Optimum length of visual back/focuser assembly C11

I have read references to having an optimum length of the visual back/focuser/eyepiece focal center assembly for an SCT, in this case a Celestron C11. It has been suggested spherical abberation can become an issue if the assembly is the wrong length. Can anyone elaborate on this or point me to an existing thread? I am considering installing a Moonlight CS SCT 2-speed focuser on the back of my Celestron C11 (screws right onto the large 3.5" threads on the back, makes a BIG hole for the light to get through, solves the truly awful focusing experience of the C11). My interest is entirely on visual, not interested in imaging.

Thanks in advance for any and all responses.

Oddog

Surely someone has something to say about this question...it's kind of embarassing to post a question to which no one responds you know. Oddog

It is easy to find out what is the level of error induced to this type of system. For 11" f/2/10 spherical-mirror SCT, OSLO gives about 1/20 wave P-V of over-correction added for each inch of focus extention beyond the optimum focus.

The problem is that manufacturers don't give
the information about where this best focus is, nor what is the actual level of correction. My guess is that best focus location varies somewhat from one unit to another, depending on the mirror match-up. And if your system is, say, under-corrected at the regular focus location, extending the focus would actually have corrective action (the opposite if it is over-corrected at the regular focus location).

So, it takes a bit of testing. If your unit is over-corrected at the regular focus, you don't want to worsen it by adding significant extentions (spherical aberration adds up arithmetically: if thescope is, say, 0.2 waves over-corrected and you add 0.1 waves by extending the focus, it nets 0.3 wave P-V error).

Vlad