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With any optical system the quality of the image always deteriorates as
magnification is increased, not too noticeable at lower powers, but very
noticeable as you approach the maximum power that the optics, and/or/
the observing conditions allow.
Firstly, SW mirrors are arguably among the best mass produced mirrors available, although it is always possible that you have a lemon.
Resolution is a tricky one, are you talking about resolution or sharpness?
The larger the aperture of a scope, the more vulnerable it is to
atmospheric disruption of the image, so that at times a smaller scope will give a superior image to a larger one, which is possibly what is happening when you stop yours down. The theoreticall resolution of a scope is purely a function of its aperture. In perfect viewing conditions a larger scope will
always out resolve a smaller one.
You will also find that focusing is easier as you stop it down. It can be
quite fiddly with really fast scopes like f/4 or faster.
Coma has no effect on the image other than the observable distortion of
the stars in the outer portion of the F.O.V.
A bit of over correction should have little negative impact on the image.
Conditions are pretty poor where I live, so I rarely go over about 250x,
but very rarely I have been able to go over 300x.
raymo
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