Posted 21 February 2013 - 09:08 AM
First, the scope doen not appear collimated.
However I believe that the flaring you see is not an artifact of mis-collimation.
Mis-collimation causes coma with is an asymmetric abberation. Your in-focus image shows a radial projection from one one side of the Airy Disk htat is off center in the in-focus image. This is a diffraction artifact.
Also, based on the fact that the tiny point at the center of the secondary shadow is not centered under the secondary shadow, I think the problem with our telescope causing the flaring is that the secondary is not centered.
Have you had the corrector off? Has did you install Bob's Knobs and somehow knock or bump the secondary mirror out of position?
In the defocused image, the tiny point with the flares coming of of it is called the Poisson Point. It is a defraction artifict that appears in all obstructed telescopes when the image is defocused.
When the secondary mirror is centered exactly over the center of the primary mirror, the Poisson Point should appear to be very near the center of the secondary shadow.
If it is not perfetly centered, it can cause the Poisson Point to appear away from the center of the secondary shadow.
This has nothing to do with the concentric alignment of the outside of the shadow and the outside of the Fresnel Pattern. The scope can be out of collimation but the Poisson Point should still be a the center of the secondary shadow.
Notice that the flares are arcs. You can easily see this. This is to me looks like some kind of diffraction, and if it were from something straight, they would appear straight. But they are curved, and that indicates that they are being causeed by somehting curved, and this is most likely the light that is grazing the secondary and primary baffles because the light from one side is hitting the edge of these baffles on the way to the focal plane.
I could be wrong. It happens. But I think your primary problem here is that the secondary mirror is not centered over the primary mirror bafffle.
Check to see if the secondary is loose or has moved (or the entire corrector). Just grasp the secondary mirror housing and tug it in different directions. If it moves in any direction with even a little pressure, it has most likely shifted.
If you removed the corrector or if you find shift, then you have to fix it.
The good news is that it is very easy to adjust centering.
Remove the diagonal and put in an eyepeice and focus on a star a comfortable altitude above the horison.
If the spot is high and right, move the secondary low and left. Just push the seconary in a manner that would push the Poisson Point back to center it. Very simple.
But the bigger question is why did it move to start with.. Is the secondary mirror loose, or did the corrector shift?
If you can't easily determine this, and the scope is a new scope, the easiest thing to do would be to return it. Normal user collimation would not cause the Poisson Point to move.
If the corrector has slipped though (dropped in shipping), you can re-center simply be loosening the corrector retaining rings and shifting the corrector in the direction required.
Anyway, your scope does show mis-collimtion, but it is also showing what I believe to be a decentered seconary mirror.
The Poisson Point not being centered in the secondary shadow (that is the way it looks to me) and the curved diffraction spikes both point to this condition.
This is not a serious condition from the standpoint of the adjustment required to correct it. I have recentered several seconary mirrors over the decades, but the quesiton once again is "How did it get decentered?"
If the secondary is not centered, you get some weird behaviors. Most notably, if you collimate on one side of focus and check collimation on the other side, you might find that the secondary shadow will show miscollimation the other side. Collimate on that side and move back, and now the first side will show perfect collimtion.
If you collimate in an in-focus star, you can get it collimmated, for that eyepeice, but you still have the diffraction from the edges of the baffles.
I could be wrong of course. It happens. But the image to me along with my own experience with the behavior of an uncentered secondary mirror point to this as a component of your problem.
My advice is to try to determing if the scondary really is decentered and fix this first if it is. Otherwise, as you may actually be experiencing, collimating a scope with a decenterd seconary is a weird thing to do because it will seem impossible to get the socpe collimated for concentric patterns on both sides of focus.
If it is a new scope, consider returning it if you feel uncomfortable moving things like this. The Secondary Mirror should NOT decenter from normal collimation. It was either shipped that way, or it was jarred out of position during shipping, or if used, the previous ownwer (or perhaps the current ownwer) made it move.
Let us know what you find out.