By the way, your software is certainly good. I cut my teeth with BYEOS. It is so nice to just punch in a RA and DEC and then go to bed.
Stars are focused. Center is best and gradually worse as you extend to the edge. This is totally expected from the description of the focus algorithm.
The edge of my field shows partial "donuts" almost like a bunch of Cs.
But I think it can be better. (unless there is something in your algorithm that puts more statistical weight to the stars halfway between the center and edge)
By excluding the middle, more stars will be in better focus and the out-of-focus stars would be less out-of-focus.
Looks like a pretty easy change with a large return.
I would think copying the "crop auto focus frames" routine and modify it slightly to crop the middle could be done. Perhaps the focus algorithm takes this into account by weighting the middle part of the image higher. This would attain the same thing.
Here is a screenshot that shows the improvement (I'll also link the speadsheet below). The simulation shows a bunch of stars with the same magnitude. The left one shows the HFR after a focus run. The right one shows the same thing except not including the center part in yellow with the algorithm. There are more 1s and 2s in the right one (mode=3). The maximum is lower and also the stdev is lower. The left one had a mode of 6.
https://www.dropbox.com/s/aw4tnnf9zdnpdfn/FocusSimulator.xlsx?dl=0
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