Celestron C-150HD reflector

I own a Celestron C-150HD reflector, the discontinued 6 inch short tube with a spherical mirror and a built in Jones-Bird corrector on the secondary. I purchased this one as a challenge, and knew ahead of time that it is hard to get good performance out of these; but, it has been fun trying. This one still has issues on brighter targets, and it has trouble resolving images at magnifications higher then 100-150x. In short it's performance is worse then I would expect from an uncorrected 150mmx500mm short tube.

What I wanted to share with you is a odd effect that I have been noticing at the eyepiece. Even when collimated and properly focused, the scope tends to distort brighter targets like Jupiter, or Saturn; however, by moving further away from the plossl eyepiece (say 25-50mm - well beyond normal eye relief) the image seems to improve. Does anyone have experience with this, and can some explain why this seems to occur? I am wondering whether spherical mirrors favor one eyepiece design over another.

n2s

By moving your eye father away, you may be
expanding the exit pupil of the telescope, and
hence changing that way it interacts with your
eye. For example, the eye has some spherical
aberration at large pupil sizes, and you might
see some cacellation of spherical in the scope
by spherical in the eye, etc. You might also
see some vignetting of the telescope pupil by
the eye pupil, effectively masking-off the outer
area of the C-150HD optics. This is only
hypotheical speculation, since I have not tried
that scope myself.