Collimation of the C-8

 

 

Collimation is the process that insures the optical elements of your telescope are properly lined up to provide the best image possible. While many amateurs are a bit wary of this procedure, it is worth taking the time to learn how to keep your scope collimated.

I had hoped to have some visuals to add to this page in hopes of aiding you in collimating your scope. Alas, my artistic ability is so limited, we are going to have to work with words rather than images.

 Collimation of the C-8 can be broken down into 2 phases: rough and fine. Rough collimation can be done under any circumstances where you have a clear night time sky. Fine collimation requires the clear sky and steady seeing. However, it is possible to get very close to perfect collimation even if the seeing isn't perfectly steady.

Rough collimation is accomplished with an eyepiece of around 20mm focal length (100x) or so. A bit more or a bit less really won't make any difference. This whole process is made a lot easier if you take the time to polar align your scope so the clock drive tracks and if you have an extra set of hands to do the adjusting while you look through the eyepiece. It can be done by one person, but it will take at least twice as much time.

Some people advocate not using a star diagonal while collimating. I have never followed this practice and prefer collimating the whole system at the same time. If your diagonal is so far out of collimation that collimating the scope with it doesn't give good results, you should consider discarding it and buying a new one.

Ok, you have the scope out, polar aligned, tracking and it's been outside long enough so that it's close to thermal equilibrium. I suggest a chair to sit in since the process is much more enjoyable if you are comfortable. Point the scope to a mag. 2 or 3 star about 45 degrees above the horizon and center it in the field of view. Slowly throw the scope out of focus until the star is about 10% the diameter of your field of view. The star should now appear like a donut - a white ring with a dark center. The dark center is the shadow of the secondary mirror and it should appear perfectly centered in the ring. If you make the image too big, the slightest adjustment of the collimation screws will move it out of the field of view and you will spend a lot of time getting it back.

Size of the image is not really important. You just want to make sure you can see the donut shape clearly. Let's assume the scope is out of collimation and the shadow is not centered in ring. The collimation screws on the C-8 are usually 5/64th inch allen screws. Pop the plastic cover off the back of your secondary holder if there is one and see what sort of tool you need to make the adjustments. If you have the usual allen screws, a good suggestion is to insert an allen wrench in all each of the screws. This helps avoid having to shine a light on the corrector to move the wrench from one screw to the other. Older C-8s had a central screw as well as the 3 adjusting screws. DO NOT loosen this screw! It holds the secondary to the corrector plate and if you loosen it, the secondary can fall onto the primary mirror!

Your goal is to turn the adjusting screws so that the shadow of the secondary is centered (as closely as possible) in the ring of light from the out of focus star. The important thing to keep in mind while doing this is to make sure the image of the star is centered in the field of view before deciding which adjusting screw to turn next. There are two techniques to determine which screw to adjust - you can either just start with any one and see what happens or you can have the person making the adjustments insert his hand into the light path in front of the corrector plate. You will see a shadow of his hand in the donut of light from the star. By comparing the position of this shadow with the angle of the off-centered secondary shadow, it is possible to make a good guess which screw should be turned.

The process of adjusting these screws should be one of SMALL motions! A small fraction of a turn can cause a large change in the appearance of the image. The three screws work in a push-pull type configuration. If you turn one and it starts to get a bit tight, you will have to back it off and turn the other screws in the opposite direction to get the movement you want.

Once you have the shadow as close to centered as you can, it's time to move to fine collimation. Slowly bring the telescope back into focus while observing the star image. Under good seeing conditions, you should see the central shadow slowly shrink and finally disappear as you approach focus. The shadow will be replaced by a small, bright diffraction disk of light surrounded by a series of bright and dark rings. These rings will grow smaller and smaller as you get closer to focus until they finally shrink down to the Airy disk and the first, and possibly, second diffraction ring of an in focus star image.

Take the telescope up to a fairly high power - something in the 250-300x range works well. Again, slowly rack the scope back out of focus until you can see the bright disk surrounded by a series of rings. Chances are these rings will be dancing all over the place with radial bright and dark lanes turning against them. You are looking at the results of bad seeing. If you can't get a fairly steady image of the rings, you will have to wait for a night of better sky conditions.

If you can see these rings and they are steady enough to clearly differentiate the bright center from the rings, you can progress with fine collimation. The process is identical to rough collimation, but you are now trying to get the bright central spot centered in the rings. This is more frustrating because the small motions of the adjusting screws will tend to throw the star's image out of the field of view and re-acquiring the star can be a pain. Try to keep the motion of the screws down to small enough increments to not move the star out of your field.

Once you have the spot centered, slowly rack the scope back into focus and, again, watch what happens to the star's image. As the rings shrink down to the 1st or 2nd diffraction ring, the rings should stay equally bright on all sides and remain centered on the dot. If the seeing will handle it, crank the scope up to the highest magnification you have. I try to perform this step with 500-600x and have used higher power when the seeing permitted. Examine the in focus star image and see if everything looks concentric. The Airy disk should be exactly centered within the first diffraction ring and the diffraction ring should be the same brightness all the way around. If the image doesn't meet these requirements, TINY tweaks of the adjustment screws will bring you to final collimation.

Here is an excellent page on collimating your telescope. It not only offers how-to information, but shows - in pictures - the effect of mis-collimation.

http://perso.club-internet.fr/legault/collim.html

Back to Celestron Homepage