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Bert
A change of focus - due to deviations of the sensor plane with respect to the plane that is orthogonal to the optical axis - would surely manifest itself as changes to the FWHM. Further more, the shape of the point spread function will deviate and become asymmetrical as one measures it outwards from the centre of the image (noting that for an aberration-free optical system the FWHM should be the same for all point sources in the system regardless of apparent magnitude).
Surely one image suffices and does not require a Bahtinov mask?
Thanks for a thought-provoking post.
Mark C.
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