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C8 optimum back focus

Started by drgenovese, 09/22/2010 10:17AM
Posted 09/22/2010 10:17AM Opening Post
Rod- do you have handy the design spec for optimum back focus for a C8 for the correct mirror spacing to get minimum spherical abberation. As I recall it's 5" from the rear cell threads but I can't find it. By the way, It has long been my contention that when people test their SC's correction they NEVER confirm that they are at optimum mirror spacing. I thought mine was mearly good (about 1/6 wave) but a few months ago in excellent seeing I was useing a 2" rear tailpiece, I think a Teleview 2" diagonal, and a TMB 4mm eyepiece and I checked collimation and the star test was PERFECT!!! By chance I found the perfect spacing. I think it is much more critical than most people think. Thanks, Charles
Posted 09/22/2010 04:09PM #1
Correction- I was using a Celestron 2" SC star diagonal-makes perfect sense.
Posted 09/23/2010 04:51AM #2
Charles R Genovese, Jr MD said:

Rod- do you have handy the design spec for optimum back focus for a C8 for the correct mirror spacing to get minimum spherical abberation. As I recall it's 5" from the rear cell threads but I can't find it. By the way, It has long been my contention that when people test their SC's correction they NEVER confirm that they are at optimum mirror spacing. I thought mine was mearly good (about 1/6 wave) but a few months ago in excellent seeing I was useing a 2" rear tailpiece, I think a Teleview 2" diagonal, and a TMB 4mm eyepiece and I checked collimation and the star test was PERFECT!!! By chance I found the perfect spacing. I think it is much more critical than most people think. Thanks, Charles

It's always been stated (by Celestron) as "distance to the film plane of a 35mm camera using a standard T adapter. That's maybe a little closer than 5-inches, but the difference as regards SA would likely be insignificant.

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