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I was going to add (Though I don't focus with a mask and never dived in to them enough to know what Stefan wrote about them) was that I would trust measured HFR more than induced diffraction spikes. Smallest stars = best focus.
Very few auto focus routines use a mask, they almost exclusively measure the HFR of a star or multiple stars in the field to determine the best focus point. The only focus assist I have seen use a mask is in APT (Astro Photography Tool) but when I was using APT I didn't have a mask, so I didn't test that function.
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