Autofocus trigger based on HFR % change

This would allow refocusing when it’s really needed rather than guessing how much temperature change or elapsed time will cause the need for a refocus. It would seem fairly easy to implement since SGP already calculates HFR.



I requested this a couple years back and it was turned down. Good luck.

I read the previous thread on this topic. @Jarod, your point is that HFR cannot be relied upon in all situuations. While true, that is also true of using a temperature trigger or time trigger. Depending on where you’re getting your temperature reading, the air temperature can change quickly while the equipment temperature can lag far behind. A time trigger is only a guess, and can have you focusing when it’s not needed. Any trigger method is not foolproof.

It would seem like the best approach is to give users the option to use the trigger that works best for their situation and their gear. Adding HFR as a trigger would be fairly simple to implement and would allow users another good option for triggering focus.

It would, of course, be good to offer a little more customization to make this measurement more robust and less subject to random variations in HFR. For example, you could allow the use to set a % change, and a number of images/measurements for that change to have occured before triggering an AF. This is similar to your dither settling where the user can say they want the guiding to settle to less than 1 pixel for 5 seconds, for example. In this case I might say I want to AF if the HFR increases by >10% for 3 exposures.


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  • one for me, it is what I use to refocus manually. I keep an eye on the image history and HFR then refocus when it strays upwards. Having a feature that triggers autofocus automatically would be great. Its a direct indication of seeing, compared with guessing with temperature changes
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This actually comes up pretty often and, years ago we implemented it, but it just caused more bad images and headaches than it was worth so we removed it. The primary issue is that a specific change in HFR means different things at different altitudes and it was hard to be consistent while tracking a target for long periods of time. Folks with short FL rigs tended to fare better, but at around 750 mm, this method fell apart.

The method to do this may be with observing trends and not values, but this would require some careful thought and may be a candidate when we refactor the way in which SGPro captures data for AF.

Yes, I agree that a trend is much better and a consistent change of 10% or greater change in HFR for 3 successive images could trigger a refocus?
I find that refocus on temperature change is innacurate and is a very indirect way to monitor focus position.
Maybe we could try again?

Possibly, but there are some things on the list in front of it. That doesn’t mean we can’t discuss it here so another trial run may be less painful. This is, I think, is not a simplistic issue and this thread is just for spit-balling (and not a commitment to build this feature).

Maybe, but, as I recall one of the problems we faced was that the tendency for HFR to grow smaller as a target gets higher in the sky can effectively cancel any rise in HFR we would have otherwise observed. The HFR, numerically, will stay consistent, but the image has gone out of focus. The “Image History” features present in SGPro now were originally to support this feature, but now they have been demoted to just “interesting” data.

Because of this, I think that directionality of the target would need to be considered which is not possible unless a user profile with lat / lon is created and assigned to the sequence. Then ask: what is the (change in) HFR we might expect, in perfect focus, with respect to natural target “movement” and then compare that number to the actual change.

This is kind of a sticky issue and would require a training of sorts. Not everybody will observe the same natural change in HFR for altitude, but maybe this could be a property of the equipment profile.

Are you over thinking a bit? SGP has helped me enormously for unattended imaging, but I still often watch what is happening and I keep a close eye on the HFR graph and will manually intervene with an autofocus if I see it rising. Should the HFR reduce as the object gains altitide, I would be pleased! Deterioration as the object loses altitude could be confused with defocus, but so long as refocusing isnt happening every 5 minutes, is there much to lose?

I’m aware that NINA has this function and it is often used.

Well, I don’t think so. I am speaking from years of experience doing complex support. If we introduce a feature that doesn’t work for all people, it is a huge drain on resources. You are speaking from your experience with your rig. That’s a fine perspective… it’s just not the only one.

It doesn’t mean your focus is better, it means the goalposts have moved because there is less atmospheric interference in the HFR analysis. The HFR measurement at the lower altitude was not sufficient to begin with and now it’s used as the baseline.

From our perspective, maybe the solution could be simpler if we disallow use of this trigger by itself. The trap is “not” flagging focus when it’s needed and not the other way around (too frequent focus is not great, but it won’t ruin the entirety of all data collected). Because:

  • An increasing HFR could mean loss of focus or a lower altitude and you may get an extra focus here. Ok… not ideal, but not terrible
  • A decreasing HFR would, in almost all circumstances, mean: “in focus”, but at a higher altitude
  • A steady HFR could mean a) the target is in focus or b) target is out of focus but at a higher altitude

Given these scenarios, it seems the worst case outcome is not flagging focus when it’s needed. So… maybe, if we add a condition where simple analysis of delta-HFR is coupled with the “safety net” of another trigger like time or temperature, then this may create a situation where the support load from this feature is tenable (which in all honesty is why it was removed last time… and not because it didn’t work for anyone).

Apologies, the wrong thing to say!

For the different scenarios you quote:
Reducing HFR is an indication of better seeing and a higher resolution image, what we strive for and this situation wouldnt trigger an autofocus

Steady HFR indicates a stable condition, a good situation and also wouldnt trigger an autofocus. It maybe more complex like better seeing mixed with defocus balancing each other out and it sounds like this is your area of concern. But if you have a trigger that is intiated with time over the night, like refocus every hour, then you are covered. And I know you will say if you are focusing every hour why have an HFR trigger!

Increasing HFR indicates detriorating seeing that can be caused by defocus from optical and/or OTA changes or atmospheric changes - high clouds, approaching weather front etc. From my experience of 30 years of CCD imaging (I started with an ST4 in 1992), its defocus causing the change in the majority of cases. A falling HFR caused by reduced altitude is not significant until it reaches a level that I would not image the object anyway, say around 30 degrees above the horizon.

Most of the optical and OTA changes occur early in an imaging session as the temperature cools. Maybe we could have autofocus triggers that occur at specified times? Like hourly for the first part of the night and only on HFR change after midinight?

The moral of the story is to have a rig with zero expansion optics, a carbon fiber tube and refocus on increasing HFR :slight_smile: