Some Collimation Myths and Misunderstandings
addendum dec 7, 2004 - see about the autocollimator below
It seems that the collimation of Newtonian reflectors is a subject full of widespread myths and misunderstandings - this webpage attempts to put some of them in their proper perspective - to the best of my ability.
myth: You have to square the focuser very accurately
I'm not quite sure of even what "square" is supposed to mean - likely it means set perpendicular to the tube, or possibly to the optical axis - or both, always assuming you have made them coincide. There is nothing wrong with doing it, of course, but the secondary is optically flat, and the angle of reflection is not critical. Most secondaries are made to look circular when tilted 45 degrees (to reflect 90 degrees), but if the angle deviates from this by a few degrees, the only consequence is that the secondary will appear slightly elliptic - it won't affect the image.
The important thing is that the focuser and the secondary are lined up, as seen from the focus. If not, you should adjust either, as appropriate - if the secondary seems to be off in a direction from or towards the primary, you can usually move the bolt that holds it in the spider. If the error is "sideways", and the secondary is indeed well centered in the optical tube, it must be the focuser that is off to one side and needs shimming. If this error is left alone, the penalty is that the fully illuminated field at focus won't be centered - a small error here is no disaster, but the center of the field should always cover the center of the field of view, and best with a small margin to avoid the effects of a possible narrow turned-down edge on the secondary. Check with a sight tube in daylight when the telescope is collimated - when the peephole of the sight tube is close to the focal plane (top of the drawtube at a "normal" position), the whole edge of the primary should be visible in the secondary. If the focuser is tilted a little "down" or "up" the tube, this will be compensated for when adjusting the tilt of the secondary - the only consequence is that the fully illuminated field will be slightly elliptic - a thing you will never notice.
myth: The primary mirror must be exactly centered within the tube
No harm in it, of course. But unless you also offset the secondary correctly (and I believe not many do), the optical axis won't be centered in the upper end of the tube, anyway. What is vitally important to good collimation, though, is that the mirror stays in place, centered or not. The common 180 deg sling, properly designed, will do a fine job of balancing the forces that might cause astigmatism by "potato chipping" of a thin mirror. But such a sling will not prevent the mirror swinging "sideways" when the tube is moved - and with a f/4.5 mirror, 1 mm of swing will cause 1 mm of miscollimation at the eyepiece. This is enough to affect performance noticeably with critical viewing. (For the ATMer, I have a modification that takes care of this problem)
myth: The secondary mirror must (or must not) be offset
"Offsetting" the secondary is done to center the fully illuminated field. A fully offset secondary has the geometric center of the elliptic face offset parallel to its face in the direction away from the focuser and down the tube - the optical center, that is the point that the optical axis (of the fully collimated telescope) hits. If you use a laser collimator, its beam outlining the optical axis, you might see (unfortunately not easily with a closed tube!) that it does in fact hit the elliptic face a little off center, towards the edge nearer to the focuser. If you center the secondary as seen in a sight tube, the offset toward the primary will be accomplished automatically, and you may not even be aware of the fact that it is optically offset. The optical axis of the telescope will (once the collimation is done!) start at the center of the primary mirror and pass this spot on the secondary going toward its celestial target. If the secondary mirror is properly offset away from the focuser, this spot is centered in the tube - if the secondary is centered within the tube, it is the spot that is offset instead (a little toward the focuser). If both the primary mirror and this spot are centered in the tube, fine. But unless either is off so far as to make parts of the tube get into the optical path, you won't tell the difference. The offset of practical mirrors, even fast ones, does not exceed some 1.5% of the aperture diameter, so offset is not likely to matter much.
myth: A laser collimator is the most accurate collimating tool
It is easy to be impressed by the accuracy of laser beams in e.g. construction work, and jump to the conclusion that the laser is as accurate for collimating telescopes. I agree that it is the most convenient tool for adjusting the tilt of the secondary, and you can center the beam in the donut opening to within a millimeter or a few - far better than really needed. But when it comes to adjusting the primary mirror, thismay not be good enough - the collimating error is in fact half the vector (i.e directions considered) sum of the errors at the primary and at the laser faceplate where the beam returns. If you know this (the laser manufacturers won't tell you how important it is!) and you can center the beam accurately enough, fine - but I suspect with many telescopes it is difficult or impossible to get both adjustments close enough to get this most critical part right. The good old Cheshire eyepiece shows only the error of the primary, and so does the Barlowed Laser - you have a better chance of getting close to perfect with either (for a better description of the Barlowed Laser, see Sky&Telescope, Jan. -03).
Laser collimator manufacturers tend to claim that their products have very small angular errors of the laser beam (relative to the housing). This is not quite as critical as they believe - the angular error will cause the collimation of the focuser to be slightly off, but it won't affect the critical collimation of the primary enough to matter.
myth: If the laser isn't the most accurate tool, then surely the autocollimator is
(revised Dec -04!)In "Perspectives of Collimation", Vic Menard (with Tippy D'Auria as co-author) writes: " before we decided to write this book, the autocollimator was without question the least understood of the collimation tools".
Only too true - I have long had a strong feeling that the explanations and instructions given in this booklet (4th edition), or for that matter elsewhere, don't alter this sad fact. However, recently I did (Nov-Dec -04, with help from Vic Menard and Jim Fly) an analysis of the reflections that can be seen in the autocollimator, and how they indicate the state of collimation. Note that to see the reflections, you need a bright, illuminated (and reflective) center spot on the primary.
I hope and trust the next edition of "Perspectives" will have a considerably revised section of the autocollimator. Used together with a Cheshire or Barlowed laser to set the collimation of the primary, it can be used to some advantage in checking and adjusting the collimation of the focuser axis - in particular, if this was initially set using a sight tube, but possibly also with a laser. This may not really make a visible difference, but if you like to use one, I suggest this is the best way to use it:
Do a reasonably close collimation with the sight tube/laser and the Cheshire/Barlowed laser.
Use the Cheshire/Barlow to check, and if necessary adjust, the primary (unless this is done accurately, the reflections in the autocollimator won't mean anything useful!)
Adjust the secondary, or better the focuser if it is adjustable, to make the reflections "stack". When they do, go back to 2 and check - if the primary is still accurately collimated, as it will be within a few iterations, stop there!
Thus, the Cheshire defines the critical collimation of the primary, while the autocollimator finishes the less critical collimation of the focuser axis.
What you shouldn't bother to do or try is:
Use the darkening of the autocollimator's reflection as a criterion of collimation (as suggested on Tectron's instruction page). It takes gross miscollimation to "open the light path" - far more than would be left even by a casual collimation as in 1 above.
Rely solely on adjusting the secondary - if the optical axes are not coincident here, no adjustment will bring them together. You need to alternate by adjusting the secondary and the primary, as described by steps 2 and 3 above.
The pencil "convergence" routine described in "perspectives". It may be instructive (or just confusing) but only shows the shift between the two images of the autocollimator.
misunderstandings: Using a holographic pattern generator can improve collimation of the primary
Some manufacturers (AFAIK LaserMax, Helix, Howie Glatter) offer holographic attachments that generate fairly wide angle light patterns useful for centering. The following is quoted from the Helix webpage:
The Grid or Scope (see below) hologram is perfect for adjusting the tilt of the primary mirror. The grid pattern is projected from the open end of the scope onto and adjacent surface. The primary mirror tilt is adjusted by centering the shadow of the secondary mirror within the projected grid pattern.
This procedure is also the recommendation for the LaserMax collimator. However, the de-centering of the secondary shadow is indeed the secondary offset (away from the focuser) that you carefully adjusted when you positioned the secondary - if you adjust the primary as instructed, you will miscollimate the primary by the actual offset, thereby grossly upsetting the collimation for any reasonably large and fast mirror. However, you can move the secondary mirror outwards (away from the mirror) by the required amount, then re-collimate. If the shadow is centered now, it means the secondary is indeed non-offset - but why the extra trouble to achieve a slightly inferior collimation?
Howie Glatter has a different recommendation (or rather had! he likely has revised this section - but if you happen to have an old manual, read on):
The primary can be adjusted by observing the pattern projected from the telescope upon a screen or wall, and centering the projected grid pattern in the projected aperture
This is a slightly different twist, but just as misleading - if the pattern is decentered, it can only mean that the laser in the focuser is no longer pointing towards the center of the primary. This should be remedied by adjusting the secondary - adjusting the primary won't change the centering, but it will destroy its collimation (after adjusting the secondary, you always need to re-adjust the primary).
I suggest you disregard these tips. However, the holographic grid can be used to advantage for adjusting the position of the secondary mirror! This done, go on with the rest of the collimation, and don't forget the Barlow (Howie Glatter now offers a Barlow attachment to use with his collimators!!!).
myth: You should always finish collimation by star collimation
At the focal plane of a paraboloid mirror, there is one point where coma, the one-sided aberration, has a minimum. This point is, by definition, the focal point. Locating it can really only be done by star collimation - using a real star, or a sufficiently distant artificial one, collimating the primary mirror for a symmetric star image (high magnification, and image close to focus for best sensitivity) at the center of the field of view. If you put a small peephole at the true focal point and look, you will see a distant reflection of the peephole itself (if its inside is illuminated), as if far behind the mirror surface. The spot on the mirror that the peephole seems to lie behind is - again by definition - the optical center of the main mirror.
The common thing to do is to place a spot at the geometric center of the primary mirror and use it with a Cheshire eyepiece, centering the reflection of the Cheshire behind the spot. Normally this is close enough (I am still waiting to hear about anyone with a mirror where the optical center is significantly offset from a well centered spot!). Using the Cheshire is a lot easier, quicker and better reproducible than star collimation is - particularly with less than excellent seeing. However, if you find the one-sided asymmetry of coma on a star at the center of the field, it can only mean one thing - the center spot isnīt at the true optical center. The best thing is to move it or put a new one, centered in the reflection of the Cheshire (the next best is to make a note of the direction and distance of the offset), for later collimation. Now you know you have the true optical center marked.
Thus, you should take the trouble and do a careful star collimation under good seeing to ensure that the primary is accurately marked - but when this is done once, collimating with a Cheshire or Barlowed laser is easier and more reproducible. There is no point in star collimating every time.
last updated Dec 7, 2004/Nils Olof Carlin