The only significant number here is the distance from the focal reducer to the chip. For Celestron FRs, that is 105mm. Variability is only in the +_5mm range for optimal performance. Meade and other reducers such as the AstroTech have different backfocus requirements.
The backfocus stated for the C8 is dependent on the position of the primary mirror. The spec was originally set at the focal point using the stock SCT 1.25” visual back and the stock prism diagonal. This was the point where the focal ratio was 10 and the focal length 2032mm. As soon as you mount the FR on the rear port, the position of the primary changes in order to reach focus and the focal length is changed by .63x. But what is x? It is no longer 2032 because the position of the primary has changed. This is why platesolves focal length varies from what some think it should be. The SCT focal length changes within a range depending on the position of the primary.
Any attempt at forcing a specific focal length by varying the backfocus distance of the FR de-optimizes the system. Just set the length to 105mm and accept the final platesolved focal length for what it is.
Just the ravings from an old-timer! My feelings are not hurt if you do not believe me. (Lol!)
While what you say is probably very true, there are other considerations at play.
First, is focal length/reduction: by following the 105mm BF rule, my setup is reduced a bit above f/4. Framing is a consideration, one might not want such a "wide " field; one images with a SC for the long focal length after all. This is my galaxy/planetary nebula setup...
Secondly, there is the issue of off axis performance, and I can attest that the aberrations are considerably worse at 105mm back focus than at 85mm. Using an of axis guider is made difficult and the stars are completely deformed at 105, much improved at 85.
Again, all the above is true FOR MY SETUP and yours might vary wildly. My whole point since the beginning has been that simply recommending 105mm back focus to everybody "blindly" might not be the best piece of advice since it is clear that there are tolerances at play here and that it could induce problems for certain setups that might be difficult to understand/identify for a newbie who is following the "common knowledge" as gospel.
Not trying to be controversial here...Thanks for reading.
Edited by ERHAD, 23 April 2021 - 03:02 AM.