Astronomy
My 10 inch Mirror with a 1/48 P-V Wavefront Error @ 550 nm
Now the mirror is close to perfection.   In fact it is too perfect. When the exit pupil of the telescope exactly matches the entrance pupil of the eye, alignment becomes a critical factor. The slightest movement of the observer's eye, disrupts the alignment, making observations with the telescope tedius.
The best solution to this problem is to produce an oversized exit pupil in the telescope. This presents the eye with a little "slop."  A good way to do this would be to limit the maximum useable focal length of the eyepiece to 54 mm instead of 55 mm.  Then when a 55 mm eyepiece is used, the "slop" is produced. Thus: FR = 54" / 8" = 54 mm / 8 mm = 6.75
Instead of shortening my mirror's Focal Length from 80" to 50", I would be much better off to shorten it to 54". This requires a Radius of curvature of 108" and still makes my mirror almost 1.5 f stops faster than the F:10 used for most 8" telescopes.