“HFR drop” -> HFR increase
We can’t be sure, unfortunately. I agree with Jared that it is not as easy as it seems. We can treat the measured HFR values as samples from a random variable that has a variance depending on factors such as seeing, focus quality, the number of stars in the estimate, mount movement, etc. As such, comparing a single (noisy) HFR measure (the latest value) against the mean of the previous values will sometimes result in false-positive autofocus triggers. For large fields of view with a statistically robust sample of stars the HFR measures should be reasonably accurate but for narrower fields of view with few stars the measures will be quite noisy and thus autofocus triggers will occur even if the image is at best focus.
I’m not arguing that an “autofocus after % change in HFR” option would not be useful to have in SGP, but the problem of false-positive triggers needs to be carefully considered before implementing it. Perhaps something like comparing the last X HFR measures to the last Y measures, where X << Y, may help reduce any false-positive triggers.
ASTAP can be easily called to measure and return HFD and number of detected stars for the latest image download, so it should be easy to easily “roll-your-own” AF trigger if you can find a way to programmatically press the SGP autofocus button.