As well as autofocus works for most systems, it has some challenges for long FL systems. This is true particularly during galaxy season - with galaxies having bright features. I have been struggling for several nights now to image M82 at 3910mm FL. For nearly every run, it starts out just fine, but once the central part of the galaxy comes closer to focus, the AF routine finds these features and thinks they’re stars. Inevitably, the HFR assigned by SGP is substantially greater than that for actual stars. This gets complicated by the sparse star background where there aren’t enough actual stars to dilute the contribution from the galaxy parts. I see two potential solutions to this. First, users could define an “exclusion area” for which SGP will not try to find stars. A more elegant approach, IMO, and one that would probably improve AF overall would be to allow outlier rejection. This could be a user-selectable number of sigma units and should be relatively easy to implement (in theory).
For now, I have to turn off smart focus since the center point is frequently higher in HFR than the two neighboring points, causing the AF routine to think something went wrong and invoke smart focus. Often, this causes the focus to get hopelessly lost. This means that almost every AF run results in a bad fit and SGP resorts to a weighted average of the lowest HFR points. This is not good, and I typically have to reject 30 to 40% of the subs for bad focus as a result. At f/11, every sub counts. No tweaking of settings results on any improvement. Outlier rejection would allow me to reject the artificially high HFR galaxy parts and result in successful focus.
Your consideration is greatly appreciated.