Collimating a Newtonian Telescope

Nils Olof Carlin (last updated March 2005, with a revised comment on the autocollimator, and June 2005, with a revised section on the 1B error).

"...bad collimation is the number one killer of telescopes world wide..." Walter Scott Houston

Well, maybe the questions here are not as frequently asked as they ought to be. After all, a lot of amateur astronomers own and/or use Newtonian telescopes. If you are one of us, you should ask these questions, and find out for yourself how important the answers are to the performance of your instrument.

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!

Dont 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 bought a factory collimated telescope, do I have to bother with 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, chance 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 doesnt 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 isnt 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.

Dont 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 with the manual regarding 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, dont 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.

It's going to be a great night for observing, and I've got all the tools. I'm in a hurry, so skip the details - what do I do?

I've prepared a page that you can download and print out, but unless you have tried at least once, you should take the time you need to follow the detailed instructions and get truly familiar with the procedure.

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 optical parts are:

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 wont explain that here) of the eyepiece - it is not the mirrors 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).

Focal ratio Diameter mmf/4 1.4

f/4.5 2.0 f/5 2.8 f/6 4.8 f/8 11 f/10 22

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.

T o 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 /(8Ff2*) where 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 aberrations 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/f, 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 de-centered, 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 nights 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 acrosshairs 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 wantnon-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 doublecrosshairs 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 dont 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 very accurate, 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 cant, 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.

These simulated images show (top row, left to right) the star images in perfect seeing:

at the center at half the radius of the sweet spot, the full radius 2.5 times the radius.

The lower row shows defocused images of the same. The star test is most sensitive near focus. (Simulation program: Aberrator by Cor Berrevoets. There is some spherical aberration as well as a secondary obstruction).

If you can work the collimating screws while looking in the eyepiece, fine - else find an assistant to work the screws by your commands. Doing it all alone can be frustrating.

If there is a marked asymmetry, try and see if the image improves if you place it off center in any direction. If so, try to tweak the primary mirror to move the image towards the center of the field, in steps until it looks symmetric ( If you start with a nearly perfect collimation, it will save you a lot of tweaking).

It is possible (I don't know how likely) that the optical center of the primary mirror is not at its mechanical center. If you star collimate and afterwards find that the center spot is consistently offset in the Cheshire, this is no doubt the reason. If so, first star collimate, then move the spot to appear centered in the Cheshire (or at least remember how far it is off, and in what direction). Once this is done, the Cheshire should give consistently good collimation.

8 - Check the finder and adjust if necessary

If you have tweaked the collimation, you may have shifted the optical axis slightly, and the finder is no longer accurately adjusted.

If your telescope is new, or if you have moved the spider, you can check that the spider vanes have not been tilted or twisted during collimation. Look down the tube and move your eye to see each vane look its thinnest, and check that you see it centered against the mirror (if the vane is centered - otherwise, check that you see it over the corresponding part of the mirror). If offsetting the secondary mirror makes the vanes slightly out of line, or out of perpendicular, this may make the diffraction spikes look less distinct or pretty, but it does not affect the sharpness or contrast of the image.

You can also check that the collimation holds at elevations from some 20 degrees to zenith (some primary mirror mounts, where the mirror isnt glued or clamped, will not hold the mirror firmly enough to use all the way down to the horizon). If the collimation cannot be kept within tolerances, there is some weakness in the tube. It may be the primary mirror support, but it may also be the spider or the secondary mirror mount that is not rigid enough.

Does this procedure give offset, or non-offset, collimation?

It seems that there are widespread misconceptions about secondary offset, or at least different ideas about what it should mean.

If you dot the geometric center of the secondary mirror face, and center this dot in the focuser (using a crosshairs sight tube or a laser collimator) and also center it inside the tube, you will get a truly non-offset secondary.

This has been recommended, but it will give an error (usually acceptable) of type 2, and I dont see that it would be simpler, or offer any other practical advantage. (If you use a holographic laser, the shadow of the secondary can only be centered in the pattern with non-offset - or non-existent - collimation. Some manufacturers recommend you to check this centering as a last step of collimation, but I believe it is better not to).

If you have optically centered the secondary in a sight tube as described above (step 3), it will be correctly offset along the tube. But what happens if your secondary is centered within the tube, without offset away from the focuser in step 2? This is sometimes called "non-offset collimation", or "neither centered, nor offset, but a compromise" (Menard/DAuria: Perspectives on collimation).

Once the point is set where the focuser axis hits the secondary (be it at the optical or mechanical center, or any other point), the further collimation of the primary mirror ensures that the combined optical axis will return to hit the secondary at that very point again. Thus, if the optical axis hits the optical center of the secondary, but it is the geometric center that is centered within the tube, it means that the optical axis will be offset from the tube axis - an error type 4 (it may also lead to some small error type 3). But since the offset is typically only 0.5-1.5% of the aperture, the error is of little or no significance. Thus, if the design of the spider is such that you cannot easily offset the secondary, you may leave it centered in the tube.

What tools can I use to make the collimation easy and accurate?

Here are some useful tools. You can make some (or all) of them yourself, see the do-it-yourself section (or follow the links) for details. Some of my designs there allow full control of tolerances - you can see immediately whether or not your collimation is within the accepted limits.

Tectron makes a set of 3 tools: crosshairs sight tube, Cheshire and autocollimator. Orion makes a combination tool (Cheshire + crosshairs sight tube). I have no personal experience with these. Several manufacturers make laser collimators. Whether you prefer a laser or a sight tube, a Cheshire can also be helpful.

The primary mirror center spot.

This is a small (but not too small) circle (square or even triangle is OK, too) of tape or paint placed in the center of the mirror - it is invaluable. Please note that it is always in the shadow of the secondary mirror, and does not interfere with the image. Regrettably, manufacturers often omit it, and if your mirror doesnt have one, I recommend you make one.

Hey - it isnt even my own scope and anyway I wouldnt dare take it to pieces and risk putting my fingerprints all over the mirror - isnt there another way?

Yes, I have a solution - not quite as neat as if you have a spot, but it will do the job. Breathe again.

Should I center spot my secondary mirror?

Unlike the spot on the primary mirror, a spot on the secondary could affect the image - and even obscure the primary mirror center spot. I suggest you don't (unless you have a compelling reason).

The peephole

This is a small cap that fits in the focuser, with a small center hole (1-2 mm or so) to look through. It is easy to make one. You can adjust the collimation to make the following appear concentric (from outside in):

The inner end of the focuser drawtube The secondary mirror (not so easy to see, since the outer edge reflects

the black interior of the tube) The primary (main) mirror reflected in the secondary The reflection of the secondary mirror - but note that this image (and only

this) is not concentric. The reflection of the drawtube and peephole cap (if transparent or semi-

transparent - if not, it will be too dark to see). The mirror spot

The eyeball centering (commonly described in handbooks) can be done with a peephole tube, but the precision is low and you have no control of the tolerances (that are more critical for short-focus optics). A semi-transparent peephole cap may be used as a Cheshire substitute (see later) in step 5, and with care and luck, the result may be good enough - and a lot better than if you did not try at all.

It is even possible to do this entirely without tools - you just hold your eye near the focuser drawtube, as well centered as you can. You might line up things more or less, but you are unlikely (more so, the "faster" your telescope is) to get the centering of the spot accurately enough (error 1A), so at least use a peephole when tweaking the primary mirror - or use star collimating to fine tune.

If the secondary mirror is correctly offset towards the primary mirror, it will appear centered when seen from a point near focus - but if you move your eye far away from focus, you will no longer see it centered. If you have the secondary mirror truly non-offset, it will appear to be offset away from the primary mirror, but its reflection is centered instead.

The simple sight tube

This is a piece of tube with a peephole cap. To center the secondary mirror, the length is not very critical. If the length is close to the inner diameter of the tube times the focal ratio of your telescope, you could also center the primary mirror within the inner opening of the tube. Here is how to make one.

You cannot, of course, see the inner opening and the mirror edges perfectly sharp at the same time, but the small peephole will improve your "depth of focus" and keep the unsharpness tolerably low.

To center the secondary mirror (step 3) you can hold a strip of white paper (or a L-shaped piece of cardboard, as shown below) inside the tube, below the secondary mirror (with a truss tube, you can use the lid of the mirror box). You see the whole of the secondary face reflecting the white paper - without it, the outer rim of the secondary will reflect the dark inside of the telescope tube around the primary mirror, and what you see is the edge of the primary, not the secondary. Move the tube in or out (be careful not to hit the secondary with it!) to make the secondary appear barely smaller than the tube opening, and lock it with the focuser locking screw. Move the secondary (try to avoid changing the tilt) to center it accurately - or at least to within acceptable tolerances. To center it "sideways", you may have to move the spider vanes - if this is not possible, shim the focuser sideways instead.

To center the primary mirror (step 4), take away the paper strip/cardboard (or take the lid off the mirror box) and push in the tube far enough to see the primary (main) mirror reflected well within the secondary. Tilt the secondary (try to avoid moving it) to center the primary mirror inside the tube opening (or inside the secondary, if that is easier), just as you centered the secondary.

To aid in checking tolerances, you could put small protrusions inside the tube at its inner end (I have used strips made from the tubing, 2 mm high and slightly wider and longer). These act as tolerance gauges - if you divide the height by the tube length you get the angle in radians (multiply by 57.3 to get degrees) that the gauge is seen by. My f/6 tube is 168 mm long and the gauge is 2 mm high, the angle is 0.68 degrees (appr 2/3 degrees). If I have centered as shown above, and the black rim is almost gone at one side and as high as the gauge at the other, then the error type 1B is half of the angle, or 1/3 degree - this is usually acceptable but I would try to halve this error.

To judge the centering of the secondary, you can multiply the gauge height by the ratio (secondary size)/(inner diameter of peephole tube). If secondary size is 84 mm and inner diameter of tube is 28 mm, errors should be multiplied by 84/28=3.0. In the illustration above, error is half the gauge height times the multiplication factor - that is *2*3 mm=3 mm.

On some telescopes, it may not be possible to center the mirrors as described. If the focuser is too tall and/or the secondary is too small, you cannot see the edge of the primary mirror inside the edge of the secondary. If the drawtube is too long and/or the secondary is too big, you cannot see the edge of the secondary to center it. In both these cases the basic design is flawed (unfortunately this is rumored to happen sometimes with commercial telescopes), and if you are an Amateur Telescope Modifier you might consider modifying it. Otherwise, you may use a crosshairs tube as described below - your telescope may give a very good performance, nonetheless.

The Crosshairs Sight Tube

This is a peephole tube with crosshairs at the inner end, for centering the optical parts. Here is how to make one. A single cross is fine for aiming at the primary mirror center spot in step 4 (this works even if you can't see the rim of the primary mirror). You can also use it in step 3 to center the secondary, if there is a spot at its optical center. This works with a focuser drawtube that is too long to show the edge of the secondary mirror with a simple sight tube. It is also useful in step 1.

A sight tube of the right length to center the primary mirror in the inner opening (see simple sight tube) and with an accurately centered double cross can be used to collimate a telescope that (for any reason) does not have a center spot on the primary mirror.

In step 4, center the mirror within the inner opening as carefully as possible, and then adjust the primary mirror to make the reflection of the crosshairs centered within the crosshairs themselves (step 5) - you may have to illuminate the crosshairs from within the tube using a flashlight:

A holographic laser may also be used in this situation, by accurately centering the holographic pattern on the primary in step 4 - then do step 5 as usual.

The Cheshire Eyepiece

This is a development of the simple peephole tube, with an illuminated face (white or shiny) set at 45 degrees. A hole in the side of the tube lets in daylight (or suitable night light, like your chart-reading light) to make the reflection of the center spot visible against the bright face. There may be a "field stop" to define better the edge of the bright area. Here is how tomake one.

The Cheshire eyepiece is used in step 5 (see above) to minimize the type 1A error. Collimation (usually of the primary mirror) is tweaked to make the spot appear centered within the bright area. Naturally, you try to center it "exactly", but as the following example shows, it is possible to estimate the error.

These images were taken through a combination sight tube - the offset of the secondary relative to the black center spot is obvious in this f/4.35 instrument. The spot is 15 mm dia. and the bright face 25 mm. In the first image, the edge appears to vary from approx. 2 to 8 mm - this is 3 mm offset, or 1.5 mm error type 1A at the focal plane, this is not acceptable. In the second image, the width varies from 4 to 6 mm, i.e. 1 mm offset or a 0.5 mm error type 1A. I would accept it.

The Combination Sight Tube

This is simply a peephole with the eye end rebuilt as a Cheshire - either cut it to size like a simple sight tube, or with crosshairs at the opening, or both. I prefer a home-made combination (as in the image below, cut to the appropriate f/ ratio - if you buy one, it is likely a one-size-fits-all, with crosshairs), as it can adequately handle the important steps 3, 4and 5.

The Laser Collimator

WARNING: Never look at any laser beam directly! Normally, the beam will stay within the optical path of the telescope, but if it is badly out of collimation, the returning beam may escape beside the secondary. Do not look down the tube if you dont see it return as expected - instead, try picking it up on a piece of paper in front of the tube.

This is a solid state laser, mounted in a tube to fit the eyepiece holder, and critically collimated to make the laser beam centered in (1A) and parallel to (1B) the tube. The "inner" end has a bright face where you can see the spot of the returning beam, with a small hole for it to exit. It can be used in steps 1, 3, 4 and, with reservations, step 5. Some models have the face at a 45 deg angle (a bit like the Cheshire), visible from a distance through a hole at the side of the assembly.

The laser collimator can be expensive and impressive, and favored by many. It makes step 4 very simple, as you can stand by the tube opening and easily see where the beam hits the primary mirror while you adjust the screws on the secondary holder. Seeing where the beam returns can be harder, particularly if the returning beam has been focused by the primary mirror to make the spot smaller than the exit hole. One trick is to see the scattered light where the beam hits the secondary on its two-way trip.

Some laser collimators are sold with a holographic attachment, projecting a reticule that covers a fairly wide angle. This may help in step 3, where you can see the centering of the reticule on the secondary (particularly if you use the L-shaped cardboard piece to project the pattern), and possibly also step 4 and 6. However, contrary to what some manufacturers claim, it is not very useful in step 5.

Potential problem using a laser to collimate the primary mirror:

After step 4, if the laser beam is off the primary mirror center, the return beam will be parallel to the optical axis, but displaced (by the same amount) at the return. If you then adjust the primary mirror to center the return beam accurately, you unwittingly miss-collimate and get a 1A error (of half the displacement). For instance, a critical tolerance for a f/4.5 mirror may only be some 0.5 mm or 0.02 in., so the allowable error in step 4 should be much less than twice this (1 mm or 0.04 in.). With many telescopes, it is not really possible to read the position of the spot (or even adjust the secondary holder) to anywhere near the required precision. On the other hand, if you allow a type 1B error of 0.2 degree (assuming a 70" = 1.8 m focal length), the spot can be 1/4 in. or 6 mm off!

(In other words, the 1A error after collimating with a laser is half the vector sum of the offsets at the primary and at the face of the collimator.)

For this reason, if you collimate step 5 with a laser, you should make very sure you have centered the spot very accurately in step 4 - or else check with a Cheshire (or even star test) that the collimation is acceptable, and tweak it if necessary. In the section on Experimenting, I have described a workaround.

You can make your own laser collimator from a cheap keychain laser pointer.

The Autocollimator

This is a revised version, based on a discussion over the YAHOO Collimate_Your_Telescope group in 2004.

The name may suggest that this is a device that will automatically collimate your telescope - sorry, but it wont. It is another development of the simple peephole tube, with a mirror inside the cap. There is a transparent spot in the center of the mirror, used as a peephole, and the mirror is set accurately perpendicular to the tube (there are other devices called "autocollimator" too - if you search the net for more info, don't be confused).

I have had a rather skeptical view of it, mostly because I have found the instructions unclear and to some extent unreasonable in claims. During this discussion, I tried to derive exactly how the reflections seen in an autocollimator relate to miscollimation, and here are my results

In brief, you may see 4 reflections of the center spot (if illuminated!), all of them stacked on top of each other if collimation is perfect but offset from each other if either the focuser or the primary axis (or both) is miscollimated. This is shown with high sensitivity, but since it is the combined error that is shown, it is not readily apparent what to adjust!

However, if you have collimated the primary as accurately as possible using a Cheshire or Barlowed laser, any errors shown will be of the focuser axis (particularly if aligned with a sight tube), and gentle adjustment of the secondary (or better the focuser, if adjustable) will bring down the 1B error to the same low level as the 1A error already minimized. It can also be used as a "sanity check" - if, after careful collimation, the autocollimator shows significant miscollimation, it would indicate a problem somewhere in the procedure - or perhaps a tool such as a laser collimator out of adjustment.

The centering mask

Take a piece of paper or semi-transparent plastic, large enough to fit over your tube opening. Make a circle exactly as large as your primary mirror, and center it over the tube opening. This is useful in step 6 above, when you want to check the centering of the optical axis within the tube. If you have decided on a maximum allowable error, multiply it by 2 and draw a circle so far outside the first circle (if you like, you can cut a hole inside the first circle). Look inside the sight tube (any kind) - if you can see the inner circle or cutout, the optical axis is not exactly centered, but as long as you cannot see the outer circle, the error is acceptable.

Do you have any final words of advice?

Well - be careful but not obsessive about collimation. You may calculate the tolerances, and perhaps you find that things aren't as critical and difficult as you suspected. If you just do the steps with reasonable care, fine too - just don't forget to spend most of your observing time enjoying the Universe.

Is this the end of the FAQ?

Yes, unless you want to go to the separate section about some special situations and the do-it-yourself projects - you may have followed some links already.

Special situations are:

A workaround to get more accurate collimating with a Laser collimator - the Barlowed Laser

Collimating the primary mirror without a center spot Collimating a Schmidt-Newtonian Collimating a rotating secondary cage Auto-collimating for wide-field photo

Thank you for staying with me so far - I hope you feel it has been worth your while and that you notice the difference, now that your telescope is well collimated. If not, deliberately miscollimate step 5 to make the error fall outside tolerances. Try it on a night with good seeing and see what it does to the image, particularly of planet details.

Nils Olof Carlin ( nilsolof.carlin@telia.com) - comments are welcome

With special thanks to Mel Bartels for his help in making this available on the net, and to Ken and Mike for maintaining the ATM site where this is mirrored - and to all helpful and friendly readers who have mailed me their views and comments.

Special Situations

Here is a section about some special situations and special problems that you may have to handle.

A Barlowed Laser Collimator

As described above, the collimation with a laser collimator is quite sensitive to errors in centering the beam on the main mirror. Here is one way that lets you use the laser to get the accuracy of a Cheshire, as well as its insensitivity to centering errors. Besides, it will work even if there is no center hole in the mirror spot!

Put your laser collimator into a Barlow lens, and aim it at a wall, at a distance about the focal length of your mirror. You will see a much larger spot, perhaps not quite round, but if it is large enough to cover the main mirror spot, you are ready to go on ( if it is too small, try a shorter Barlow!).

You need to put a target over the lens end of the Barlow: Cut a circle of cardboard, large enough to fit over the lens end of the Barlow, and make a center hole to let through the laser beam. Attach it to the Barlow (make it a tight fit to the lens cell, but make a tab to let you lift it off afterwards).

Do step 4 the usual way without the Barlow. For step 5, put the Barlow in the focuser and the laser in the Barlow. You should now see the wide return spot illuminate the target, and also the shadow of the main mirror spot. Now collimate the main mirror to center this shadow.

(The laser light now appears as if coming from a virtual point source, for a 2x Barlow this is near the upper end of the barrel. If used in the normal focuser position, it is well balanced for 1B error).

Collimating Main Mirror Without a Center Spot

Some commercial telescopes come without a center spot on the main mirror. I recommend you make one, but if you do not want to, you could still get good collimation using a (preferrably double) crosshairs sight tube in step 4 and 5. After step 4, you have the main mirror centered in the sight tube. In step 5, you look for the reflection of the crosshairs (at night, you need to illuminate it with a flashlight), and adjust the main mirror to center the reflection in the (unreflected) cross.

A holographic laser is also useful - in step 4, you center the holographic pattern by the degree of the primary, then proceed with step 5 as usual.

The precision is not as high as with a center spot, so do not skip the star check (step 7).

Collimating a Rotating Diagonal Cage

A telescope with a rotating upper assembly gives freedom to place the eyepiece as conveniently as possible. However, if collimation should be maintained after rotation, the optical axis must coincide with the axis of rotation of the upper assembly - this complicates matters. Here is what I believe might work:

I think a laser collimator is indispensable. If necessary, do a coarse collimation up to step 4, and put the laser in place. Rotate the cage, and see how the spot moves - it should trace a circle somewhere on the main mirror. Tilt the secondary to make the spot stationary or almost so - it now shows where the axis of rotation hits the main mirror. Next you need to center it on the mirror spot by whatever means you can, such as adjusting the truss tube clamps, to fine-tune the tilt of the upper assembly.

Now that the axis of rotation is centered on the main mirror, you need to center the optical axis - both at the top of the tube and at the main mirror. You need a second marker for the center - a strip of thin plywood might work, if you screw it to the stationary part of the upper tube, across the center and as close to the secondary as possible. Rotate the upper assembly again - the laser will trace out a circle on the strip. Draw it and mark its center, and temporarily remove the strip to drill a hole here (a little smaller than the laser spot). The tilt of the secondary is probably close enough, so don't alter it now, but you need to center the beam on the hole: sideways by shimming the focuser, and towards/away from the focuser by moving the secondary (adjusting the spider tensioning screws - if this can't be done, move it in/out of the tube instead).

Check step 3, and then by repeatedly tilting the mirror and moving it away or towards the focuser (steps 4 and 2 respectively), you should be able to aim the laser beam through the hole down to the center of the main mirror. You probably also need to fine-tune the "sideways" aim of the focuser by shimming it. Remove the strip again (until next time) and proceed as usual with step 5, tilting the main mirror (step 6 is not necessary - the optical axis is centered by now). (Thanks to Dwight Elvey of the ATM list for ideas)

Collimating a Schmidt-Newtonian

A Schmidt-Newtonian differs from a regular Newtonian in that the primary is spherical, not paraboloid. To cancel the resulting spherical aberration, there is instead a corrector plate at the tube opening, a thin glass disc figured to a complex shape. This means that coma is reduced (approx. halved) compared to a paraboloid mirror of the same f/ ratio - and in addition, the closed tube may be less sensitive to tube currents.

But it also means that the collimation procedure must differ in some respects. I don't have a Schmidt-Newtonian to try, so this is based on a theoretical analysis - so if you try to follow these guidelines, I'd appreciate a mail about your experiences.

In the regular Newtonian, the optical axis must be accurately centered at the optical center of the primary. In the Schmidt-Newtonian, it must be centered in the corrector instead (this aspect seems to have eluded the author of the manual of one common line of such telescopes!). In practice, the centering of the optical axis at the level of the secondary and the corrector is determined by the position of the secondary and the focuser. However, there is no way to decide this or do the adjustments until you have established an optical axis first.

So here is an outline of the steps I believe are needed:

Step 1: See step 4 of regular collimation - tilt the secondary mirror to center the focuser's axis on the primary's center spot, using a laser collimator or crosshairs sight tube (it has been reported that the spot isn't always accurately centered, and you may want to check this once and for all, by removing the mirror cell and measure the position with a ruler).

Step 2: See step 5 of regular collimation - tilt the primary, using a Cheshire or Barlowed laser. Now you have an optical axis established, and you can check its position at the level of the corrector plate.

Step 3 - check the centering of the corrector plate. I see no practical way of using the center, as it is hidden by the offset secondary and its holder. Instead, you can use a peephole cap (or any cheshire or sight tube that is not too long!) and check the centering of the corrector plate's rim with respect to the primary mirror's edge. Looking at a brightly lit ceiling or the sky (avoiding the sun), you should see the inside of the corrector rim, at least if you rack the focuser fully in.

You may find that the rim looks wider in one direction - identify this direction by holding a finger just outside the rim where it appears widest.

If this is towards the focuser, it means the optical axis is offset towards the focuser and needs to be moved away from it. The optical axis will always pass the point at the secondary where the focuser's axis (and laser beam, if used) strikes. To move it, you would like to move the secondary away from the focuser, but this may not be possible - the offset is factory-adjusted. Instead, you can achieve the same thing by moving the secondary in the direction out of the tube. The necessary distance is half the difference in width of the rim at its widest and narrowest, but you will have to try your best guess. Loosen the 3 outer adjustment screws just a little (all by the same amount), and tighten the center one.

If the widest rim is away from the focuser, move the secondary in the opposite direction, but be careful not to loosen the center screw all the way! Also try to keep the tube horizontal with the focuser above in case of accident, and be careful not to rotate the secondary holder.

If it is to one side or the other as seen from the focuser, adjusting the secondary won't help. You will have to shim the focuser instead, aiming it a little towards the narrower side of the rim.

If both, try first eliminating the larger of the errors.

This done, go back to step 1 and repeat - as many times as needed.

You may check the centering of the secondary as in the regular steps 2 and 3, but you will have to accept the decentering you may find - it should be negligible in practice, as the mirror should be reasonably correctly offset in the design.

Step 4: star test the collimation. What to go for is a symmetric diffraction pattern near or at focus - if you defocus enough to clearly see the shadow of the secondary, the test is likely not as sensitive as it should be.

By small adjustments of the primary, you should be able to remove or reduce coma. This is possible even if the original optical axis wasn't accurately centered at the corrector plate, but I believe you can get astigmatism that is not possible to cancel until you have got the axis centered.

Auto-Collimating for Wide-Field Photo

Using wide-field photo (with a coma-corrector), you need to ensure that the film track is accurately perpendicular to the optical axis. This may work:

As with any auto-collimator, you need to get the collimation close with other tools. You may want to make a flat Cheshire to fit the film track of the open camera back - make a cardboard piece the right size, with a peephole at its center. Draw a circle, a bit larger than your main mirror spot, leave it white but paint the cardboard outside of it black.

To make a flat auto-collimator, take a piece of mirror glass, and cut it to fit the film track (I believe any good mirror is fine - you do not need first-surface optical quality. Bevel the edges after cutting!). Make a small, centered peephole in the aluminizing (I have made a mask of tape with a small hole, and polished away the aluminium with a little cerium oxide polishing agent).

To use, attach the coma corrector and camera (no film yet!). If you are not sure step 5 is perfect, put the flat Cheshire in the camera film track (circle on the side towards the tube). Illuminate it from the inside of the tube, and adjust the main mirror to center the spot in the circle. Then put in the autocollimating mirror instead, and peek through the hole.

When correctly adjusted, you will see your own eye pupil in the peephole, infinitely magnified - that is, the auto-collimating mirror (seen reflected) will look quite dark. If not, try to tilt the camera.

Do-It-Yourself Section:

The Main Mirror Center Spot

I suggest you use a piece of dark PVC tape - it is less messy than paint. Cut to a circle of suitable diameter, not too small if you want to use a Cheshire or combination tube. 10-15 mm (1/2 inch) can be fine, or even larger to fit your Cheshire. If you cover a thin segment of the sticky side with another small piece of tape, you can easily peel it off, if you should ever need to.

To attach it, take two plastic rulers or strips with markings for the periphery and center. Make them into a cross and fasten the spot with a very small piece of tape in the very center corner. Then center the spot exactly by using the markings, and press down the spot to make it stick. Then remove the strips. Another trick is to make a paper circle the size of your mirror, fold it twice and cut off a piece at the folded corner, then unfold. If you ever consider using a laser collimator, make the spot with a center hole of at least 5 mm diameter (paper hole reinforcers are popular for this).

In step 3, if you want to use a laser collimator or crosshairs sight tube to center the secondary mirror, you need a very small spot on the secondary at the optical center). With a plain or combination sight tube, it is not needed.

The Peephole

One simple and popular design is a 35mm film can (most will fit a 1 in. focuser), with its bottom cut off and with a small hole. 1 - 2 mm (0.04-0.08 in.) will do, drilled or cut with a sharp knife through the center of the cap.

The Simple Sight Tube

Find a piece of plastic or other suitable tubing that fits your focuser snugly without wobbling easily (I have used PVC tubing for electrical conduits and for drains, and aluminum tubes from old vacuum cleaners). Make a peephole cap from any material that you can make a nice hole in, that can be cemented to the tube end and is reasonably dark. Make a hole about 1 mm, centered as well as you can (if you find it too small, you can make it a little larger later). The length of the tube should be the inner diameter times the focal ratio: I have a tube with 28 mm inner diameter for my f/5.6 telescope, so I cut it to 28*5.6 = 157 mm length.

If the inside of the tube is shiny (you may put some glossy tape inside to make it shiny), you will see a bright reflection around the secondary/main mirror, separated from it by a thin, dark rim. This will let you center it accurately.

It is a good idea to put a few turns of tape around the outside of the tube, to prevent it from inadvertently hitting the secondary mirror with the focuser racked fully in.

If you have several telescopes of different focal ratios, you need one sight tube of the right length for each - no problem, they are easy to make. Why not make one of them (for the smallest f/ ratio) a combination sight tube?

This simple sight tube can be enhanced in several ways:

The Crosshairs Sight Tube

If you put crosshairs at the inner end of a sight tube, they will help you center certain things along the line of sight: the secondary mirror (if the optical center is spotted), or the main mirror (particularly if you can't see its edge), or if you don't have a main mirror spot. You could use white thread (fairly thick, to be easily visible at a distance) glued to the rim of the tube.

If you use a pair of parallel threads instead of a single one, you will get a "double cross" with a clear square in the middle, and you can see the centered reflection unobscured - this is particularly useful for collimating without a main mirror center spot.

The Cheshire Eyepiece

This is also simple to make - but why not make the more useful combination tube? The difference is just the length of the tube!

The Combination Tube

Start out as to make a simple sight tube of the right length for your focal ratio (if you just want to make the Cheshire, the length isn't important). Cut a "V" with 45 degree cuts of half the tube diameter near the peephole end. Cut a piece of cardboard to a half ellipse to fit inside the tube. Make a hole in its center (elliptical if you can). Make it narrower than your main mirror center spot, but if you want the combination tube, not so narrow that you can not see the inner end of the tube through it! Cement it to the tube and cut away the excess. To get a clean edge to the bright face, you can make a field stop. A simple one can be made from a narrow ring of the same tube, with a piece cut away to let you put it inside the tube, and blackened. To compensate for a possible error 1B, the field stop should ideally be as far inside the focus as the peephole is outside it, but this is not critical.

On commercial Cheshire eyepieces, the bright face is often made of reflecting or semi-reflecting material. This can be helpful to see the faint reflections in the lenses when you collimate a refractor, but is hardly needed for a Newtonian.

If you make your own Cheshire eyepiece and/or main mirror spot, you can make a field stop to act as a built-in tolerance gauge: Decide the acceptable error type 1A, and multiply it by 4 to get the difference in diameter between field stop and mirror spot. For a f/4.5 mirror, 0.5mm is a strict tolerance.

The difference in diameter is then 2 mm, so make the inner diameter of the field stop 2 mm larger (or the center spot 2 mm smaller, if that is easier), and the outer diameter to fit inside the tube. If you make the field stop of black cardboard, you can make two retaining rings (like the simple field stop above) to hold it in place.

Now, if you collimate step 5 and make the mirror spot appear fully inside the bright field of the Cheshire, you know that the 1A error is acceptably small, and you will have no trouble with coma.

A Laser Collimator from a Cheap Keychain Laser Pointer

You can find other descriptions of laser collimators on the web, but here is how I made one - the precision is not great, but adequate. I used 2 pieces of plastic tubing, from electric conduit - the sizes, o.d. 20 mm and 32 mm (more like 31.7=1.25", actually), are what I happened to have handy.

Keychain laser pointers come cheap (I bought the one you see below in Greece). There may be laser hologram tops to make nice patterns, but the ones I got are too small to be of much use in collimating.

The inner tube holds the laser pointer with a little space between (about 1 mm). I put a few turns of tape near the output end of the laser pointer to hold it centered here, but not too firm to allow adjustment. At the back end, 3 small set screws (3 mm, or 1/8" if you prefer) in threaded holes allow accurate centering of the beam. A threaded hole sits right over the push-button and another screw in it turns on the beam. Normally, this is one with a large, protruding head, but when I adjust the beam alignment, I use a small, recessed screw instead.

Between the outer and middle tubes is tape along a large part of the length. There are holes for the set screws that are fully inside the outer tube and thus well protected. Another hole admits the push-button screw - in normal use, I have one with a large head that I can turn from the outside.

To see the return beam, a "front" is needed, with a center hole to let out the laser beam. I found a white bottle cap that fit nicely, but cardboard would be fine. The length of the outer tube is not critical, but you may want to make it long enough to make the "front" visible from within the tube. To assemble the whole outfit, I just push fit the components, and add the screws.

Adjusting it is straightforward. I use the small screw to turn on the beam, and place the tube in a V block (actually, a corner in a bookshelf). When I turn the tube, the spot on the opposite wall traces a circle - I then adjust the 3 setscrews to keep the spot stationary. Last, I substitute the large screw to work the switch.

Here are the parts: at the top is the laser pointer with tape. From left to right is the outer tube, the inner tube, the bottle cap above the complete collimator seen from the back end, and the collimator seen from the side. You can see I had to go almost to the limit of adjustment with this pointer!

The Autocollimator (AC) and its Reflections New comments, July 2010

The original version of this webpage was, I believe, the first reasonably complete analysis of the operation of the autocollimator. Since then, the understanding of the autocollimator, its operation and its use - and also its design! - has been further enhanced by the work of Jason Khadder, "Jason D" on the Cloudy Nights' Reflectors forum:

Understanding the importance of having the AC reasonably near the focal plane of the telescope

Understanding the Case of the Missing Reflections (see below) Solving the Case of the Missing Reflections by adding a second,

decentered pupil Designing the "HotSpot", a better center spot than the more traditional

triangular one Designing the CAM, or COC alignment mask (this is AFAIK not

commercially available yet and is not commented here)

This analysis attempts to describe the reflections seen and their positions as functions of miscollimation and builds on a discussion in the autumn of 2004, in the Yahoo Collimate_Your_Telescope forum.

The autocollimator is a flat mirror mounted in a short tube made to fit a Newtonian telescope focuser, and set accurately perpendicular to the tubes axis. (The mirror should be placed reasonably close to the actual focal plane of the telescope. In practice, if you position the focuser in the range you use for observing with any of your eyepieces, this is close enough). Centered in it is a small peephole or pupil that you look through. The primary mirrors center is marked with a bright or reflective spot, and you can see the spot (reflected in the secondary) and a few more reflections of the spot after several reflections back and forth.

This picture shows the model XLK autocollimator, with the ordinary, centered pupil (to the left) and the added, decentered pupil (INFINITY, by Jim Fly). The mirror is highly reflective, and thus almost impossible to see, but it is placed midway between what appears to be the open ends of the aluminum barrel!

To use it, you first do a fairly close collimation with a sight tube and a Cheshire collimator (or, if you prefer, with a laser - with or without a Barlow attachment), otherwise the reflections may fall well outside where you can see them.

Then insert the autocollimator and fine-adjust the collimation. Change between the AC and the Cheshire as in the example below, until the reflections show that you are close enough. But what exactly are the reflections, and what do they show?

This figure (click this or later images for a larger version!) shows, greatly exaggerated and not to scale, the primary and autocollimator (the secondary mirror is ignored, not to introduce even more confusion). The primarys optical axis starts at the center mark P and goes to COC, the center of curvature of the mirror. Midway between P and COC is F, the focus of the primary mirror.

The autocollimators axis is perpendicular to the mirror and centered in it, at the pupil. There is a combined collimation error shown: the primarys axis misses the autocollimator pupil by a distance A (elsewhere I have called this a 1A error!), and the autocollimator(=focuser) axis misses the primarys center by B (1B error).

To Trace the Reflections:

Draw a line (green) from P, parallel to the autocollimator axis, to V1 via H5, and another parallel line from F to H3.

Also, draw a line parallel to them from COC to H2, and a third line V2 to 2, parallel and with a distance V2-COC equal to COC-V1.

At V1 is a virtual image of P by the autocollimator. This virtual image is reflected to a focused virtual image (the details are not shown) V2, also at the level of the COC. This virtual image is again reflected in the autocollimator down to a real image 2 at the primary. This reflection can be seen as an inverted, or more accurately rotated by 180, but (just like P) sharp when seen with an eye or camera focused at one focal length.

Also another reflection 1 can be seen (this will be shown later), by intercepting the light going towards the image V2 its projected position on the primary is found by drawing a line V2 to autocollimator pupil, extending it down to the primary.

The real image at 2 is projected by the reverse path back to P, accurately on top of it after another 180 rotation, thus ending for good the series of reflections.

But before this, a reflection 3 can be seen, formed from 3 in analogy to how the reflection 1 is formed from P. Both 1 and 3 are seen as bundles of converging light, as if focused at minus one focal length if imaged with a camera focused at P, they will appear noticeably defocused (if the camera is focused at infinity, all images will appear defocused by the same amount).

Now we can determine the relative positions of the reflections (positive to the right in the figure):

The distance P to H3 is A+B, as is H3 to H2. H2 to 2 equals p to H2 =2A+2B, the total displacement P to 2 is 4A+4B.

By similar reasoning, the distance from H4 to the autocollimator pupil is 4A+3B, and from 2 to 1 twice this thus the displacement of 1 from P is -4A-2B. The displacement of 3 from P is 2B, remarkably enough independently of any miscollimation of the primary.

This figure shows the paths of reflections 1 (bold red) and 2 (bold green) seen in the autocollimator after 2 and 4 reflections respectively (note that the reflection at H6 should rightly have occurred much farther to the right!). The reflections of 3, (if you include the secondary, there are 13 reflections in all!) are left as an exercise to the reader.

Let us also regard the reflections of the autocollimator:

Draw a line from COC through the ACP, it reaches the primary at X, displaced -2A from P. This is where the autocollimator will be seen by its first or "foreground" reflection in the same manner as the face of a Cheshire!

Draw a line (green) from F parallel to the autocollimator axis down to G1.

Trace a ray from ACP parallel to the primarys axis down to G2 it will be reflected toward F, and then to G3 where the distance G3-G1 = G1-G2=2A+B, and then parallel to the primary's axis up to G4 where the distance G4-F=G3-P=3A+2B. Thus, there will be a real "background" image of the autocollimator pupil at G4, displaced from the "foreground" reflection by 4A+2B (and the whole autocollimator mirror around it, rotated 180).

The reflection at G4 will be imaged back to the ACP itself, at least as long as G4 still falls on the mirror face! Thus, the light path will be closed and the autocollimator face will be dark regardless of collimation darkening is not a useful collimation criterion as has sometimes been claimed.

The reflections of the pupils will be visible against the background of more or less coincident spot reflections - but not easily so, if it were indeed possible to stack the reflections perfectly.

The "background" reflected pupil at G4 will appear at a displacement of 2A+2B from P (not shown).

Miscollimation errors:

Given a miscollimation (in one dimension of two!) of A of the primary axis and B of the focuser/eyepiece axis, the displacements of the reflections are:

P to 1 -4A-2B

P to 2 4A+4B

P to 3 2B

P to X (the foreground reflection) -2A

P to the background reflection 2A+2B

foreground to background reflections 4A+2B

You can also use the angular error expressed as C=A+B:

P to 1 -2C-2A

P to 2 4C

P to 3 2C-2A

P to X (the foreground reflection) -2A

P to the background reflection 2C

foreground to background reflections 2C+2A

Here, you see that while the sensitivity to angular errors is high (P to 2 = 3 to 1 = 4C), the sensitivity to errors of the primarys axis is not greater for the autocollimator than it is for the Cheshire or Barlowed laser.

What it looks like in the Autocollimator: These images by Vic Menard show eloquently what can be seen: The camera is focused at P, so it and the rotated image 2 are sharp, while 1 (non-rotated, bright) and 3 (rotated, fainter) are defocused.

In the image above, the focuser (or secondary) is intentionally misaligned (A=0), so you see from left to right:

1 at -2B P 3 at 2B 2 at 4B

Also note the reflection of ACP at +2B, falling on 3 making the center hole appear sharper than the edge.

In the image above, it is instead the primary that is miscollimated, so you will see from upper left to lower right:

1 at -4A P and 3 almost coincident 2 at +4A

There is also a smaller miscollimation of B at almost right angles to the main one - when the reflections are well separated as here, it is easy to see that "3" is slightly below and to the left of "P". What you can do here is adjust the secondary to bring "3" to coincide with "P" (never mind what happens to "1" and "2"!) This done, you know that any error left is in the collimation of the primary. Adjust it until the reflections ("1" and "2") merge with "P" and vanish - now collimation is done! This is, in fact, how the "carefully decollimated protocol" works. To use this method, you may have to first slightly decollimate the primary in order to separate the reflections and identify "3".

In principle, it is possible to solve an equation system for each direction: call X=P to 1, Y=P to 2:

A=-X/2-Y/4; B=X/2+Y/2;

But this doesnt seem very practical out in the field! And the appearance of the spots doesnt in general suggest what needs adjusting trying to stack P and 2 will leave you with A=-B and 1 and 3 slightly off by 2B in opposite directions.

One theoretical solution: offset the primary collimation enough to separate the spots, then identify the faintest reflection 3 and stack it with P by adjusting the secondary (thus setting B=0), and finish by adjusting the primary until all spots stack. Vic Menard reports that it is indeed a useful method! But you might want to finish by checking with the Cheshire, anyway.

During the 4 years from the first version of this webpage, this method (sometimes known as "the Carefully Decollimated Protocol" has gained popularity as a quick and useful method.

One practical approach is to use the sight tube (or laser) to collimate (at least roughly) the focuser axis (by adjusting the secondary mirror), then use the Cheshire (or Barlowed laser) to adjust the primary as accurately as possible.

Thus, with A=0 set as closely as possible, you adjust the focuser tilt (if adjustable!) or the secondary to stack all images, finishing by checking that A=0 still if not, repeat. This will converge quickly, quicker if you can adjust the focuser tilt rather than the secondary tilt.

What precision could be expected in practice?

The fundamental weakness - often touted as the main advantage - of the autocollimator is its "multi-pass" nature: the errors of the axes are intermixed (but for the faintest reflection "3", only separately discernible by intentional miscollimation of the primary) and cannot be separated for separate adjustments. Assume you have an error A after collimating with the Cheshire, and succeed in bringing the reflections 1 and 2 accurately together (they will be removed from P by a minimum distance of A): thus -4A-2B=4A+4B or B=-4/3A). The residual error after bringing P and 2 together is of the same magnitude.

What about reading accuracy? If you use a Cheshire with a fine peephole, you see the spot (at one focal length) and reflected bright face of the Cheshire (at infinity) sharply, independently of the eye position, but if you enlarge the peephole, you will find the lineup shifting slightly if you move your eye (in the Barlowed laser, the very small point source of light corresponds to a very small peephole of a Cheshire).

With an autocollimator with a fairly large peephole (see the first illustration above), you will see the reflections P and 2 at one focal length, their separation independent of the eye position, but the reflections 1 and 3 are defocused (twice as much as in the case of the Cheshire) and may be affected by the eye position with some loss of reading accuracy. However, the distance between the foreground and background reflections of the autocollimator is equal to the distance between P and the defocused 1, but here they are both focused at infinity and free of parallax - thus, an autocollimator with the inner edge of the barrel coated with reflective material may show this separation to higher precision by eliminating the "lune" from the "background" reflection of the autocollimator edge.

If the "foreground" reflection of the autocollimator pupil itself can be seen against the background of slightly displaced and defocused spot reflections (and I expect this is unavoidable in practice - but there is also a "background" reflection of it that may interfere), it can of course be read in the same manner as a Cheshire (with some reservation for the size of the peephole, see above), eliminating the swapping of tools at least until the very final stages. (The CAM, briefly mentioned above, makes the AC work like a Cheshire, thereby eliminating the need for swapping).

Given one of the axes (usually the primary's) is collimated to within a small residual error with an independent method, the autocollimator can be expected to bring down the collimating errors of the other axis to the same order of magnitude, but not better - even considering the limits to its own reading precision.

One trap to avoid: adjusting the secondary will affect the collimation of the primary's axis to some extent, and if you correct a large error B this way, you may introduce a significant error A - thus, do not forget to check with the autocollimator/Barlowed laser after any (unless minor) adjustment of the secondary.

The Case of the Missing Reflections:

When you come close to convergence, the "inverted" reflections (2 and 3) seem to vanish - you would expect all reflections to stack neatly and add up to a regular hexagram star pattern, but instead you only see one triangle (P, actually). Why?

The simple answer: at convergence, the reflections have to pass the AC twice along quite symmetric paths. The first time outside the pupil in order to be reflected further, the second time inside the pupil in order to be observed.

This is impossible of course, and the reflections will vanish. However, this problem can be solved by adding a second, decentered pupil: looking through it instead, you see the reflections P and 2 stacked, and separately 1 and 3 stacked.

For a complex answer, look at the figure: we will trace a sample ray from the tip of the triangle to its expected (but missing) reflection 2 and further to the observer's eye. Call R= th