I have a couple of questions about the corrector plate and secondary in a C8.
A few years ago, when loosening the threaded retaining ring for Fastar installation, the entire secondary instead came loose inside the corrector plate hole. I had to remove the corrector plate to properly tighten the secondary. I also found NO shims -- none between secondary and corrector and none between corrector and OTA. I also found no marks I found meaningful on the corrector plate. I was very stupid and didn't add any clock marks between corrector plate and OTA, and didn't even mark the 'out' direction of the corrector. Even more stupid was that probably this could have gone back to Celestron as the root cause was the internal retaining ring on secondary assembly was loose and the scope was under warranty when this occurred. Again I acknowledge the stupidity.
Anyway I reassembled by centering the corrector in OTA as best I could and likewise the secondary in corrector as best I could, leaving uniform air gaps around each. I measured between the OTA and secondary with calipers as well. I wasn't 100% certain the corrector plate 'out' side was the same as original. I expected to reassemble and then have to do some trial and error fine tuning:
- clock position of secondary within corrector
- if things were uniformly bad, try flipping corrector so inner face is out
- if needed, try clocking corrector plate within OTA
When I put it back together and collimated though, it was actually pretty good. No worse than it was originally, in my estimation. So I left it as-is and have continued to use it for a couple more years. I couldn't help but think at the time though that:
- the optics are pretty uniformly made and spherical, so collimation will correct any alignment problem
- OR the OTA isn't super well tuned off production line, so reassembly without fine-tuning yields same performance as stock OR
- OR I got really lucky (doubtful)
My questions are:
1. Is it possible for me to fine-tune the corrector/secondary assembly/alignment to get better performance? What kind of tools/procedure/information would I need to reassemble with best possible alignment of primary, secondary, and corrector plate?
2. Can I do a test to verify which side of the corrector plate should be out? I don't have an optical flat, but I do have lasers, straightedges, and a granite surface plate.
3. Is there anything I can do to tune the primary? It would be nice to reduce focus backlash and tune grease/bushings etc. for a perfect sliding fit
4. (maybe not related to above reassembly) I notice sometimes one one side of focus things tend to blur out vertically, and on the other side they blur horizonally. Collimating doesn't fix this. Other times the issue seems to be gone. I also have some chromatic fringes sometimes. Other times things are very sharp and crisp -- could this be differences in eyepieces? The primary mirror slopping around a bit?
Just looking to discuss this with people smarter/more experienced than me -- feel free to chime in.
Thank you kindly!