I've owned this Orion SkyQuest XX14g for a couple of weeks now, and it's really been a great performer. Optics star tested well. I posted in a previous thread that I needed to modify it to work with a MaxBright II Binoviewer / 2" 1.7X Glass Path Corrector / Coma Corrector. I ended up getting longer collimation bolts and just moving the mirror 15mm up the tube. All of that went well, and reassembly was a breeze. The problem I'm having is that now that it's reassembled, the views last night were wildly inferior to everything I've seen with it so far. The optics exhibited a strong astigmatism, clearly changing direction 90-degrees inside and outside focus, and stars at any magnitude above 200X did not come to sharp pinpoint focus. I realize there are several possibilities for what the cause is, but I'm hoping for more input from everyone!
Here are my theories:
1. Collimation: I believe that collimation, if poor enough, can cause a reflector to exhibit astigmatism. The problem is, I really don't believe the collimation was bad at all. I've been using passive tools and a laser long enough to be able to get everything pretty close, and that was no different last night.
2. Cooling: I know that last night was significantly cooler than the last two weeks around here, so it was a larger temperature drop than anything I've had since owning the scope. Still, the mirror had been outside acclimating for 2+ hours before I decided to star test it. I did not have any cooling fans on it.
3. Mirror Cell: This scope has the conical-back mirror that attaches to the cell via a fastened threaded central cylinder. There is a small amount of play between the attachment cylinder and the mirror cell, and before I removed the mirror it was loose enough that the mirror could move laterally by up to 1-2mm. When I reinstalled the mirror, I snugged the system up enough to remove that play. There's no correlation between the torque on that attachment system and the mirror itself, as it's just fastened to the top of the attachment cylinder and not impacted by how tight you put it in the cell. I'll try to get some pictures of this, but I don't think it's the culprit. In fact, once I tried other remedies, I loosened this retaining ring so that the mirror again was able to move, slightly, and it had no impact on the astigmatism.
I'm leaning towards insufficient mirror cooling, and then maybe collimation errors, as the culprit here. The only thing I forgot to try while I was out was to rotate the primary mirror and see if the astigmatism rotated with it. If the astigmatism rotated, I assume this would have helped eliminate collimation as the main problem.
Also, because of that small 1-2mm play in the mirror to cell attachment, it was really hard to try to get the mirror perfectly centered. I stressed about that awhile, but then remembered that it was loose enough to move around before I ever touched it and had star tested very well. I don't know how important that 1-2mm play is, and I also don't have a great plan for perfectly centering the mirror anyway.
What have I done? I appreciate any input on helping me get the optical performance back to what it was 3 days ago.
Edited by austin.grant, 26 November 2020 - 10:42 AM.