Optimal Back Focus for C14

I've heard from Celestron that the optimal back focus is 4" behind the mirror cell. This makes it inaccessible using a rack and pinion focuser and a 2" star diagonal (which itself takes up about 4" of back focus).

How sensitive is the system to changing the back focus?

Has any one experimented to see if there is a change in the quality of the image with changes in back focus?

My thought is to fix the mirror and use a Feathertouch focuser.

Dan McConaughy

Hi Dan,

Much academic discussion has occurred on this subject over the years, but I think the difference in image scale is the most obvious effect. I have an old 1965 C10 which at f15 is essentially smae focal lenght as the C14. It had a firror flop problem which caused me to add a WO SCT crayford which is fairly long. I might detect some slight difference in the secondary shadow breakout on a star test, but it still deals with planets and lunar detail superbly, and in focus star images are still extremely sharp. At 14" aperture on yours, the degradation will be far less impacting than the atmosphere, so go ahead with the setup is my recommendation.

Dave

For a C8, I once calculated the depth of focus is about 8 inches, i.e., 1/4 wave of spherical abberation for a perfect system is generated by moving the focus from nominal +/- 4 inches. The C14 should be similar. Since no system is perfect, it implies that the best corrected position could be found by trial and error using the star test.