But surely if orthogonal alignment is off, then the PSF will not be circular and will appear coma-like - depending on position from centre of image - hence visual cue (I acknowledge that FWHM alone does not allow for correct interpretation of deviations from orthogonality).
What I suggested will probably require more software assistance than looking at a Bahtinov pattern - but I still feel that one image without a Bahtinov mask provides the necessary information. (I'm curious - maybe even ignorant - how can you correct for orthogonal deviations?).
The "inverted" bit requires system-specific interpretation and doesn't discriminate between your or my perspective on possible approaches - it applies to both scenarios.
Mark C.
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