Quote:
Originally Posted by Paul Haese
Hi Lee,
don't you just love Newtonians? 
So this is some of the ideas I have used to find solutions to problems. - Check the rotation of the secondary to focuser. That is certainly one cause of tilt or elongation. You don't need to be far off to have this problem.
- Next check the collimation thoroughly. Both the above and collimation have huge effects on the appearance of tilt when combined with a coma corrector.
- Then check the actual corrector (threads etc). These suckers are prone to tilt and paying a lot does not guarantee you don't get tilt. Use a laser through the corrector to see if it is centred. I found it was not the case on the OOUK corrector.
Just remember this is going to be something very minor, only fractions of a millimeter that is causing this. |
Thanks for the advice Paul, it's much appreciated... gotta say, life was a lot simpler when I used refractors
I squared the focuser to the tube the other day... or at least I did so to the best of my ability. Looks like it was fine going down the tube, but off a bit across the tube.
After squaring the focuser I recollimated using Catseye kit. This time I did something different; I didn't use any of the thumbscrews on the focuser adapter whilst using the AC, but held it very tightly against the focuser adapter at all times. Doing this I was then able to rotate the AC with maybe the barest hint of "destacking" visible via the offset pupil. I've never been able to do that before, it usually unstacks significantly as I rotate it. This also confirms the AC was correctly placed at the focal plane, otherwise it would have unstacked as I rotated it. The unstacking I saw as I rotated it was at most 0.5mm and if I recall correctly the offset pupil magnifies errors 8x, which should mean the error was approx 0.06mm which I imagine is as close to perfect as you can get.
Given the above I'm pretty confident my collimation, both primary and secondary.
Now, please correct me if I'm wrong, but when I did the rotation test, I kept the relationship between the camera and the coma corrector unchanged, so if it were an issue with the coma corrector, wouldn't it have stayed fixed with respect to the image?
The one thing I can think of at the moment is tilt introduced by tightening the thumbscrews (from the focuser adapter) onto the coma corrector...
I'm not entirely sure how to check the coma corrector with the laser... can you elaborate on that? Note also that my coma corrector has no eyepiece adapter or anything, it's just got an (M42?) thread on it