The effect of an off axis stop is an interesting one. The star image size will remain the same (assuming seeing limited) so the maximum theoretical resolution is unchanged. The defocussing effect of field curvature should be less due to the increased depth of field with the higher focal ratio and the chromatic coma should also be less with the less steeply convergent beam. I did suggest this to someone recently though in a similar situation and they reported no obvious improvement. Best approach I would say is to see what you can get on the existing setup and then try a stopped down aperture. (What spacing do you have currently?)
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