s telescope which saw the four biggest moons of Jupiter four centuries ago, and 26 times more than a single VLT telescope. “The E-ELT will tackle the biggest scientific challenges of our time, and aim for a number of notable firsts, including tracking down Earth-like planets around other stars in the ‘habitable zones’ where life could exist — one of the Holy Grails of modern observational astronomy,” the ESO said. ESO said that early contracts for the project have already been placed. Shortly before the Council meeting, a contract was signed to begin a detailed design study for the very challenging M4 adaptive mirror of the telescope. This is one of the longest lead-time items in the whole E-ELT program, and an early start was essential. Detailed design work for the route of the road to the summit of Cerro Armazones, where the E-ELT will be sited, is also in progress and some of the civil works are expected to begin this year. These include preparation of the access road to the summit of Cerro Armazones as well as the leveling of the summit itself. ## THE ORIGIN OF COSMIC RAYS V.L. GINZBURG , S.I. SYROVATSKII , in The Origin of Cosmic Rays , 1964 ### Some data from the field of extragalactic astronomy The region of the Universe which can be observed with the most powerful optical and radio telescopes is called the Metagalaxy and has a radius of less than 10 10 light years. This whole region is at present (here and below we are thinking of clocks on Earth) in a state of expansion. Here in the first approximation the velocity of recession of the galaxies u is where r is the distance (from our Galaxy) and Hubble's constant h according to present data is approximately 100 km/sec · Mpc ⋍ 3·2 × 10 −18 sec −1 . † The characteristic time of the metagalaxy's evolution is Relationship (13.1) is valid only as long as uc = 3 × 10 10 cm/sec. In this case the Doppler shift of the wavelength λ0 towards the red end of the spectrum is The condition z ≪ 1 must also be observed, generally speaking, so as to be able to use Euclidean geometry. Below, moreover, we shall use terminology corresponding to Euclidean space right up to z ≲ 0·5 (the value z ⋍ 0·5 corresponds to the maximum distance which can be reached by existing optical telescopes ). There is even more reason to proceed in this way since there is still no clarity in the question of the terms that are non-linear with respect to z (i.e., outside the region where z ≪ 1). The available preliminary data indicate that the curvature of the Universe is relatively small and is possibly non-existent (this fits in with Euclidean geometry allowing, of course, for the expansion of the system). † In the region with z ⩽ 0·5 there are approximately 3 × 10 9 galaxies, 387 which for h = 100 km/sec·Mpc corresponds to a galaxy concentration The value of (13.3) is probably close to the lower limit of NG since the number of galaxies assumed may be too low because of an underestimate of the number of dwarf systems. As has already been indicated in section 12 , the mass of the Galaxy is approximately 10 11 M = 2 × 10 44 g. The mass of the galaxy M31 is 4 times greater but there are dwarf galaxies with a mass of only 4 × 10 8 M. It is difficult to give a modern mean value for the mass of the galaxies but in order of magnitude M ¯ ∼ 3 × 10 10 M ⊙ (according to Allen 315 M ¯ = 5 × 10 10 M ⊙ ). With NG = 5 × 10 −75 cm −3 and M ¯ = 10 44 g the mean density of the matter in the Metagalaxy is ρ = 5 × 10 −31 g/cm 3 . This value is, of course, a lower limit since only the matter concentrated in the galaxies is taken into account in obtaining it. There are no more or less direct data on the density of the gas in intergalactic space, as has already been mentioned. However, it is often considered that the most reasonable value is (the conversion to the concentration n is made upon the normal assumption that the gas consists largely of hydrogen). The bases for the estimate (13.4) are as follows. In relativistic cosmology a certain critical density ρcr = 3h 2 /8πκ, where κ = 6·67 × 10 −8 g −1 cm 3 sec −2 is the gravitational constant, is of great importance. In a homogeneous cosmological model, when there is no Λ-term, 165, 382–386 the value ρ = ρcr corresponds to a Euclidean metric, i.e., the Einstein–de Sitter model (if ρ > ρcr space has a positive and if ρ < ρcr a negative curvature). The available preliminary data are in favour of ρ ∼ ρcr ⋍ 2 × 10 −29 g/cm 3 (for h = 100 km/sec · Mpc). The value of (13.4) thus corresponds to a density close to ρcr. As a guide Smorodinskii 384 gives the more definite value ρ = (1 to 3)ρcr. Not to mention the inaccuracy of the observational data this estimate of ρ depends on the selection of the model (in the present case models without a Λ-term are under discussion). We note that in stationary cosmology 386, 388, 389 (see also below) the gas density is ρ = ρcr and does not depend on time. The density ρ in Euclidean expanding space changes in accordance with the law where ρ(TMg) is the present value of the density, R is the characteristic scale (the distance between remote galaxies) and the scale for the time t is selected so that now t = TMg (see below). Law (13.5) corresponds simply to preservation of the total mass of gas in the system connected with this expanding gas. An estimate of the emission energy density in the Metagalaxy was given in section 8 (the index 3/2 in (13.6) is obtained below see (13.11) ) This calculation proceeds from the assumption of the continuous emission of light, the brightness of the sources not changing in time. As for the emission at the moment t1 the density of its energy varies in accordance with the law wph(t2) = wph(t1)[(R(t1)/R(t2)] 4 . The energy density of any ultra-relativistic gas, in particular cosmic rays and neutrinos, 384 varies in the same way—in accordance with the law (R1/R2) 4 . The appearance of an extra term (R1/R2) when compared with the case of a non-relativistic gas (in the latter case the energy density 3nkT/2 is proportional to the density of the mass ρ(t2) = ρ(t1)[R1(t1)/R2(t2)] 3 ) is connected with the change in energy of a relativistic particle as the result of the Doppler effect. The same thing (the appearance of (R1/R2) 4 in the expression for the energy density) occurs for cosmological models in which the pressure is determined by relativistic particles, i.e., p = w/3 (w is the energy density). Unlike a non-relativistic gas, when the pressure p is small when compared with the total energy density w = ρc 2 , the expansion of a relativistic gas is linked with so much work done by the pressure forces that the extra degree R1/R2 appears. 165 In the case of cosmic rays (for a constant number of particles) the law by virtue of what has been said may also always be applicable. In actual fact this does not have to be so. Of course, if ultra-relativistic cosmic rays move in a straight line in the Metagalaxy like photons or neutrinos the relationship (13.7) is valid. The idea that cosmic rays diffuse in the intergalactic magnetic fields is probably closer to reality. In this case the change in their energy density w is not universal and depends on the actual conditions. However, for an isotropically and homogeneously expanding medium the energy of a particle is E V −1/3 , where V is the volume (see section 10 ). Since the particle concentration is N V −1 the energy density is w = NE and V R 3 we come once again to formula (13.7) . Galaxies have a tendency to come into multiple systems and clusters. We have spoken about one such cluster—the Local Group—in section 12 . Other clusters are generally considerably larger than the Local Group (it consists of approximately 15 galaxies) and on the average there are about 200 galaxies in a cluster the mean diameter of a cluster is about 10 25 cm. Larger clusters are also met with (the cluster in Virgo consists of 2500 galaxies and the cluster in Coma of 1000 galaxies). There are probably even larger combinations of galaxies—super-clusters or supergalaxies. For example the Local Group together with our Galaxy, according to a series of data 387 , are part of the Local Supergalaxy. The nucleus of the latter is the large cluster in Virgo already mentioned which is at a distance of 12 to 15 Mpc ⋍ 5 × 10 25 cm (1 Mpc = 3 × 10 24 cm). The Local Supergalaxy takes the form of a disk or a very flattened ellipsoid with a diameter of about 30 Mpc and a thickness of about 6 Mpc. The volume of the system is V ∼ 10 77 cm 3 . The mass of all the galaxies in the supergalaxy is M ∼ 10 14 M, but for stability of the system, according to Burbidge 387 , the mass M must be of the order of 10 15 M. If this estimate is true and the system is stable then there should be † an intergalactic gas with a density ρ ∼ M/V ∼ 2 × 10 48 /10 77 = 2 × 10 −29 g/cm 3 . This value is in accordance with estimate (13.4) so is quite probable. The relative velocities of the galaxies (without allowing for the rate of overall expansion) are of the order of 100 to 500 km/sec. The intergalactic gas probably has velocities of the same order. If ρ = 2 × 10 −29 g/cm 3 and u = 300 km/sec, then Here the field strength Heq is estimated from the condition H 2 eq/8π = ρu 2 /2. The values in (13.8) are clearly the maximum ones which are reasonable at the present time. ‡ For a gas velocity u ∼ 10 7 cm/sec we clearly have ρu 2 /2 ∼ 10 −15 erg/cm 3 and Heq ∼ 10 −7 oe in addition, even the quasi-equilibrium value of the field H may be slightly less than Heq if it is connected with small-scale turbulent motions. 396 In the Local Supergalaxy there are about 10 4 galaxies which corresponds to a concentration of Nsuperg ∼ 10 −73 cm −3 . This value is 20 times higher than the mean concentration (13.3) but it is hard to judge how accurate this estimate of Nsuperg is. ## GMT to be World’s Biggest Optical Telescope The Gran Telescopio Canarias in Spain is currently the world’s largest optical telescope with the aperture of its main mirror 10.4 metres across. Despite roughly 8 meters being the maximum size for an accurate single mirror to be built on Earth, such a large size was achieved by joining 36 segments together, but now the Giant Magellan Telescope (GMT) currently under construction at Las Campanas Observatory in Chile is set to claim the title with its huge 25 meter diameter mirror. The feat will be achieved by stitching seven single-cast mirrors together, one central and six off-axis, which will then have five times the light gathering capability of Gran Telescopio Canarias, and ten times that of the space-based Hubble. Moreover, the$1 billion telescope project scheduled to come online in 2021 will be able to eliminate most atmospheric distortions, and be so powerful that, as Astrophysicist Ethan Siegel explains:

“Distant galaxies will be imaged out to ten billion light years. We’ll be able to measure their rotation curves, look for signatures of mergers, measure galactic outflows, look for star formation regions and ionization signatures. We’ll be able to directly image Earth-like exoplanets, including Proxima b, out to somewhere between 15-30 light years distant. Jupiter-like planets will be visible out to more like 300 light years.”

Equally exciting is the prospect of GMT advancing our understanding of the universe by revealing secrets scientists don’t yet know exist. In the 1920’s, for instance, the 2.54-metre Hooker telescope located at Mount Wilson Observatory lead to Edwin Hubble’s discovery of an expanding Universe while in the 1990’s/2000’s, NASA’s Hubble Space Telescope captured the Hubble Ultra Deep Field (HDF) and Hubble Ultra-Deep Field (HUDF) that took us to within 400 million years of the Big Bang, and added to our understanding of the number of galaxies that exist. There are therefore great hopes that the Giant Magellan Telescope will lead to seeing the Universe in ways never imagined, and elaborating further, GMT’s director, Pat McCarthy, explained:

“There’s a huge, rich world of unknown things out there to be discovered. It’s things like GMT, its cutting-edge new facilities, that bring out these great discoveries. We hope, once GMT is built, some clever young person comes along and does something completely unexpected with it that changes everything. That would be success.”

## THE EUROPEAN EXTREMELY LARGE TELESCOPE ("ELT") PROJECT

The European Extremely Large Telescope (ELT) is a revolutionary scientific project for a 40m-class telescope that will allow us to address many of the most pressing unsolved questions about our Universe.

The ELT will be the largest optical/near-infrared telescope in the world and will gather 13 times more light than the largest optical telescopes existing today. The ELT will be able to correct for the atmospheric distortions (i.e., fully adaptive and diffraction-limited) from the start, providing images 16 times sharper than those from the Hubble Space Telescope. The ELT will vastly advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, super-massive black holes, and the nature of the Universe's dark sector (more).

The ELT project is included in the European Strategy Forum on Research Infrastructures (ESFRI) List of Opportunities. It has also been ranked in the 2010-2025 ASTRONET European strategic planning as one of two clear top priorities for future ground-based astronomical infrastructures.

The project has completed the ELT's detailed design, which passed the Final Design Review successfully in September 2010. Between the end of 2010 and the summer of 2011 the ELT project extended the detailed design phase in order to consider the recommendations of the ELT Design Review. The main goals were the reduction of risk by optimising the cost and constraining the schedule in order to ensure that ESO can further expand its leading role in astronomy by constructing the world’s first extremely large telescope.

In June 2011 ESO Council endorsed a revised baseline design for the ELT. This led to a significant cost saving and to a reduction of risk on major items such as the secondary mirror.

Also in December 2014 a new policy for the awarding of GTO time on the ELT was approved by Council at its meeting. See the Policy Document.

The ELT is planned to start operations as an integrated part of the Paranal Observatory in 2025.

The European Extremely Large Telescope — the World’s Biggest Eye on the Sky

The following is a list of largest single mount optical telescopes sorted by total objective diameter (aperture), including segmented and multi-mirror configurations. It is a historical list, with the instruments listed in chronological succession by objective size. By itself, the diameter of the primary optics can be a poor measure of a telescope's historical or scientific significance for example, William Parsons, 3rd Earl of Rosse's 72-inch (1.8 m) reflecting telescope did not perform as well (i.e. gather as much light) as the smaller silvered glass mirror telescopes that succeeded it because of the poor performance of its speculum metal mirror.

Optical Telescopes (List by Overall Aperture)
 Name Aperture Type Built by Location Year Meter Inch Gran Telescopio Canarias (GTC) 10.4 m 409″ Reflector – Segmented,36 Spain (90%), Mexico, USA ORM, La Palma, Canary Islands, Spain 2009 Keck 1 10 m 394″ Reflector – Segmented,36 USA Mauna Kea Observatory, Hawaii 1993 BTA-6 6 m 238″ Reflector Soviet Union Zelenchukskaya, Caucasus 1976 Hale Telescope 5.08 m 200″ Reflector USA Palomar Observatory, California 1948 Hooker Telescope 2.54 m 100″ Reflector USA Mt. Wilson Observatory California 1917 Leviathan of Parsonstown 1.83 m 72″ Reflector – metal mirror William Parsons Birr Castle Ireland 1845 Herschel's 40-foot telescope [1] 1.26 m 49.5″ Reflector – metal mirror William Herschel Observatory House England 1789–1815 John Michell's Gregorian reflector [2] 75 cm 29.5″ Reflector - Gregorian John Michell Yorkshire, Great Britain 1780–1789 Father Noel's Gregorian reflector [2] 60 cm 23.5″ Reflector – Gregorian Father Noel Paris, France 1761 James Short's Gregorian reflector 50 cm 19.5" Reflector – Gregorian James Short Great Britain 1750 James Short's Gregorian reflector 38 cm 14″ Reflector – Gregorian James Short Great Britain 1734 Christiaan Huygens 210 foot refractor 22 cm 8.5" Refractor – Aerial telescope Christiaan Huygens Netherlands 1686 Christiaan Huygens 170 foot refractor 20 cm 8" Refractor – Aerial telescope Christiaan Huygens Netherlands 1686 Christiaan Huygens 210 foot refractor 19 cm 7.5" Refractor – Aerial telescope Christiaan Huygens Netherlands 1686 Hooke's reflector [3] 18 cm 7″ Reflector Robert Hooke Great Britain 16?? Hevelius refractor 12 cm 4.7″ Refractor Johannes Hevelius Gdańsk, Poland 1645 Hevelius Scheiner's helioscope 6 cm 2.3″ Refractor Johannes Hevelius Gdańsk, Poland 1638 Galileo's 1620 telescope [4] 3.8 cm 1.5″ Refractor Galileo Galilei Italy 1620 Galileo's 1612 telescope [4] 2.6 cm 1″ Refractor Galileo Galilei Italy 1612 Galileo's 1609 telescope [4] 1.5 cm .62″ Refractor Galileo Galilei Italy 1609 Hans Lippershey's telescope ? cm .?″ Refractor Hans Lippershey Middelburg, the Netherlands 1608

Chronological list of optical telescopes by historical significance, which reflects the overall technological progression and not only the primary mirror's diameter (as shown in table above).

## LUCI in the Sky?

However, the amusement didn’t last long, for as word of it spread, many critics of the Roman Catholic Church made the connection with the LUCIFER instrument for the LBT sharing a mountain with the VATT, which very quickly (but incorrectly) morphed into the Vatican owning a telescope called LUCIFER. The firestorm that ensued probably was responsible for renaming the instrument LUCI in 2012, six years before LUCI was completed and went into service (such a complex instrument can require years of planning and development). Note that LUCI is not a telescope but rather is an instrument that attaches to a telescope. Also notice that the LUCI instrument attaches only to the LBT, not the VATT. They may be at the same observatory (MGIO), but they are separate telescopes, owners/collaborators and systems.

Despite the rechristening of LUCIFER as LUCI and the fact that LUCIFER never was the name of a telescope, let alone a telescope owned by the Vatican, this false rumor keeps making the rounds. So, if you know someone repeating this false rumor, please refer them to this article for correction.