Subframe for autofocus

I have been using SGP for a year now and have had little trouble with focus until tonite.

The reason is that I am imaging a globular cluster tonite and have not done that with SGP before.

It appears that autofocus is massively confused by the dense cluster of stars. I have tried various settings
of the sample and nebulosity filters to no effect.

It occurred to me that if one could select a portion of the image away from the offending object (subframe) for
autofocus to use, it would fix this problem.

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Or could the focus and target position be written into a sequence?

I think you mean focusing in one FOV and then slewing to another FOV with the target in it. If so, that is the way FocusMax/ACP does it but they also need a “good” single star. My take is that involves too much mucking about and I was not fond of that method when I was using ACP. Focusing in the target FOV like SGP does is much better, IMHO.

That is why I suggested a user-selected subframe (and this selection mechanism already exists for “frame and focus”) and allowing the user to use this subframe during autofocus. As long as the subframe was nowhere near the object that is causing the focus to fail, dithers and such should have little or no negative effect and the user could select a region with an appropriate level of usable focus stars. The subframe would need to contain enough stars to be valid but this would be up to the user and would be better, in any case, than having an object in the focus region that causes fails.

It was pretty bad last night on two globulars (M3 and M5 at 1000 mm with my STT-8300), nearly impossible to get a reliable autofocus regardless of what I modified. I tried modifying pretty much everything one can including sample size, nebula filtering, filter used for focus, and exposure time. It maybe gave decent focus one out of 5 times and it was not predictable or reliable. As soon as I moved the cluster just out of the FOV, it started working great again so I know that was the issue.

Of course one could also take a reverse approach and allow the user to select an “exclusion zone” where SGP would not look when doing focus, but it seems more logical to use the existing subframe mechanism.

Yes - that’s exactly what I meant. The benefit of slewing off is that you would then get the whole field of view to calculate the HFRs (which I assume) would give a more robust calculation than limiting the fov to exclude a cluster…

At the expense of the added time for the slew, the added solve and synch to get back into place for imaging, and the added complexity of involving PhD (to relocate the guidestar) and the mount and maybe it’s software (such as for Paramounts). Since those are the things that typically go wrong, it is asking for trouble, IMHO. Been there and done that with ACP/FocusMax and it is more trouble than it is worth even if it is theoretically a bit better.

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I’m facing this exact problem now. SGP picks part of the galaxy as a starfield and miscalculates the focus position. I would like to either exclude this troubling part or select a subframe where there would be stars to be used by the autofocus routine. I have been away from the forums for quite some time and I was wondering if any solution to this problem has been found.