Alternative Autofocus trigger suggestion


#1

This was previously buried in a Temperhum discussion but I think is worth considering as a feature request

One of the autofocus triggers is a frame count. That’s kind of arbitrary. An alternative consideration, linked to event image parameters, is to trigger if the image HFD degrades by a certain percentage (I sometimes monitor the very useful image history box and pause and do an AF if the HFR starts going up).


#2

Generally a good idea.

The other thing that can cause HFD to deteriorate (and is probably at least as common) is deteriorating seeing and the often attendant poor tracking plus “seeing caused atmospheric de-focus”. That is something that can’t be fixed short of laser star AO so I wonder if there is some way to eliminate seeing/tracking from the picture. In other words, how does one separate crappy focus on one’s system from crappy skies?

Maybe if the HFD has gone up, look at the RMS tracking error and if that has also gone up, then do not add another focus but if RMS tracking has not gone up (but HFD has) then do the focus. Since guiding is on a centroid, I suspect tracking would not be affected much by focus until it got way off but that should be looked at.

One could also compare the new (triggered) focus HFD to the previous HFD (that triggered the refocus) and if it did not improve, one could assume that it was not focus that was the issue and proceed accordingly.

OK, that does add a lot of complexity but I figured that the skies parameter should at least be pointed out.


Autofocus trigger based on FWHM (or HFR) of previous image
#3

Interesting. That potentially ties-in to the question that just came up in a separate thread of SGP getting guider RMS error information from the guiding app.
Andy


#4

I really like this idea, although I realize there could be pitfalls. I’m sure it is temperature and cool down related (although my scope is in my garage, so it is pretty close to ambient on setup) but have noticed my focus seems to drift less as the night progresses.

Aside from the SGP log, is there any functionality to review AF history? You could use that to help get temp offsets but also correlate other factors.


#5

Unfortunately there are a couple pitfalls with this approach… As mentioned, temperature is one, but more commonly altitude. HFR changes with altitude. Issues I see:

  • It is possible for HFR to stay within band and be out of CFZ. This is especially true if the baseline HFR is captured with the target at lower altitude. The same HFR, but higher in the sky, may not be in focus at all.
  • This is a complicated user setting… we try to avoid that when possible. It would be difficult to calculate the “bounds of HFR” with respect to CFZ and I think we would spend a lot of time supporting this.

Autofocus trigger based on FWHM (or HFR) of previous image

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