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SCT Collimation under Adverse Circumstances

Posted by Greg Nowell   06/27/2009 11:39PM

SCT Collimation under Adverse Circumstances
Collimating an SCT is almost never done under ideal conditions. There are a number of resources on the Internet on how to collimate an SCT, but few of them are geared to the conditions that matter most to users: when the scope is grossly miscollimated, causing feelings of dread and fear, and when the sky is so bad one thinks: “What’s the point?”

SCTs sometimes lose collimation during shipping, were never collimated well at the factory, and sometimes are taken apart by their owners for a variety of good and not-so-good reasons, which we need not go into here. If you need to take your SCT apart, you can follow the pictorial essay on SCT disassembly, here:

And I note, that if you pull the corrector and secondary together as a unit, and replace them carefully in the same orientation, this usually results in the need for ordinary collimation, not in gross miscollimation.

The basic idea of collimating an SCT is this: an out of focus star, seen through a medium to high power eyepiece (20mm to 3 mm), will show a doughnut shape with a black dot in the middle (this is the shadow of the secondary mirror). On a perfectly collimated SCT the doughnut is perfectly round. When the doughnut is off center, you use the three screws on the secondary support, in the center of the corrector plate, to adjust the image so that it gets centered. Where this gets tricky is during gross miscollimation, when you won’t see a doughnut, and during bad seeing, when it looks like the cook was drunk and dropped the doughnuts on the floor, getting every conceivable shape except round.

You evaluate your collimation only when the star you’re using is centered in the field of view.

A profound source of psychological confusion about star doughnuts, whose main function in life is, and ought to be, collimating, is the entire industry of people who are busy obsessing about their optics because of Suiter’s book on star testing. And then there are the precision optics buffs. They have their place, but these people are not your tribe. You’ve got a nice SCT. Run along and buy an apo if you must. But right now you have an SCT to collimate, so stop worrying about anything else. If you collimate your SCT, you will do much to confound the critical opinions about them that you will find on the Internet. And in so doing, you will bring honor to your tribe. Here, too, is a tribal secret:

An out of focus star is rarely symmetrical on both sides of focus, because the temperature differential between the outside of the corrector plate and the inside will cause one side to look really bad, and the other, pretty good. Don’t obsess about that. In the procedures that follow, use the side of focus that gives you the best doughnut.


When your SCT is grossly miscollimated stars will have no focus point and will appear like comets or flares. This will show in any eyepiece: y’aint got no doughnut. For fixing gross miscollimation medium power eyepieces are typically needed, say 20 to 14mm. Higher power eyepieces are difficult to use because your star will rapidly travel out of the field of view and you will need to move the scope to get it back in again.

For fixing gross miscollimation I strongly recommend the inexpensive artificial stars currently sold on Astromart. These will allow you to do the work during the day when you won’t feel like you’re wasting precious sky time, or under pressure from star party attendees to show what you’ve got in the scope (which would be, to be precise, a crummy out-of-focus star). During strong daylight the out-of-focus rings of an artificial star may be difficult to see, so there are some advantages to doing it at night, but once the procedure is understood daytime collimation is certainly easy enough. However, for large SCTs, such as C11s and C14s, collimation achieved during the day with the scope in horizontal position may be only approximate, since the scope is rarely horizontal in real night time use, and the big mirrors are stressed in the horizontal position. A tune up on a bright star at 45 degrees may be needed later. Figure 1 shows a daytime collimation device purchased from an Astromart vendor.

The first thing to do is to make sure that all three screws are in fact gripping the secondary plate. Usually this is not an issue, unless the secondary has been removed or one has been careless in following the instructions that come with Bob’s Knobs. With Fastar configurations, removal inside the Fastar housing does not “count.” What counts is removal of the secondary itself from the actual baffle. If you have the corrector off and the secondary out, use the opportunity to verify that the line drawn on the back and/or side of the secondary is pointing at the serial number on the corrector, as this is part of the optics adjustment. Sometimes the line on the mirror is hidden so the only way to see its orientation is to remove it entirely from the baffle. I recommend complete removal to check the orientation ONLY if you have serious reason to believe that the secondary mirror may not have been installed in its correct position—for example, you have an old SCT and just got it recoated. In addition, the orientation of the number on the corrector should correspond to the marking on the back of the primary mirror, if there is one. The usual “default” position is with the printed serial number visible on the outside of the secondary “down” (numbers right side up) at the same position as the dovetail, which is 6 o’clock. The other serial number is engraved in the corrector, is not always easy to see, and can’t be seen at all when the scope is assembled. It should be at 3 o’clock.

Figure 2 shows what a star in a grossly miscollimated SCT looks like. It is in the nature of a grossly miscollimated SCT to be heavily skewed to one direction; once this is understood, it is a question of finding the right screw to adjust. You want the screw that lets you travel in the direction “away” from the head of the comet-like star. As you do so the out of focus image will gradually form an illuminated crescent; keep traveling “away” from where the comet head was. Eventually you will arrive at a position where the out of focus star forms an evenly illuminated circle. At that point your scope is no longer grossly miscollimated. It is simply miscollimated. If you travel too far in the opposite direction you will gradually form a reverse illuminated crescent and a reverse comet. Don’t do that.

You may find that you are “traveling in the right direction” and run out of screw adjustment. That’s unfortunate. You need to back out the screw that has “bottomed out” (or screw it in, if it has gotten too loose) and then move the other two screws in the opposite direction. Take a look at what your comet head is doing and resume traveling away from it. When you have a fairly circular, evenly illuminated out of focus star you are ready to do your fine collimation. At this point a second axis of adjustment may further improve the circularity and evenness of illumination of the nascent doughnut's outer ring.


Bumpy vehicle rides and mild bumps in ordinary use are the leading causes of the need to touch up collimation or “ordinary collimation” on an SCT, from time to time. For SCTs ten inches and over, the stress of being horizontal may compromise the usefulness of the daytime collimation, so this is a nighttime exercise, unless you have some means to run an artificial star out about 60 yards and hoist it till your scope is at a 45 degree angle. After a few tries you will find that ordinary collimation is easy.

The main problem with star collimation is that the stars don’t hold still. One very rarely sees the textbook perfect circles. Here the basic procedure is not hard. You find the side of focus which gives you your “best doughnut.” At high power—say a 10mm eyepiece—break slowly away from best focus till your doughnut shows a dark center and three rings. The rings are deforming because of the seeing. Using your adjustment screws, your mission is to try to adjust things so that the middle ring, no matter where everything deforms, does not touch the other rings. Or, failing that, it seems to touch the other rings in almost every direction with no special “favoritism” for any one spot. If your middle ring is not touching the other two, you are pretty well collimated. You will find that even when the sky steadies you may not have anything else to do. For this exercise you use a 10mm eyepiece, but not a 3mm eyepiece because the seeing is bad. If the seeing were excellent you would push the magnification with a 3 to 5mm eyepiece—whatever yields 50x per inch of aperture—and just center the black dot in the middle of your nice doughnut. But the seeing is not excellent. In fact, it is almost always bad, which is why you’re reading this article. So forget the roundness of the doughnut and try to keep that middle ring from contacting the other two. See Figure 3.

It is helpful to know which screw to use to make the tiny adjustments. You can use trial and error, but holding a hand from the edge of the corrector to an adjustment screw will cast a shadow. Doing this on all three screws will cast three different shadows. Each shadow is an ADJUSTMENT AXIS, and the screw nearest to that adjustment axis is the one you need to adjust. You must use much smaller movements than you did in the gross collimation process. Sixteenths of a revolution and even less are sometimes all that is needed. But if you go too far it is easy enough to come back.

Your collimation is excellent if you really can’t tell, with the doughnut only slightly out of focus, which direction to adjust it to make it better. If you think a tiny tweek is needed, odds are, it will improve your telescope’s performance.

You can also use the SHEER GENIUS COLLIMATION DEVICE ™, which is so brilliantly conceived that it is too easy to make and cannot be sold for a profit, not even in the booming worldwide SCT collimation industry. I am the inventor. The SHEER GENIUS COLLIMATION DEVICE, or SGCD, is in Figure 4.

LASERS. In my view lasers sold for SCTs are worthless, but artificial stars are quite useful. Don’t let the Newtonian owner’s fixation with the perfect laser trouble you. This is a different tribe. You are not of this tribe. Let them do their weird dances and pay their outlandish fees for lasered light, tubes, collimation caps, and what not. You need stars, natural or artificial. For regular collimation you should use real stars, because now you can have the scope angled at close to a “real” position of use: about 45 degrees, so that you get the average of the mirror stress between vertical and horizontal. If you are a neophyte with an SCT, do your first night time star collimation during a full moon, when you will be untroubled with other projects.

WHEN ALIGNMENT IS NOT KNOWN. The guts of an SCT follow a twofold orientation: the mark on the secondary points to the engraved number on the corrector. The number on the corrector is lined up with a line on the back of the primary. Thus all three components, secondary, corrector, and primary mirror, have a correct orientation point. Usually by thinking and looking for the marks you can get things to where they ought to be. I know of some conditions where the orientation to the primary is not known via a mark, but I have not heard of an unmarked secondary. In any case you may need to run barefoot through your own data. Collimate as best you can for example with the secondary oriented to the corrector plate serial number. Make a not of any flaring or asymmetries in your in focus and out of focus stars. Then rotate and whole corrector and secondary 90 degrees and collimate as well as you can. Compare the results. Then rotate 90 more degrees and compare again. If one of these positions looks noticeably better than the others, that’s the position to use. I’m not sure I have the patience to try this on more than four positions but I certainly would not use more than eight. You’ll typically be OK if you’re within 90 degrees.

HOT MIRROR. You’ll see lots of wavy lines swamping your doughnut. The direction of travel will always be “up” no matter where you point your scope. (Heavy wavy lines from bad seeing may follow a high prevailing wind.) Do the best that you can, don’t sweat the fine details, and come back to the job later.

STOP AND NOTE: Do you see a flare in the eyepiece? Is it your telescope or did you forget that your eyepiece has a dioptrx astigmatism corrector on it? The dioptrx will show flares except when it is correctly oriented for your eye.

JUPITER, SATURN, and MARS. If you are in a do or die competition against a high class dob or apo you need your collimation at its best. Your general purpose 45 degree collimation may not be optimal, especially in larger scopes where the mirrors tend to shift a bit depending on where they are pointed. On Jupiter, use one of the Galilean moons as a star to refine your collimation. That lines things up in the exact position needed. On Saturn or Mars, use some nearby star. I note, however, that on my C14 the big guy holds collimation very well from low to the south to all over the rest of the sky, which I check every time I set up, because I build for the computer a ten star pointing model with a high power eyepiece. Nonetheless, a “local tweak” of collimation for Jupiter, Saturn and Mars will tease some extra performance out of your scope, assuming that you can detect any off centeredness in the out of focus doughnut.

BOB’S KNOBS: I use them, but I’m not sure they’re better than OEM defaults. They may keep you from trying to tighten past the point where your screw has bottomed out, which is good. They are harder than factory defaults to use in winter when you’re wearing gloves, which is not so good.

HOLDING COLLIMATION. If you find that your SCT does not hold collimation well, and are certain that the secondary mirror is being held snugly by its screws and is not loose in the corrector in any way, then you may need to tighten the primary lock nut. A major symptom is that on equatorial mounts a double star is “cleanly split and round” with the eyepiece on one side of the meridian but flared and a mess on the other side of the meridian. A detailed discussion with photos is in an appendix to the document “How to Clean Your Corrector Plate” which is in the FILES section of two different Yahoo groups:


The document on both sites is maintained by the present author.