I bought a used C90. Star testing at a 0.6mm exit pupil (138x) shows fairly good diffraction pattern. Rings look concentric. As it is brought to focus, when there are only one or two diffraction rings visible, the light collects at one point on one of the rings. Bright spot on one ring gets brighter and brighter until it reaches focus. Going through focus, the light stays collected at that point, and the diffraction rings open up again, away from that point of light.
Is this a collimation error, or a different aberration? There are 6 collimation screws on the back mirror.
Planetary views at 138x are OK. I can see the Cassini division and some cloud bands on Saturn, and several cloud bands on Jupiter. Going up to 208x, the image significantly breaks down. Images from an 8" Newtonian at the same time are fairly good, so it is not poor seeing. I understand the resolution of a a 90mm Maksutov should be less than that of an 8" Newtonian, but I have read others get sharp views up to 200x and beyond on a C90, and wonder whether there is a collimation error that could be improved.
I have tried the star test without the diagonal and see something similar.
Assuming good seeing, what would be a reasonable maximum magnification for a 90mm Maksutov for planetary detail?