Posted 20 May 2013 - 05:47 PM
My 12" gives very sharp and contrasty information, however, it does not give a nice concentric set of rings on both sides of focus. I have had many CATS in my day, and have been blessed with perfect collimation all around. I know that I should not be concerned with this, as if it works well, it is better to just leave it. But as our Uncle Rod says, there may be "mo' better" in the telescope, so why not try to at least understand what is going on.
The difference in the pattern is not huge, but it is definitely noticeable, the off side being easily regarded as horrible collimation to an experienced user, a circumstance that we would find intolerable. It can be one or the other, it does not matter. I favor collimation on the CCW side, forcing the mirror up, and this is always the one I check and/or adjust. It is guaranteed that the other will be out. So it's either perfect intra or perfect extra. Never both. But as I sit here staring down the "barrel" of this very nice instrument (I say this because I do not want to open her up and contaminate things) I try to wrap my mind around the fact that (in a perfect CAT,) there is a traveling focal point that moves with the mirror, depending of course, on what eyepiece is being used/where the mirror is at a given time. Got that. But if this is the case, how then, is it that the optical system, which is obviously out of alignment, is finding a perfect point of focus down its entire path? Wouldn't there be just one point of focus? The point where the path is no longer linear? The point where that line veers off?
Is this the mark of an inferior telescope? Or does it matter, as long as you are able to have perfect alignment in one particular spot? Because if the optics are aligned in that spot, and they obviously are, is it important to have perfect alignment in other places, even so close to the focal point? I have always understood that the alignment of catadioptric optics do fluctuate a bit because of this huge mirror moving back and forth as we focus and as the instrument tilts about its mount. But the misalignment is negligible, and should not be this obvious.
My first thought is to turn the secondary in small increments while observing a star in collimation mode and finding better position. The outer secondary housing, however, is snugged up, and I just do not feel good about messing with it, unless I get some feedback strongly suggesting I do.
I am very happy with this telescope, but it's just one of those mysteries that I would like to solve.
Thanks a bunch, Chris
Posted 20 May 2013 - 08:13 PM
Have you tried using Metaguide to test the collimation. It may be an impartial arbiter for you as I know how frustrating it can be to determine whether or not the diffraction rings are concentric or not especially with skyglow, poor seeing and vibration!!
Posted 20 May 2013 - 08:57 PM
If the change is related to the center of the secondary shadow position, the other potential problem is that the secondary mirror is not centered over the primary.
Next time you have the scope out, aim at a bright star and defocus so that only a couple of rings are visible.
Ignore position of the secondary shadow in the Fresnel pattern for now.
Instead, look carefully at the very center of the secondary shadow.
You should see a small point of light. If you do not see it, try moving a bit closer to best focus.
This point of light is called the Poisson Point and it is a diffraction artifact.
Now pay careful attention to the position of the Poisson Point with respect to the ring immediately around it to see if the point is centered in the inner bright ring.
It does not matter at all about whether it is centered in the outer ring. You want to see if the point is centered only in the bright inner ring.
If the Poisson Point is not centered in the inner ring, this could be an indication that the secondary mirror is not perfectly centered over the optical axis.
This may also give the symptom of having a scope that is in collimation at best focus but be out of collimation on either side of focus.
What happens is that when you collimate in this condition, you are actually collimating on a point on the secondary that is not exactly in the center of the secondary.
Now it seems counterintuitive, but the scope can actually give a very high quality view at the center of the field of your eyepiece, but what occurs in this condition is that the focal plane is tilted. When you achieve collimation at perfect focus at what is slightly away from the center of the field, but because the secondary mirror has to be tilted off in one direction to do this, the rest of the focal plane is tilted with respect to the field stop of the eyepiece.
I cannot guarantee that this is your problem, but the symptoms you report are consistent with this condition.
If you do find the Poisson Point to be uncentered, then you may want to experiment with trying to move it.
For this, do not use a diagonal. Use an eyepiece on a bright star.
If the point is high and right in the inner bright ring, then you move the corrector/secondary towards low and left. In other words, the Poisson point shows the direction of the displacement.
Over the years, I have helped at lest three people on this forum with this condition and all have reported that the collimation problem was related to an un-centered secondary mirror.
Please understand that I cannot guaranty that this is your problem and that if you attempt to move the corrector or secondary, you do so at your own risk. Mark everything so that you can return it to its original location if you don't resolve the problem.
Posted 21 May 2013 - 04:50 AM
It sounds like the secondary may be slightly off center - but that may not matter much as long as you collimate based on the in-focus star appearance. If you collimate just based on "centering the donut" then you may actually be putting it out of collimation for when it matters - at focus.
If you don't like the aesthetic aspect of the donut not being centered then you may have to adjust the optics - but if you are just concerned about the image quality then normal collimation may be fine. For that I would recommend MetaGuide, which I wrote, since its whole purpose is to allow collimation of an actual star, in focus, based on the appearance of the Airy pattern.
When collimating a cassegrain you can use the appearance of the "donut" for rough collimation, but for fine collimation you want to use a high power view of the in-focus star - and its Airy pattern - for final collimation.
If you have an SCT with an aspheric secondary, rather than spherical, there may be more need to get the secondary centered - but for spherical secondaries it is less imporant because any offset of the secondary can be largely compensated for by a small amount of tilt that will avoid coma on axis. There is little need to have the donut perfectly centered, in that case, inside and outside focus.
Posted 21 May 2013 - 09:54 AM
Thanks for the link, but it's more of a physical problem than anything else.
And I got your PM on that other thing, thank you very much. You are correct when you say that buying and using new stuff is one of the fun parts!
Posted 21 May 2013 - 10:22 AM
I hoped you would contribute since you are the one person who could figure this out.
I recall bending over awkwardly in one of my first tests in trying to solve this mystery. The straight-through view exhibited the same problem.
I am going to print this out and get to work when I can. Taking the secondary out, or any other part is not a problem at all. I have been tinkering with LX200s for a while (just never came across this particular problem.) If I seem apprehensive, it's only because the inside of this tube is perfectly clean and I wanted to keep it that way.
I can't see any abberations in the field, in any direction, but I never specifically looked for them. It makes perfect sense that the mirror is tilted or not centered, which is what I suspected, but needed confirmation of. What you are saying is that the center can be (and is) perfect, but the outer field is not. I will check it carefully. If it does not bother me, which is unlikely, because I am as sensitive as you are when it comes to flatness of field, then I will leave it. But I will probably take the plunge anyway. Now I can clearly envision the problem riding up and down-and not changing. This is the dilemma, I can see it! (I actually think I had a mental block when it came to the thought of contaminating the tube.) Maybe I can get away with not removing it-just turning and manipulating.
So it's actually, the more I think about it, the centering, not the tilt. Got it-
This is exactly the response I needed, thank you!
Posted 21 May 2013 - 10:30 AM
I am going to go back and check out your Metaguide more thoroughly. When I perused it after Al suggested it, I got the impression that it was to be used on perfect optics and would not help in my instance, because my problem is physical. But now I realize that using the Metaguide AND a screwdriver will be necessary.
What you say also makes perfect sense and I thank you very much for your input.