I’m very interested in the focusing ability of SGP. In particular how it manages to focus without slewing away to a focus star. I understand that it looks at FWHM values over the entire image which i principal sounds like a good approach. However, I can imagine there might be situations where this might have complications such as a naturally curved field of a refractor without a field flattener, or imaging a subject such as a globular cluster dead center. Does SGP have any way to compensate (if necessary) for such situations?
Yes, you can crop the analysis area of AF frames if you’d like… this can help remove uniform portions of the image from consideration of the mean HFD calculation (although it probably doesn’t matter much… even stars that show vignetting, still have a smaller HFD when focus is better). I have several test runs with M13 at about 750mm FL and SGPro has no difficulty with regions like this. Difficulty might occur at higher FL for DSOs like globs though. You might be able to use the nebulosity rejection and manually override the number of stars used for the calculation to your advantage here. So many permutations… we cannot test them all.