Some Collimation Myths and Misunderstandings
At last updated per march 27, 2015 – after ten years
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, the first thing to check is the rotation of the secondary holder. If rotated some, you can still tilt the holder to reflect the focuser axis to the center of the primary, but the holder will look tilted. Correct the tilt and rotation – if still not centered, it may 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
There are indeed two separate “offsets” of the secondary, and both are really centering in two directions – even though the distances of offsetting are equal.
“Offsetting” towards the primary mirror is done to center the fully illuminated field. By centering the secondary mirror as seen in a suitable sight tube in the focuser, this “offset” is automatically done, and you may not even be aware of the fact. Without such a sight tube, collimate the focuser axis (using a laser or crosshairs combination “Cheshire” tool). Now, the primary is by definition centered on the focuser axis, so look in a peephole device such as a collimating cap to see if the secondary is centered on it. If so, the secondary is correctly placed (“offset”) along the tube. If not, move it and re-try.
If you want the optical axis (that is aiming at a star that you see in the center of the field of view) to be centered in the tube, at the level of the focuser, you need to offset the secondary away from the focuser. This offset may even be built into the secondary holder. If not, you can safely ignore it, the optical performance is not affected, and the aim is usually within a single degree from the tube axis.
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
In the 4th edition of "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".
Since then, Vic has written a 5th edition, greatly revised and improved: see http://www.vicmenard.com/telescopes/
See my analysis of the reflections that can be seen in the autocollimator to find out what it shows – in particular, with Jason D’s eccentric pupil.
Note that to see the reflections, you need a bright, illuminated (and reflective) center spot on the primary – such as Jason D’s HotSpot.
For both, see http://www.catseyecollimation.com/
Used together with a Cheshire (including Catseye’s BlackCat) 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:
1. Do a reasonably close collimation with the sight tube/laser and the Cheshire/Barlowed laser.
2. 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!)
3. 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.
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 isn´t 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. Recently, Jason D, in a thread on Cloudy nights forums, has added valuable knowledge: start with the star centered and focused, then carefully defocus outward to see where the diffraction rings first emerge. The more common way to see the centering of the emerging secondary shadow is far too insensitive.
last updated Mar 27, 2015 /Nils Olof Carlin