Quote:
Originally Posted by multiweb
My reasoning behind this is that the light cone reflected by the primary will be "cropped" by the secondary and not totally reflected so although I'll get less light I'd be using the better part of the primary (no the edges?). Does this sound right? Any toughts? Obviously the scope is a newtonian. 5"/FL650mm F/5. I mostly image at prime focus with a QHY8 [which is a faily large chip] and a 2" nose piece. Thanks for any pointers.
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You shouldn't be getting any objectionable coma at F5 and an MPCC.
Moving your primary forward will just increase vignetting not reduce the size of the comatic image. Coma is a function of F ratio.
Just put an aperture stop on your scope. Put a 4.1" aperture in front of your scope to make it F6, but experiment with F5.5 also. Remember that as you stop the scope down you'll be decreasing the ratio between the secondary and mirror diameter which will increase vignetting in the field ( which I guess you can fix by 'flat fielding' in image processing ) .