I've updated the charts in the OP (now ~1 year, 350 autofocus runs at night). The temperature data now slightly favors a weak quadratic fit (but I'm still showing a linear). The temp ranges from 20F to 70F. I've also examined focus sensitivity for different systems against seeing (1.5 to 3 arcsecs, f/2.2 to f/10 setups). For ease of understanding, I have attached two graphs, one showing a CFZ perspective (microns), and the other using an EAF quality focuser (5760 counts per revolution) combined with a 750um focuser thread pitch (resulting in focus counts). In case it's not obvious, f/10 or f/7 setups won't need Adaptive Focus Control (AFC for you football fans). In good/best seeing, f/4 might benefit. In all cases, f/2 and f/3 need AFC. Those of us with f/2 or f/3 systems know intrinsically what the charts are saying; the current focus adjustment controls (think scheduler, time/temp deltas) do not suffice. I've also attached a temperature chart from this month to support that assertion (my site/gear).

This month, I used a stand-alone program to manage an autofocus "integrator", and manually offset focus between exposures as temperature and elevation changed. Manually intensive, but the results were very satisfactory! There appears to be no Ekos "gotchas" for updating focus between exposures. Adjustment can coexistent with download, dither, or exposure delay timing. I was easily able to maintain focus in my f/2.2 rig as temperatures rapidly changed. One good seeded autofocus and then occasional updates between exposures will keep focus well managed. Only 1 autofocus needed per target!

So a couple of thoughts. My 7 night January run had some bad seeing in it. Autofocus has difficulty when HFR readings are bouncing all over the place. This adds to spread in the logged data. Linear autofocus tends to pull up short of the bottom of the V curve in bad seeing, especially if the last HFR reading (averaged or not) is marginally higher than the prior HFR reading. Getting one good autofocus result (even if manually adjusted), followed by AFC updates, seems like a better approach to managing focus than risking N funky autofocus results. Finally, while I started this idea thinking that a seeded autofocus was the goal, it now seems clear that AFC between exposures IS the goal. A seeded autofocus start is desirable, but realized AFC between exposures is a "can't live without" feature for f/2 & f/3 systems! So work will continue.... cheers, Doug

Very interesting! Thanks for sharing.
One thing that keeps me from 'blindly' adjusting focus based on temperature is focuser backlash. How do you handle this, especially how did you measure it (assuming you do have some)? One of the reasons I do like Hys focus routine that is insensitive to (at least small) BL.
An EKOS module to automatically compute focus backlash would be a nice thing

I haven't yet measured my EAF focuser backlash, but it's on the list of things to confirm this month. I'll use a caliper and manually drive focus to check it. I'm a bit uneasy about the flexible coupler ZWO uses (between motor shaft and focuser shaft), but I think it likely is ok and shouldn't add significant BL. I suspect EAF, Pegasus, and other similar focuser offerings are going to be well enough behaved to ignore BL. Just thinking out loud, I would guess that BL related skew would bias measurements without altering overall trendline slope. The integrator already addresses prediction bias (back to autofocus position); it should cover BL bias too. It's a beautiful theory that could be wrong, but early testing seems to bear this idea out.

Honestly, I'm much less worried about BL than I am with position errors introduced when linear autofocus pulls up short. That kind of error needs to be addressed. Beyond the obvious logged position error, introducing AFC would have the effect of "freezing" that bad focus (relative position offset), resulting in AFC being less ideal than if multiple sloppy AF runs were done (assuming that some of those AF solutions would be good). I need to talk to Hy offline about what these charts suggest regarding sufficient precision of linear AF solutions, and how that might lead to some improvements. More later.... Cheers, Doug

Edit: One important thing to remember about AFC is that it's operating a trendline. Generally speaking, the temperature trend will be downward (bumps excepted). This means that BL isn't a factor because the adjustments are all unidirectional. If you were really worried about BL, you could avoid adjustments during reverse bumps (they typically don't last long). Guess: the magnitude of the BL probably needs to be less than 1/2 of the CFZ to be safely ignored otherwise.

I'm afraid you are over optimistic there. The BL of my EAF is 84 units. An error of 20 already produces a slightly visible loss of sharpness (I do have good seeing though). But 80 produces doughnuts (well, not really as I don't have central obstruction...). So a correction that reverses direction would completely set off my focus without proper BL compensation. Well, rather it wouldn't do anything before BL is eaten up by corrections.
As for the value - I measured that with a piston micrometer (no idea if that is the proper word). The value should be fine, although I did that when the telescope was horizontal. Usually the scope will point up. I measured it from the slope of the up and down scans, so that shouldn't matter. But I have no real feeling how well that compensation works for corrections that are smaller than the BL....

As for the flex coupler: I was woried about them too, and (with my older Pegasus DMFC) did a test replacing it with a torsion-free version. It didn't affect BL. That of course depends on actual load and stick slip in the focuser gear which might be stronger at low temperatures. But for me it's indeed no issue.

Uh, you lost me there. I might have to read through this thread again...
It has improved substantially IMO. And it is insensitive to BL. Without compensation, at least for me the polynomial AF routine is close to unusable, and leads to errors larger than the position errors I get from linear. That one quite sometimes is 10 units short, very rarely 20.

Looking forward to it

Thanks for the BL report; I'll definitely check mine for comparison. What focal ratio are you operating at? It would seem that 84 counts on an EAF shouldn't produce that visible of an impact unless you're operating at low f ratio....but it's an interesting data point....so thanks for commenting!

About poly focus alg, we totally agree. I can't use it at all.... In that sense, the linear alg is head & shoulders above the others. I do think there's still room for improvement however, even given how much better it already is.

That's on a Sharpstar 140, F/6.5. So not fast at all. 1 step corresponds to 2.8μ movement, so 84 steps is 0.235mm. That is a R&P focuser, maybe it just has a coarse gear ratio.
IIRC, around 30-40 steps of that BL are internal of the EAF, based on various forum posts here and there.

I'm going to need to think about what you've said. Using the CFZ formula from here (www.goldastro.com/goldfocus/ncfz.php), it seems you should not be able to tell any difference in 20um of change if already well positioned in the CFZ. Your CFZ size for 1, 1.5, 2, and 3 arcsecs seeing (10% tolerance) is 42um, 63um, 84um, and 126um respectively. I guess the real question is how did you arrive at 2.8um / step for your system? We're both using EAF. My Celestron focuser thread pitch is 750um/rev (0.13um/step). You've calculated 21x coarser threads (seems hard to believe, but I couldn't find the thread pitch for your focuser to confirm). Were you forced to not use the 10/1 fine focus on your setup, or have I screwed up somewhere in the calc?

Doug,

I'm indeed not using the 10:1 fine focus. The reason is that those are friction-driven(*), and, like crayfords, are sensitive to slip. One can usually hold/fix the coarse knob on the opposite side and then turn the fine knob, and will be able to move it. I think I even read somewhere in a manual (maybe from the Pegasus? don't remember) to explicitly not use the fine gear for motor focusers.
And if the CFZ is around 50μ, a step width of 0.13μ sound like clear oversampling, isn't it?

(*) at least those that I know; they use an outer cup, an inner axis, and three steel balls as "gear". Don't know if there are others that have a real planetary gear for this.

Now that you mention the 10/1 avoidance, I think I read that too somewhere. Still, that's a big thread. 0.13um for my setup is more resolution than I need, but I preferred this to the Celestron offering which was only ~1/5th (or less) the resolution. I wasn't thinking about HFR instability on V curve fit when I did this.....but regardless, I'm still happy with the EAF (or will be until I measure the BL).

I can now confirm DerPit's backlash comment. My EAF has tested today @ 95+/-5 counts of backlash. I definitely didn't expect that much! This has likely added spread in my dataset, reducing sensitivity, but even so, the story is pretty clear. It should be easy to tune and get a tighter dataset from here forward. I guess focuser BL tuning should be considered mandatory for low f-ratio systems!

@DerPit: Re: "One thing that keeps me from 'blindly' adjusting focus based on temperature is focuser backlash. How do you handle this, especially how did you measure it (assuming you do have some)? "

I wanted to return to this question just for a moment (and thank you for helping me to get my BL sorted out). I've measured my BL now and set it in the INDI control. I've also verified reversal behavior is as sensitive as expected. For very short f-ratio setups, I can see that BL tuning is very important (but less important as f-ratio increases). Using the INDI BL control settings, it seems BL can be well managed. Am I missing something more important from your question?

No, I think things are sorted out fine. Especially, for you the effect of the BL might be much less than for me, as you use the 10:1 fine control. It seems most of the BL (also mine?) is intrinsic to the EAF. In itself a good thing IMO, as then it very confined and one should be able to reproducibly correct it.
Maybe just one note: Watch out for drifts in your focus position that could indicate issues from creep using the 10:1 drive.
Cheers,

Pit