I would like to share my recent tests with autofocusing a FSQ85 at native focal length.
I have posted in a couple of threads recently documenting the problem with autofocusing my FSQ85 as the best focus by HFR is different from best focus for round stars. It is a documented problem with some Tak and the standard takahashi response is to focus 2/3 of the way from centre. However this is not currently implemented by SGPro. A feature request has been previously lodged for user selectable subframe focusing as a potential solution.
Last night I had clear sky with relatively full moo so I performed more testing. When bin 2x2 previously I found that I needed a step size of at least 15-18 to get consistent V curves but I know that these step sizes are larger than the CFZ of FSQ85 (by calculation it allows 5 steps for my setup). When bin2x2 the best focus by HFR is always a few click out from best focus for round stars. When I critically analysed the images manually, the best focus images actually looked quite good with round stars at the corners. In fact the central 3 AF samples all show round stars at the corners. However when actually capturing at bin1x1 the ovoid corner stars reared their ugly head.
Last night as I last resort I tried autofocus bin1x1. In doing so I can reduce the step size to 8 which is about half what I have been using. In doing so I can see that the stars go from radially outwards thru to round to circumferential about centre, this time with best focus coinciding with round corner stars! However at bin1x1 I needed 10 second exposures even with lum filter and it was very very slow. Best results also needed to reduce the min star size to 2 which I suspect also slowed things down. I seemed to be able to get good results with a few different targets using these settings with a Lum filter so I am happy.
The same thing does not quite hold for colour filters. I suspect that has something to do with the longer duration needed to get all the corner stars (which are usually dimmer for most targets) included in the calculation. I increased exposure lengths to 20 seconds and the results were still not consistent.
Anyway I am happy at least I can get good auto focus with Lum filter and I may be able to use offsets for the other filters which is probably needed anyway otherwise it may take 15 minute per autofocus run every filter change. I still think user selectable subframe focusing (not just excluding the centre) is the way to go as I can continue bin 1:1 (which seems to be the essential ingredient for success) but have the subframe at the “worst” corners only to speed up the image download and preferentially optimise the corner star focus.
Next time I will try to use half stepping to see if I can get finer granularity in the step size to optimise the focus even further.
I should also add that having guiding on ruin the autofocus run (I use OAG). So long exposure AF exposures are no go beyond 10-20 seconds for my set up.