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This is getting interesting!
Steve000, not sure what the point of your image is. No offense intended, but in that image, it looks like Betelgeuse is saturated so I'd be surprised if you can get a reasonable FWHM reading.
Anyhow, Peter has clarified he's not trying to focus using Betelgeuse, but a fainter star, so OK on that score.
Peter, sounds like your question is specific to the Maxim software you are using. I use the Artemis software, so I can't give specific instruction, only general advice.
What works for me is to use a star that is below saturation (40000 counts is probably still OK) with a short (say 2s) exposure time, then adjust your focus so that FWHM is minimized. At that point, peak ADUs will be maximized.
So you need to work out what quantity your software is reporting, and optimize for that.
-Ivan
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