I'm so glad I have found this thread as I have been battling with back focus issues ever since I bought new my Celestron F6.3 Reducer/Corrector for my C8.
My C8 (XLT) was bought new in Sep 2020 and the Celestron Reducer Corrector F6.3 #94175 (made in china) was bought new in Dec 2020. All my measurements are taken with a vernier calliper and from vendor manuals.
I get perfect round stars in the center with very good stars from halfway from center to edges when using my Nikon D80 (APS-C) or Nikon D600 (used in either DX/FX mode) with the Celestron T-Adaptor-SC screwed to the back cell with a Celestron Nikon (m42) T-ring. This setup is Camera (46.5mm) -> T-Ring (8mm) -> T-Adaptor-SC (50mm) = 104.5mm
When I keep the image train the way it is above and just introduce the celestron Reducer/Corrector my FOV has increased (as expected), but my stars from halfway of center to edges are so egg shaped it makes me wonder if this is a corrector or not. In fact the stars are worse in most of my FOV with the reducer in place. I have the warp speed look but not consistent in all corners.
What I have done
I have spoken to the shop I bought his from and they say its a back focus issue and said I need to have 85mm from the reducer rear lens (towards camera). From that time I have bought many M42 and M48 extensions and adaptors to give me from 50mm through to 115mm. In all my efforts I can not get round stars for 30% of my FOV and get better images without the corrector.
I understand using an APS-C or full frame camera will give me vignetting, but I dont expect it to give me worse egg shaped stars through my FOV.
As of recently I have added the Baader 2" SCT click-lock which after I measured is 47mm (with reducer in place). As this is in place I cant go any less than 102mm from the rear lens of the reducer.
This setup is Camera (46.5mm) -> Skywatcher M48 T-Ring (8.5mm) -> M48 3mm extension -> 2" Click Lock (47mm) = 105mm
If anyone can provide some assistance to help me out it would be appreciated as I'm kind of annoyed that I really cant use this scope with the reducer and its now almost just used for planets on a different setup.