Okay, it's clear to me now that "collimation" her just means misalignment, still don't understand why/how that became the case, but so be it.
Yes, there's nothing you can do about trefoil since that is not an alignment error but a mounting error. Also you won't measure it with a 120 deg symmetry anyway. I have no experience with SCT's, but yes, if you can only adjust the tilt of one mirror then you can't correct both astig and coma. Astigmatism comes from attempting to correct for coma with the wrong motion, for example trying to tilt coma out when the coma was due to a decenter error. I guess my point was that you can see both coma and astigmatism with this type of mask whether or not you can correct them both.
The big reason I brought this up, btw, is because I am polishing up a paper for publication about using the Bahtinov mask to measure collimation (in the parallel rays sense) so I've been looking around to see what else might have been looked at with the B-mask and collimation (again, the parallel rays sense).
I assume that when you say " you can see astigmatism" is when you look at the effect of the diffraction pattern from the Tri-Bahtinov inside and outside focus. I can't see why you can see an asymmetry in the Tri-Bahtinov spikes when you are focused which is where it is normally used.
Do you mind posting a link to your pre-print when you are done? Hiding among us here are mathematicians, physicists, optics engineers and advanced amateurs who would appreciate (or criticize ) your insights.
Edited by cytan299, 10 July 2020 - 07:32 AM.