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Laser Collimator For Dobsonian vs Newtonian

Laser Collimator For Dobsonian vs Newtonian



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Will a laser collimator like the Gosky 1.25 Metal Laser Collimator, https://www.amazon.com/gp/product/B01M4IVUYG/ref=crt_ewc_title_dp_1?ie=UTF8&psc=1&smid=AHQ6VCR020F8X, work for a dobsonian even though it says it's for a newtonian?


A Dob is a Newtonian, so yes it will work. Strictly speaking the Dob bit is the mount, but it's nearly always a newtonian that's mounted.


A step by step guide to Collimation

If like me you own some sort of reflector telescope, whether this be a Newtonian, Dobsonian, Ritchey Chretien or as I have a Hyperboloid Astrograph then you’ll know that there is a very strong importance on collimation, the faster the optics the more critical collimation becomes, especially for imaging. After recently removing the rear mirror assembly for cleaning, as well as changing from the QHY183M to the QHY268C-PH amongst onther stuff in the imaging train, I wanted to share my experience and knowledge around collimation. Let’s start off with the details on what I use


Laser Collimator For Dobsonian vs Newtonian - Astronomy

A couple of questions: First, did you use the whole Cheshire collimation tool set (including the auto collimator) or just the Cheshire tool? Did you also use the sight tube (I don't understand everything in this yet so if the question seems stupid it probably is)?

Also, Zhumell has a deluxe collimator. Would it be any better than the one included with the scope, do you think?

I don't currently have an autocollimator, but the Cheshire I have is a sight-tube/Cheshire combo tool. It functions both as a sight tube and as a Cheshire, so I can use the same tool to do everthing from centering the secondary to aligning the primary with one tool.

As far as the Zhumell deluxe laser, I'm sure it might be a little nicer, but the inherent problem with a laser collimator is that it needs to be collimated, just as the scope needs to be collimated. Putting the laser in a Barlow will diffuse the beam, which can be quite useful. By placing a paper target on the Barlow, you can center the shadow of the center dot on that target. Then you know that you are perfectly collimated, as this is not dependent on the laser's collimation.

Hope this helps. Once I get a hold of a Barlow, I'm going to do a write-up on Barlowed laser collimation.

DO you have a recommendation as to where to buy the collimation tool set from? I'm looking at getting an order turned in so I can really see!

I got mine at the Cloudy Nights Classifieds section. This item at telescopes.com is similar to my Cheshire eyepiece.

Your headline would have been more accurate if it had said: Why Not to Simply Rely on a Laser Collimator.

Perhaps a good follow-up post would be how to collimate your laser collimator. Assuming it has a round body, this is fairly straightforward.

Clamp a piece of angle iron or aluminum in place and lay the collimator in it (pointing at a wall). Now roll the collimator (rotating it in place 360 degrees). If perfectly collimated, the laser should stay at the same point on the wall. If it travels in a circle, it is out of collimation.

Good laser collimators can themselves be collimated. I have an Orion Lasermate Deluxe. I'm told the collimation screws are under a self-adhesive label.

You bring up a good point, cheekygeek, as most laser collimators do have a means to collimate them. I've been meaning to make such a jig, but school lately has been hectic!

On my Zhumell laser, the collimation screws are set into the body, inside some little holes. If I remember correctly, they are allen screws.

Anyways, another way to collimate a laser is to actually put it in the telescope, and adjust your secondary WAY out of whack, so that the laser beam misses the secondary entirely on the return trip and exits the objective. Then your telescope has become the jig, and you can get a decent collimation of the laser in this manner. If I get time this weekend, I'll do a write-up of the process for future use.

I had the same mechanical slop problem with my 10" LightBridge using the LaserMate collimator until I found a company solves the slop problem with their new SCA technology. I went ahead and bought the SCA laser collimator last week, and the laser works like a charm.


What about Laser Collimators?

If you’re set on using a laser collimator, then I’d avoid buying the cheap $15 ones made in China. Most of the time, they’re not collimated, which makes them very ineffective. You’ll need to collimate the laser collimator yourself – if you do decide on a cheap laser collimator, then check this video guide out on how to collimate it yourself

As you can see from the video, you need to be a bit ‘hands on’ to be able to do this properly. So, that’s why I’d avoid this, especially if you’re a beginner.

Hotech Laser Collimators

Generally, some people will say Hotech Collimators are overrated. Personally, I wouldn’t even think about opting for a cheap collimator after getting a Hotech. If you want a quality laser collimator, then this is my number 1 choice.

Hotechs are pretty well known amongst astronomers as some of the best laser collimators available. They’re definitely a little expensive, but you get your moneys worth with a Hotech. The good thing about this Hotech is that it’s an SCA collimator – SCA stands for self centering adapter. If you want a guide on how to use this collimator properly, then here’s the best one that’s currently online

Another one of the main benefits of a laser collimator is obviously that you can do it in the nighttime, which means you don’t need to do it in advance. With Cheshires or if you’re trying to collimate without a tool, you’ll have to do things completely in advance – which can be a bit of a nightmare.


Harrisastronomy

What is collimating, anyway?

Collimating a telescope is lining up its optical components (lenses, mirrors, prisms, eyepieces) in their proper positions. This should be accurately done, or else the image quality will suffer. Different telescope types, like Newtonian, Schmidt-Cassegrain, or refractors all need good collimation. However, they have quite different optical components, and here I will talk about Newtonian telescopes, the simplest mirror telescopes (but in this revision, I have added some thoughts on Schmidt-Newtonian telescopes)

Newtonian? My telescope is supposed to be a Dobsonian!

Don’t worry. A Dobsonian telescope is a special kind of Newtonian, with a simple but very efficient mounting that distinguishes it from other Newtonians. Optically, and as far as collimating is concerned, they are the same.

I have bought a telescope, and it is factory collimated. Do I have to bother about collimating it?

Yes, most likely. If you have a factory-collimated refractor, Schmidt-Cassegrain, or Maksutov, you could very well leave the collimation alone and have a good chance of enjoying the excellent performance of your telescope for years to come. With a Newtonian, chances are less, for several reasons:

The main mirror must be held in place without stress that could bend it and change the optical figure, and cannot be rigidly held – it may shift slightly whenever you transport or shake the tube. The secondary mirror is also held by a “spider” that may change its position ever so slightly, and as we will see, it doesn’t take much to disturb the collimation enough to really matter. If you move your telescope to darker skies and back, and particularly if you have one with truss tubes that you assemble and disassemble, you must be able to check the collimation each night out, and you must be able to tweak it whenever needed.

Even if your telescope was factory collimated before shipping, it may have been on its longest journey ever before it reached you, and chances are great that it has lost much of its collimation. If you learn how to check the collimation, you will know whether or not your telescope is ready to deliver its best.

If the situation is that bad, maybe a Newtonian isn’t for me. Should I trade it in for something better?

My advice is: think again. There may be other good reasons for you to prefer another type of telescope. But a good Newtonian is a great performer when it is well collimated, and can come out close to or maybe ahead of any other instrument of the same size (aperture). Before you decide to trade it in, ask how much extra you would have to pay to get an alternative.

Suppose you have bought a fine guitar with a lovely note, and you are learning to play it. Now you notice it seems to get slightly out of tune. What do you do – learn how to tune it, or trade it in for a piano?

I believe that the reason Newtonians have a dubious reputation for critical performance is that too many Newtonians are never even collimated at all. Poor optics may not be easy or cheap to fix. Poor collimation, however, is something you can learn how to handle, and chances are good that you will be able to turn your scope into a star performer.

Don’t forget – a complete collimation of all the optical components is a bit of work – but the nightly checking takes a few seconds, and the tweaking, if needed, may take a minute.

OK, I am willing to give it a try, at least. How do I do it? Read the manual?

When I tried to figure out the hows and whys of collimating, I had very little help of the manual for my 6-incher. I tried reading the sections on collimating in a few magazines and handbooks, without really understanding what should happen, and why. I kept on trying, and in due time I felt my efforts paid off. That is why I write (and re-write) this – I hope I can make it easier for you. But let me point this out: Much of what I write here is common knowledge, even if it is not easy to come by, but some is the result of my own studies and experiments – particularly the error analysis and some of the tools – and my recommendations here are very much my own (and very controversial in some places). I believe it is sound advice, but I may be wrong on some accounts – if you really find fault with what I say, don’t hesitate to email me. Let it also be said that collimation is a subject of much heated discussion among us diehard Telescope Nuts, and I doubt that this will put an end to it (on second thoughts, I know for certain it won’t!).

I believe it will be easier for you to learn how, if you know why. By all means read your manual! Telescopes differ in design details, and your manual probably contains valuable information on how to adjust the screws and things on your particular instrument.

What are the parts of a Newtonian, what do they do and what parts can or need I adjust?

This is basic stuff, and if you know it well already, just read quickly.

The primary, or main, mirror.

This is the large mirror in the bottom of the tube, with a concave, aluminized face figured to an extremely accurate paraboloid surface. It concentrates the light from a star into a sharp image – not really a point, but a diffraction pattern with a small circle of light surrounded by small, faint rings.

It is held in some kind of mirror cell, fancy or simple, that rests on 3 set screws. By adjusting these screws, you can finely adjust the tilt of the primary mirror, this is an important part of collimation (you only need to adjust 2 of them – it might be wise to leave the third in a middle position). Often there are 3 extra screws (or else springs) for locking the mirror cell in place, once it is adjusted. It may look something like this:

The secondary, or diagonal, mirror.

This is a smaller mirror with an elliptic face (its size is given as the length of its minor axis, i.e. its “width”). It is suspended by a spider with one or several vanes inside the tube near its opening, and the face is at 45 degrees to the tube. It is used to deflect the light from the primary mirror sideways, so that you can see the image without having your head in the way of the incoming starlight.

The secondary mirror holder, and often the spider itself, is adjustable. It can be (more or less easily) moved sideways and along the tube, and it can be tilted (or rotated) slightly. Commonly, the mirror holder has a center bolt and three screws for adjustment.

The eyepiece

This is a more or less fancy magnifying glass, used to see the image of the star or whatever else you look at. It has a certain focal length, and with several eyepieces of different focal lengths, you can select the magnification (often called “power”) that you want. The focuser is where you put the eyepiece, it has a drawtube that holds the eyepiece and can be moved a little bit in and out, as needed to “focus” to get the sharpest view.

These optical parts are held in mechanical alignment by a tube of sorts. The tube, in turn, is supported by some mounting that lets you aim it at your chosen celestial object, and perhaps track its apparent motion as the Earth rotates.

How are they supposed to be aligned when the scope is well collimated?

There are two optical axes in a Newtonian telescope: the optical axis of the primary mirror, and the optical axis of the eyepiece.

The axis of the primary mirror is perpendicular to the mirror at its optical center – for practical purposes assumed to be the center of the circular glass mirror. For convenience, this is often marked with a spot of paint or tape. The light from a star in the exact direction of the primary mirror axis will be reflected and “focused” to a sharp image at the focal point or focus on this axis. Other stars will form images around the focus, in the focal plane (actually, the focal “plane” is part of a sphere, with its radius equal to the focal length). The distance along the optical axis, from the mirror center to the focus, is the focal length.

The axis of the eyepiece is usually taken as the center of the focuser drawtube. The secondary mirror reflects the incoming light to the side of the tube, and here the focused image forms, and is seen with the eyepiece. The secondary will also “reflect”, or rather deflect, the optical axes – it has an optical center, but no optical axis to concern us.

The main purpose of collimating is to align the two axes to form one common axis.

In most instruments, the focuser is fixed (or at least not readily adjustable), so it is practical to use the focuser axis as a reference. You first adjust the position and tilt of the secondary mirror to center the (reflected) eyepiece axis on the primary mirror, and then adjust the tilt of the primary mirror to center its (reflected) optical axis in the focuser. This done, the two optical axes are brought together.

Here comes some heavy theory – do I really have to read it?

Glad you asked – if you read this for the first time, you will probably find it a bit difficult to chew and swallow in one bite. So if you like, skip to the “End of heavy theory” for some more practical stuff. But I am sure the theory will make you understand the practical things better, and you may go back to read it any time later.

A systematic background:

I propose the following system of requirements for collimating Newtonians, and the corresponding errors, to facilitate understanding of the process (for illustrations, see the section on the corresponding errors)

The first and foremost requirement is:

1 – The two optical axes should be coincident, forming one common axis.

To simplify the error analysis, this can be broken down into two parts, giving separate kinds of error if violated:

1A – The optical axes should intersect at the common point of focus.

1B – The optical axes should be parallel.

When these requirements are met, and we can consider one common optical axis, the following supplementary requirements should also be met:

2 – The optical axis should strike the optical center of the secondary mirror.

3 – The optical axis should be deflected 90 degrees by the secondary mirror

4 – The optical axis (between the primary and secondary mirrors) should be centered in the tube.

A Newtonian telescope can be collimated to meet each of these requirements more or less closely, but as with mechanical adjustments in general, they can not and need not be met exactly.

If we understand the effects of the separate errors, we can decide on the maximum error tolerances. We can then be sure that the telescope will perform as well as it should, if the collimation is done to within these tolerances. Here is a discussion of the effects of the errors:

Error type 1A – The optical axes are separated at the focal plane

The eyepiece focus and the primary mirror’s focus lie separated in a common focal plane.

This is the crucial error for visual use. Images by the paraboloid mirrors of Newtonians can be close to perfect near the focal point, but suffer from increasingly severe coma at increasing distances from it. Coma is an optical aberration that causes loss of contrast and detail resolution. It is approximately proportional to the distance from focus, and inversely proportional to the third power of the focal ratio f (this is the focal length of the primary mirror divided by its diameter).

Any good eyepiece gives a very sharp view near its focus – that is, in the center of the field of view. Towards the edge, however, all eyepieces cause more or less unsharpness of star images. This is mainly due to astigmatism (I won’t explain that here) of the eyepiece – it is not the mirror’s fault – but shows up worse with a short focus mirror (with a low focal ratio). For most eyepieces, the coma of the primary mirror gives much less contribution to the unsharpness.

If the focus of the primary mirror is in the focal plane away from the eyepiece focus, there will be some coma at the center of the field of view, where the image should be sharpest, and the image is not quite as clear and crisp as it could have been – particularly when you use high magnification to catch the subtle details of planet surfaces.

The “sweet spot” in the focal plane around the optical axis, where coma has limited effect even at high magnifications, could be surprisingly small. Also surprisingly, a large telescope mirror does not have a larger “sweet spot” than a small mirror – the diameter is a only a function of the focal ratio of the primary mirror, not its size. This table gives the diameter of the “sweet spot” where coma is less than 1/14 wavelengths RMS, and the Strehl ratio is lowered by no more than 0.2 (this corresponds roughly to the rather obsolete Rayleigh “quarter-wave” criterion of diffraction limiting).

At the edge of this “sweet spot”, coma may be noticeable in good atmospheric seeing, but within a circle of half this diameter, it would be very difficult ever to detect it (and the requirements for “good” seeing are stricter, the larger the mirror is). The small size of the “sweet spot” is of course one price you must pay for the convenience of a telescope with a large mirror, yet short, manageable tube, but it may not be as serious a drawback as it appears. In a “fast” f/4.5 telescope with high magnification, giving an exit pupil of 0.5 mm (this is 50x per inch of aperture, a reasonable upper limit at least for small telescopes), an eyepiece of “standard” field (some 50 deg apparent) has a true field of approximately 2 mm, the same size as the “sweet spot”. But obviously, with short focus telescopes (low f/ numbers), centering the “sweet spot” within the eyepiece field of view is very much more critical than with the smaller, “slower” telescopes that were common before the “Dobsonian revolution”. The “eyeballing” collimation, even today often found in manuals, may be good enough for a 6 in. f/8 telescope and yet fail badly when applied to a modern, large and fast instrument.

To decide on a reasonable tolerance for the 1A error, 1/4 of the diameter (half the radius) for your f/ratio could be a reasonable goal – for a small, dedicated planetary scope, you may want a little closer tolerance such as 1/6, but for a large, fast Dob where the seeing will usually limit the resolution, you could allow 1/3 or even 1/2 of the diameter. See later how you can use the Cheshire (the barlowed laser is similar in this respect) or the laser collimator to decide when you are within your tolerance.

If you have a center spot that happens to be at a distance D from the true optical center, and do a perfect collimation against it, the collimation error 1A at the focal plane is half the distance, or D/2 . Thus, for critical collimation, the allowable miscentering of the center spot should not be more than the diameter of the “sweet spot”, and preferrably less than half the diameter.

Error type 1B – the optical axes are not parallel, but form an angle.

This type of error means that the focal plane of the eyepiece, or the plane of a film or electronic detector, will be tilted relative to the focal plane of the telescope.

Let us assume that collimation of the primary is perfect at the center of the field of view, but the focuser’s axis misses the center of the primary by a distance b. This means an angle of tilt = b/F where F is the focal length (in radians – multiply by 57.3 to convert to degrees). At a point a distance m from the center of the field of view, the defocusing distance d between the planes is dm/F. The P-V defocus error in wavelengths is (Suiter): dm /(8Ff 2 *λ) where λ is the reference wavelength of 550 nm – to obtain the RMS error, this is further divided by √12. If this error is no more than 1/3 of the unavoidable coma, it means it will not contribute more than 10% to the total wavefront deviation – even disregarding other contributing aberreations such as curvature of the focal plane of the primary but also of the eyepiece, as well as off-axis astigmatism of the eyepiece itself. Coma will give an RMS aberration of 6.7m/f³, this leads to the tolerance d=0.034D where D is the mirror diameter, (rather surprisingly, the focal ratio and focal length are eliminated from this expression!) or about 1/30 of the mirror diameter. This should not be difficult to meet.

However, to reduce coma in large, fast telescopes, a coma corrector such as the Paracorr (by TeleVue) is often used. It will reduce coma by a factor of about 6. In this case, the tolerance should be 1/180 of the mirror diameter. This can be met with a laser or an autocollimator, but may be difficult with just a sight tube.

An error of type 1B may cause an error of type 1A, if the collimation tool is used far from the focal plane, making the optical axes intersect at a point far from the focus. If the tolerances above are met, this should not be a significant problem.

In this estimate of tolerances, the aberrations of the eyepiece itself have been ignored, although they can be the dominant aberrations visually. Any eyepiece will have a spherical aberration that increases rapidly with decreasing f/ ratio. If this spherical aberration is strong, a tilt of the eyepiece may cause astigmatism – this is not readily calculated, but I believe the effect is insignificant if the tolerances above are met.

It has been suggested that an eyepiece in a Barlow lens, or an eyepiece with a similar negative first element built in, may be more sensitive to an error type 1B. This is not so – if the optical axis was centered in the focal plane without the Barlow, it will also be centered in the new focal plane of the Barlow, even if the combination is slightly tilted.

Error type 2 – the optical axis strikes the secondary mirror at a point away from the optical center.

The secondary mirror has an elliptical surface with a major to minor axis ratio equal to the square root of 2, for 90 degree deflection. Depending on its size, it lets some of the focal plane be fully illuminated, that is any point within the area of full illumination sees the whole primary mirror reflected in the secondary. Outside of this, some light is lost.

Due to the 45 degree tilt, the elliptic surface appears circular when you see it with your eye centered on the optical axis near the focus. However, due to the perspective, the center of the circle you see is offset from the geometric center of the ellipse, towards the edge nearest to the focuser. To be optically centered, the secondary mirror must be offset both in the direction away from the focuser and towards the primary mirror. The offset in each direction can be calculated with very complex formulae, but the formula offset=minor axis/(4*focal ratio) is accurate enough for practical purposes (it is exact if just the center is fully illuminated – with a larger fully illuminated field, the error is insignificant anyway). The distance along the mirror face from the center of the ellipse to the optical center is the offset multiplied by 1.414 (the square root of 2).

Example: with a 33 mm secondary mirror (the size refers to the minor axis) in a f/6 Newtonian, the offset is 33/(4*6) mm = 1.3 mm.

An error of type 2 causes the fully illuminated field to be offset relative to focus, and will cause an uneven light loss near the edge of the low power field. For wide-field photo, the secondary mirror should be large enough to let the whole film frame be fully illuminated, but for visual use, a secondary size of no more than 20-25 % of the primary mirror diameter is commonly preferred, in order to minimize unwanted diffraction effects. This means there is usually some light loss by the edge of the field, but at least the focus should always be fully illuminated – the tolerance should not be larger than the radius of the fully illuminated field. At least for short focus instruments, light loss is very gradual outside the fully illuminated field, and an offset of a few millimeters should have little effect visually. Sufficient accuracy is easily achieved with suitable tools.

To calculate the secondary size or the size of the fully illuminated field: let D be the diameter of the primary mirror, d the diameter (minor axis) of the secondary, F the focal length, b the distance from the optical center of the secondary to the focus, and x the diameter of the fully illuminated field: x = (Fd-Db)/(F-b) or d = x + b(D-x)/F

Error type 3 – the common optical axis is not reflected at 90 degrees.

Standard secondary mirrors and holders are designed for 90 degree reflection, and seen from the focus the elliptical mirror appears circular. An angle of more or less than 90 degrees will make it appear slightly elliptic – and the fully illuminated field will also be somewhat elliptic. If you have collimated, but the holder is not parallel to the optical axis, the reflection of the secondary holder may look visibly skewed. If so, you should consider shimming or otherwise “squaring” the focuser. An error of type 3 will have no other effects on the image (contrary to common belief).

Error type 4 – the common optical axis is not centered in the tube.

If the axis is grossly decentered, the tube opening may cause some (very mild) vignetting, and this should of course be avoided if possible. Otherwise, it will have no optical effect. It may cause problems with some mounts, as an offset axis will not trace a great circle when the tube is moved in declination. This might introduce some error when using digital setting circles. For exact centering of the optical axis, the secondary mirror must be correctly offset, and the primary mirror must also be accurately centered.

End of heavy theory – at least most of it.

So what steps do I take to collimate my telescope?

The lining up of the optical components should be done in a sequence that is as simple and ordered as possible. Ideally, you would start at one end of the optical chain and then proceed in steps to the other, without going back to readjust what once was adjusted. With real telescopes this is not possible, the adjustments affect each other in different ways depending on design details. For instance, with common secondary supports, it is not possible to adjust the tilt without moving the optical center significantly.

One practical way is to do it in the order described below (you could perhaps do it in the opposite order, but I believe it is much more complicated). Remember that this refers to a full collimation, like when you assemble the telescope from parts – you do not have to go through all of this just to get your telescope ready for the night’s observing! With a truss tube that you assemble at the observing site, you should do step 4, else step 5 (and maybe step 8) is usually quite sufficient for this.

The tools will be described in a later section, with details of their use. The error numbers are explained in the section on theory that you may have skipped.

In the first three steps, you place the focuser and secondary within the upper end of the tube.You can use a simple or combination sight tube, as described below. You may also use a crosshairs sight tube or a laser collimator.

1 – Square the focuser

If the focuser appears to be squarely mounted on the tube, it is not likely to be badly off – however, if you find it impossible to perform step 3, this could be the reason. You can make a small mark directly opposite the focuser hole. With the secondary mirror out of place, use a sighting device in the focuser. Shim the focuser to center the mark. A piece of tubing that fits your focuser, long enough to reach across the tube, will make it even easier, and so will a laser collimator or a crosshairs sight tube. (This minimizes the error type 3)

2 – Center the secondary mirror within the tube

You should check the centering of the secondary mirror side-to-side as seen from the focuser – if it is off by more than you think it ought to be (a few mm perhaps), adjust it and go back to step1 and shim the focuser as needed. If you like, and if it is simple to do with the spider you have, you can also offset it from the center of the tube, in the direction away from the focuser. Calculate the offset, and use a ruler, or else wait and adjust (and re-collimate) after step 6 is done. If you cannot offset the secondary, e.g. because of the spider design, you may leave it centered in this direction, too, without serious consequences. (This minimizes the error type 4)

3 – Center the secondary mirror along the tube

To center the fully illuminated field of view, the secondary mirror should be offset towards the primary mirror, as seen from the center of outer rays (this is the point where the primary mirror appears to exactly fill the face of the secondary). If you center it as seen in a sight tube, it will automatically be correctly offset towards the primary. A holographic laser collimator with a wide enough pattern could also be used.(For fully offset collimation, the secondary should also be offset away from the focuser as described in step 2.) If you want non-offset collimation, you could put a small spot at the geometric center of the secondary and center it on the laser beam.

If you find that the secondary is offset “sideways”, away from the tube axis despite your efforts so far, you may have to go back to either step 1 and shim the focuser sideways or to step 2 and adjust the spider setscrews.

(This minimizes the error type 2)

4 – Tilt the secondary mirror to make the extended optical axis hit the center of the primary mirror .

Use the appropriate setscrews on the secondary mount – depending on the design, you may also rotate the secondary holder to center “sideways”. You can use a single or double crosshairs sight tube, by centering the spot on the crosshairs. You could also use a simple or combination sight tube, by centering the primary mirror within the sight tube end (if you don’t have a center spot, this is one way, another is with a holographic laser). Perhaps the most convenient tool is the laser collimator, making the laser beam hit the center spot. If you plan to use the laser also in step 5, it is imperative that the centering is veryaccurate, else you only need to make sure the error is no more than perhaps 1/300 of the focal length, or 1-2 percent of the diameter of the primary.

If you can rotate the secondary, you could get it skewed by tilting it one way, and rotating it the other. If you see the secondary or its reflection looking skewed, try straightening it up, then rotate it to get the primary mirror roughly in line. Then start over with this step, but do not rotate.

If you have made major adjustments, go back to step 3 (and possibly step 2) and check that the adjustments still are OK, or adjust if needed.

If the primary mirror is badly out of adjustment, part of its edge may appear obscured by the tube opening. If this makes centering difficult, go forward to step 5, and make a coarse adjustment before going back to step 4 again. (This minimizes the error type 1B.)

Going forward from here, do not skip step 5!

5 – Tilt the primary mirror, to center its optical axis in the focuser.

If you have a mirror cell that holds the mirror very loosely (this is particularly common in Dobsonians), you may make the mirror settle by tilting the tube nearly horizontally, and then raise it, before you go on.

Here you use the set screws to adjust its tilt (use 2 to adjust, and leave the 3rd – else you may find, after some time, that all are near the end of their range), and thus the tilt of its optical axis. You could use a Cheshire or a combination tube, by centering the primary mirror spot in the bright spot of the Cheshire (if you use the calibrated Cheshire, you will know that the error type 1A is within tolerances, when the black spot is surrounded by an unbroken ring of light). You may even use a peephole with a semi-transparent lid as a primitive Cheshire, if you illuminate it from the outside. To minimize any error from a possible miscollimation in step 4, do not place the Cheshire far from the focal plane – near the edge of the focuser drawtube at its usual position.

If you can reach to adjust the collimating screws while looking into the Cheshire, this simplifies things enormously! I have built my own telescopes so that I can, but with most commercial telescopes, this is not possible. The next best thing is an assistant to turn each screw in each direction while you note the effects. A simple trick is to put a sticker near the focuser, where you draw two arrows to mark the direction that the spot appears to move when you turn each screw inwards – this way, it is easy to decide what screw to turn in what direction.

You can use a laser collimator (and a perforated center spot) to get close, but as the precision is entirely dependent on the accurate centering of the spot in step 4, it would be wise to check – and if necessary, fine tune – with a Cheshire, unless you are quite confident this was done very accurately. Another way to get high precision is using a combination of laser collimator and Barlow lens.

If you have no center spot, you can use a double crosshairs tube – see here how.

I like a fairly large spot, not very much smaller than the bright face of the Cheshire. What I see is a thin ring of light, and I can readily detect even a small asymmetry. Others like to align against the center hole of the Cheshire – for instance by using a square mirror spot, its side a little smaller than the opening, its corners protruding outside it. A donut-shaped ring may be fine if its inner diameter is a bit larger than the opening in the center of the Cheshire – any way is fine as long as you can match the positions accurately.

(This step minimizes the critical error 1A)

6 – Check the centering of the optical axis in the telescope tube and in the focuser drawtube.

A coarse test is to look through the empty focuser tube and check if you can see the outer tube end reflected in the primary mirror from any point within the focuser. If you can’t, the centering is OK optically.

If you have reason to do better, you can make a centering mask, and check with a peephole (or Cheshire or sight tube) whether it is well centered relative to the primary mirror. If you need to adjust, move the spider the required amount away from the visible part of the centering mask (put your finger inside the tube opening to see which direction it is), and start over from step 3. (This checks the error type 4)

7 – Star test

The whole purpose of collimating is to get the best images of stars and other celestial objects. You may want to check the collimation by looking at a star – use a magnification of 1-2x per mm of aperture (25-50x per inch). Do not trust this step unless the seeing is good enough to clearly discern the diffraction rings.

Center a star in your field of view (the centering is important! You may use Polaris if you live far enough North and have a telescope with no tracking facilities). Gently rack the focuser from one side of focus, passing the focus, going to the other side.

The Orion SkyQuest Dobsonian line has been one of the best Dobsonians for beginners for many years. While it may not offer the most bells and whistles, and it may not be the least expensive, and it may not come with the best accessories, it is a serious workhorse of a telescope and is my choice for the best Dobsonian for most beginners.

What the Orion SkyQyest has in spades is reliability. This scope has been produced in the same configuration with only minor changes as they found and fixed things for years. No other dob in this roundup has been beaten on for this long by this many people, and that means something that always works. That is how they made one of the best Dobsonian telescopes ever made.

I know people who had one of these for years, then sold it to someone else who used it for years. Go talk to the people who sold theirs and they will tell you it was one of the best Dobsonians they have ever used, always ready, always capable.

Orion also has excellent technical support and parts availability. The one downside to this is that they don’t want to sell parts to someone who is not the original purchaser. While this stinks, it also isn’t really that much of a problem as there are plenty of after-market people who can supply virtually anything you may need. Besides, this is a simple scope so there just are not too many complex pieces. For some of the best Dobsonian parts and upgrades out there, check out www.scopestuff.com

Best for beginners who need portability: Sky-Watcher 8″

When you need the best Dobsonian that will fit in the back of your VW Beetle convertible, this is the telescope you want and is the best Dobsonian for portability and storage. Not only does the tube come off the mount, but it then collapses down making it even more portable. It is also the best Dobsonian to fit in the bottom of virtually any closet making it perfect for someone who lives in an apartment or just does not have much storage space.

The Sky-Watcher is a bit more expensive than the Orion xt8 but of course, it has the ability to compress down. It also has a much nicer finder and a tension clutch on the bearings, which the xt8 lacks. Overall this scope will feel nicer and has a much more refined quality to it. If you want something sensible in size but that also feels like the best Dobsonian, this is it.

The only downside is that since it is not a solid tube design, you may need to collimate it more often. It, unfortunately, does not come with a laser collimator but that is something that is easy to fix. My favorite, and what I consider the best Dobsonian laser collimator, is the Astromania Alignment Next Generation Laser Collimator which is around $25.

Best for more advanced users: Orion 12″ XT12i

Almost everyone who buys a Dobsonian as a beginner gets one without a computer. That’s fine for general observing for newcomers to the hobby. Once you have been doing this a while and want to find more and more difficult targets you may need two things a larger aperture telescope and a computer. This is the best Dobsonian for solving those problems.

The Orion Intelliscope push to system is the best Dobsonian electronics package and bridges the gap between full manual with no computer to a full go-to system. It tells you where to push and then confirms you are on the target once you get there. Not only does that make finding objects easier, but substantially faster. Besides, one of the things that makes the best Dobsonian for you is if you will use it. Being able to find targets quickly absolutely makes some people more likely to use their telescope and therefore, makes it the best Dobsonian for them.

Another excellent use for the computer on a dob is for outreach. Set it up, align it, and take requests on what people want to see. With a 12″ aperture there is virtually nothing you can’t see, assuming it is up. This is probably the absolute best Dobsonian for outreach.

This is much like the xt8 as far as reliability and simplicity is concerned. Orion also added a much nicer finder and improved substantially on the base. Overall this the best Dobsonian for any user if it is in your budget.

Best for advanced users who need portability: Orion 12″ XX12i

This little guy is on the best Dobsonian list because it will allow you all the capabilities of the standard 12″ Intelliscope Dobsonian telescope with the added benefit of being far easier to transport, even in a small car. It also has an upgraded focuser but not quite as nice a finder when compared to the standard 12″ Intelliscope.

To really make the portability work, Orion even makes a set of padded cases, the Orion 15094 Case Set, that each piece goes into. While there are other cases for telescopes, and some for dobs, these Orion cases are the best Dobsonian cases out there. This makes it one of the best Dobsonians to carry out to the dark site.

The base on this and the standard Intelliscope are the same, making the tube the only difference. The components of the truss tube variant seem to be made of heavier gauge metal making it not only stiffer, but a lot nicer to work with. Everything seems amazingly stable.

As with most truss tube setups, you will want to make sure you get the shroud that fits it and Orion makes a specific 15097 Light Shroud for this model and it really is the best Dobsinain shroud you can get for it.

If you need portability, a reasonable price, push to capabilities, and excellent views, this is the best Dobsonian for you.

Best top of the line mass-produced: Orion 14″ XX14g

Most of the portability of the 12″ truss tube with more seeing power and a full go to computer system, this guy will provide amazing images of any target you choose to point it at and still not give you a hernia trying to get it out to the field.

If you want the best views you can get while still staying portable in a mass-produced telescope, this is the best Dobsonian for you.

One of the advantages of the 14″ over the 12″ is that the base also collapses. This makes it as easy to transport than the 12″ model, just a little different.

This telescope also has the full goto controller that they use with their EQ mounts. In my opinion, this is the absolute best Dobsonian GOTO package available from any manufacturer.

Off and on Orion, as well as other manufacturers, have produced larger models than this 14″. They did not make this list because they are not always in production or lack some of the features of the XX14g. They also tend to get exponentially harder to transport once you get over this 14″ model. Unless you drive a full-sized SUV or don’t mind putting a telescope in the back of a pickup, this is about as large as you want to go.


Collimation Tools: What You Need and What You Don’t

Aligning the optics of your reflector telescope is crucial for optimal performance — all the more so if you have a telescope with a focal ratio of f/5 or less. A good tool can make the difference between successful collimation, and an exercise in frustration that encourages you to settle for “good enough.” But selecting the right tool can be more confusing than actually using it. On-line discussions offer a bewildering array of opinions and experiences — some of which posted by people who make and sell the products they (naturally enough) recommend. So what do you really need to collimate your scope?

Here is a rundown of the various collimation tools commonly available, and their relative strengths and weaknesses. My evaluations are based on several decades of making and using reflector telescopes. All the devices discussed below can produce satisfactory collimation. What generally distinguishes one from another is not accuracy, but rather, ease of use and cost.

Option #1: No Tools

Yes, it is possible to collimate your reflector without any tools. It’s called the “star test.” The detailed ins and outs of this method are beyond the scope of this article, but essentially you centre a bright star in the eyepiece, throw it out of focus, and note where the shadow of the secondary mirror is positioned within the expanded disk of light. It should be centred. The test becomes progressively more sensitive the nearer you get to focus. Regardless of what other collimation method you use, the star test is the final arbiter of optical alignment. If it looks right in the star test, it is right.

Best features: You can do it without spending a single dollar. No centre dot is needed on the primary mirror.
Worst features: The method takes some experience and isn’t the best choice for absolute beginners. It’s also usually more time consuming than other methods and requires a star (or point-source light). It’s also not the best way to ensure the secondary mirror is correctly placed.
Accuracy: Dead accurate.
Ease of use: For the highest accuracy you’ll need a night of good, steady seeing. Experience will make the method more reliable and effective.

Option #2: Collimation Cap

A simple, inexpensive collimation cap.

Possibly your telescope came with one of these. Orion Telescopes supplies them with their reflectors, as do some other manufacturers. The device is simply a plastic cap with a small hole in its centre and a reflective underside. If your telescope didn’t come with one, you can make one with an old plastic film canister. For 90% of the collimation I do, this is the tool that I use. The only time I usually need something more is when I’m assembling a scope from scratch.

Best features: Cheap and effective.
Worst features: Not the best tool for aligning the secondary mirror (though it can be done). Requires the centre of the primary mirror to be marked.
Accuracy: Very accurate if your mirror’s centre dot is correctly positioned.
Ease of Use: Very easy to use.

Option #3: Cheshire Eyepiece

This combination tool from Orion is a Cheshire eyepiece and sight tube in one.

Not an “eyepiece” in the usual sense of the word, a Cheshire is a sight tube with a small hole at the top that you look through, and a shiny surface tilted at 45° and aimed at a large hole in the side of the tube. The version Orion (and others) sell also has a set of cross-hairs at the bottom of the tube for aligning the secondary mirror. This “all-in-one” collimation tool is excellent. Indeed, if you have one of these, you need nothing else.

Best features: One tool that does it all. Relatively inexpensive.
Worst features: In the dark you’ll probably need a red flashlight to illuminate the shiny surface of the collimation eyepiece. Requires a centre-dotted primary mirror.
Accuracy: Very accurate if your mirror’s centre dot is correctly positioned.
Ease of use: Easy to use.

Option #4: Laser Collimator

A laser collimator tool for 1¼” focusers.

Laser collimators have been around for many years now and seem to be especially attractive to those who equate lasers with precision. Unfortunately, it’s been my experience that beginners all too often end up de-collimating their scopes when using one of these. Why? The Achilles heel of the laser collimator is that its accuracy depends on how carefully you’ve adjusted your scope’s secondary mirror — a procedure that is far more difficult than it is important to image quality. In other words, if your scope’s secondary mirror isn’t set correctly, you can actually achieve a “pass” by putting your primary mirror out of alignment — a situation that can have disastrous consequences when it comes to image quality. That said, I have a laser collimator and find it a useful tool for adjusting the tilt of the secondary mirror. I don’t recommend it for adjusting the primary, however.

Best features: Can be used in the dark. Useful for adjusting the secondary mirror.
Worst features: Can lead to miscollimation. Batteries required. Expensive relative to benefits. Requires centre of primary to be marked.
Accuracy: Potentially accurate if used correctly. Accuracy dependent on mechanical alignment of the laser within its housing and how the device seats in the focuser. Accuracy highly dependent on positioning of the secondary mirror.
Ease of use: Relatively difficult to use successfully.

Option #5: Barlowed Laser

These views show the target for a Barlowed laser setup. The images show the telescope nearly collimated (top) and fully collimated (bottom).

The Barlowed laser is the newest approach in the collimation game. Most people heard about it the first time when Nils Olof Carlin’s article appeared in the January 2003 issue of Sky & Telescope (page 121). As editor of the telescope-making department, I had the privilege of working with Nils to bring this to the pages of the magazine. Essentially the setup consists of an ordinary laser collimator used in conjunction with a Barlow fitted with a target attached in front of the lens. You can also purchase Barlowed lasers from commercial sources such as Howie Glatter and Kendrick Astro Equipment. Unlike a plain laser, the Barlowed version works very well and avoids the pitfalls of the former. This is my favourite method for collimating in the dark.

Best feature: Works well in the dark.
Worst features: Can be relatively expensive. Requires the centre of the primary mirror be marked.
Accuracy. Very accurate.
Ease of use: Very easy.

Recommendations

The five options described above cover those most commonly available and frequently used. With varying ease, all of them can help you accurately collimate scopes — even those with fast (under f/5) focal ratios. There are other tools and systems, but mostly they are either variations of those covered here, or devices that increase the complexity of the operation without a corresponding improvement in accuracy.

For most people, a simple collimation cap is fine. The Barlowed laser is also a good option, especially if you already have a Barlow lens in your eyepiece box. If you do most of your collimation in the dark when you arrive at an observing site, this is the way to go. Nearly as convenient and useful is the Cheshire eyepiece. The important thing to remember is that you don’t have to get a bunch of tools — one chosen with care is all you need. Take the time to learn how to use it well and you won’t need another.

I’m purely a visual observer and mostly use scopes that are shade over f/4. For collimation I use either a Cheshire or laser to position the secondary mirror (something that rarely needs adjustment) and a simple collimation cap for tweaking the primary. That’s it. My scopes are always perfectly aligned, something I can quickly verify with a star test. Collimation rarely takes me more than a minute and most nights all I do is check to see that everything is okay since I last used my scope. There’s really no reason to spend any more time on it than that.

If you want to read more about collimation, I can recommend Nils Olof Carlin’s excellent piece, Some Collimation Myths and Misunderstandings That article should fill in most of the gaps arising from the brevity of this overview.


114 Newtonian or 6" dobsonian?

  • topic starter

This is my first post ever on the net so here goes! We are trying to find a scope to buy my father for Christmas. He is not new to astronomy but hasn't owned a scope before. We are in Wellington, NZ and the selection of scopes isn't that great here. I have done quite a bit of research over the net but the exercise of choosing one of many scopes is rather dauting! Due to price limitations and what is available here (too expensive to import) we have got it down to the Konus or AstroNZ (Auckland Astronomical Society retail arm) 114 Newtonian or the AstroNZ 6" or 8" dobsonian. Can anyone advise on whuch would be best for a beginner? Someone told me that the dobs are not that easy to track with etc and the Konusmotor 114 comes with a tracking motor. Also he will want to be able to take photos and we want something that will be good for planets and deep sky viewing. We can only afford around $650NZ (I think this is about $350US). Would a refractor be better? Any advice would be greatly appreciated ASAP as we don't have much time left to buy something before Christmas.

#2 imjeffp

2. Me? I'd look for a 6" newtonian on a GEM, like this one. The extra aperture vs. the 114 is a good thing, and the GEM with the addition of a drive motor will track the sky for you. And if you ignored #1, you could use a webcam with acceptable results on the GEM.

#3 kiwisailor

Take a look on Trademe, there have been a couple of 6" & 8" dobs for sale lately- stay away from the short tube newts (one currently for sale). The small EQ mounts are pretty shakey, the 8" dob would be the best for viewing, your dad could take some snaps of the moon, but for serious photography, you are talking serious money.

#4 Guest_**DONOTDELETE**_*

  • topic starter

#5 Scott Beith

First thing: Welcome to Cloudy Nights .

An 8" Dob will show you so much more than the 4.5" that you would be amazed. Forget astrophotography for a while - a minimal setup will cost big $$. Even though I use refractors, I would suggest an 8" Dob in a heartbeat for a first scope. It will bring enjoyment for years.
If you chose an EQ mounted scope - go for a 6" f/5.

To all of you who know me - yes that hurt to say it.

#6 kiwisailor

The 8" with the laser collimator would be a good call, make sure your dad signs up to Cloudy Nights and he'll get plenty of bang for the bucks

#7 dgs©

¼°per minute, and the typical high power (250×) eypiece will have a Field Of View around ½°. You don't really want to watch a planet from edge to edge, as typically the best quality image will be away from the edges. Kind of a long winded way of explaining the benefits of tracking with an Equatorial mount.

Also in favor of the Dobsonian. He could use the Dob for a couple of years and save up (or you could) for an Equatorial mount to put the same Newtonian on. Makes your Christmas shopping easier in the next year or two.

#8 Guest_**DONOTDELETE**_*

  • topic starter

#9 Guest_**DONOTDELETE**_*

  • topic starter

#10 chazcheese

#11 Scott Beith

Most people find tracking a Dob easy.

I unfortunately am not one of them - 99% have no problem.

I am also a Slobbering Refractor Freak, so the advice I give will be adjusted to the needs of your Father - not necessarily my needs.

Aperture and ease of use is a great combo for a beginner - and the 8" Dob offers both.

#12 Mitrovarr

The general advice given is to get the largest scope you can afford and can carry, and there's a good reason for it. A 8" scope is noticably better than a 6", and a 6" is noticably better than a 4.5". A 8" scope just blows away a 4.5" scope, there's no comparison.

The aperture difference matters most with regard to diffuse and planetary nebulae, galaxies, and globular clusters. Planets will look a lot better in a larger scope, but only when conditions are right. Open clusters generally look good in anything.

The difference really is impressive, an 8" telescope can get partial or total resolution on about half the messier globulars, a 6" can only get resolution on some of the brighter and looser ones, and a 4.5" is hard pressed to resolve any but the brightest and easiest (you guys are lucky down there, with 47 Tucanae and Omega Centauri.) The 8" will show all the messier planetary nebulae quite well and many others as well, the 6" will show the messiers (some faintly) and a few others, and the 4.5" will only bring in the showpieces, and then dimly.

With regard to astrophotography, I wouldn't let it influence your decision. There isn't anything that $350 can buy that is suitable for long-exposure through-the-scope astrophotography. Any telescope, even a dobsonian, can be used for eyepiece projection photography of the planets. The only kind of photography a telescope like the Konusmotor can do that a dobsonian can't is piggyback astrophotography, where the camera is mounted on the back of the telescope for a long-exposure wide-field images of the sky.

Tracking with a dobsonian is not really a big deal, except at very high power when looking at the planets. Even then, you have to consider that the dobsonians can handle much higher power, because of the aperture. Plus, a new user will have to figure how to align the equatorial mount, which is kind of a pain. Cheap equatorian mounts also usually aren't as stable as cheap dobsonians. The only time I really miss tracking is when I'm observing in a group and other people are looking through the scope.


Cheshire eyepiece vs. laser collimator?

Both are good, but both take some practice and understanding of each's limitations. You can get very close with a Cheshire eyepiece, and the price is lower. I also think a Cheshire works better for the alignment of the secondary. At least it's less prone to errors caused by miscollimation of the tool itself. For the alignment of the primary, a Barlowed laser makes the job pretty easy.

#3 lamplight

#4 obin robinson

I have both. I use both. I should say though I use the laser MUCH more often.

#5 csrlice12

#6 *skyguy*

#7 csrlice12

Not really, I just turned 60 and use the Glatter exclusively. Did a lot of research first though buy once, spend once. but the Cheshire will give you just as good results for a lot less price. It's just a bit more involved to use, but does a fantastic job.

Remember, us old farts are from the "Star Wars/Star Trek" Generation. Lasers are cool! Those two guys using their green lasers as light sabers and making deep breathing sounds. look at their ages next time.

#8 Feidb

#9 bigstormgirl

If you're over 50 . a Cheshire. If your under 50 . a laser. It's a generation thing!

#10 FoggyEyes

Cheshire: When you initially put the scope together or major problem like dropping the OTA and it rolled down the hill. This gets the secondary dead on and everything well lined up.
Laser: All other times for fine tuning the collimation, you can also point out the OTA rolling down the hill.

#11 csrlice12

#12 SkyGibbon

#13 Muleya

#14 Tim D

#15 Jarrod

Both. Cheshire for getting the secondary centered in the tube and for getting the initial secondary tilt. Then the laser for fine-tuning the secondary. Then add a barlow to the laser to do the primary. I then like to check a final time with the cheshire to get visual confirmation that everything looks perfectly aligned because if there is an issue with laser centering/parallelization, it should show up here.

A good laser is more accurate, and is quicker and easier to use. The cheshire is more foolproof.

#16 CeleNoptic

#17 DaveG

#18 Diana N

Well, not clear why to use both? If your laser collimator holds its alignment well and it's more convenient than why to bother with the Cheshire?

#19 Diana N

How about a $3 plastic cap with a reflective inside?

A reflective interior? My, my aren't we getting fancy .

(Says another old fart who's a graduate from the Kodak Film Can School of Collimation.)

#20 CeleNoptic

I use both, for the reasons already stated.

Stated where? Here I can see mostly statements like the Cheshire for the secondary, the laser for the primary without clear reasoning.

Because the first step in collimation is getting the secondary holder properly positioned under the focuser, and the laser collimator can't help with that.

#21 FoggyEyes

#22 CeleNoptic

#23 Vic Menard

Exactly! That was one of the first videos I downloaded after I got my Dob a year and a half ago. I'm puzzled why laser collimator can't help with the positioning of the secondary mirror . I've been using laser collimator only .

"Positioning" or placement of the secondary mirror is accomplished by aligning three circles:
the bottom edge of the focuser or sight tube,
the actual edge of the secondary mirror, and
the reflected edge of the primary mirror.

When the secondary mirror tilt is adjusted to aim the laser beam at the primary mirror center spot, the focuser axis is being collimated. Adjusting the primary mirror to cause the beam to return on itself back to the laser emitter provides a coarse primary mirror axial alignment.

Secondary mirror placement is usually assessed before the focuser axial alignment. The two alignments interact and change each other, so to achieve optimal alignment, the two alignments are repeated systematically reducing residual errors of both.

If the secondary mirror placement isn't assessed as part of the axial alignment procedures, it's quite possible that a significant secondary mirror placement error can be propagated over time, potentially impacting image performance.

#24 CeleNoptic

Secondary mirror placement is usually assessed before the focuser axial alignment. The two alignments interact and change each other, so to achieve optimal alignment, the two alignments are repeated systematically reducing residual errors of both.

Vic, thanks for the detailed explanation. Now it's clear what is the duty of the cheshire .

( after checking that the laser beam is not exiting out the front)

#25 tezster

I have a potentially silly question - everyone always warns about a laser collimator potentially being miscollimated itself, but I've never heard of the same thing said about passive tools i.e. Cheshire/sight tubes.

Does this mean that all cheshire and/or sight tubes, no matter what they cost or who produces them, are 100% accurate? Is there something inherent in their design and production that makes them so easy to make?