Bart wrote: With elevation is meant the altitude of the object (as if using an alt-az mount), this -does- change during a session.
Also, if you take refraction into account, will you then -not- take it into account when using narrow band filters? There the effect isn't measurable.
Now a question, would you consider to focus during imaging or quickly adjust the focus in between lights?
Personally, I don't use very long exposure lights, especially with Starlink up and screwing up our images.
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.
dmsummers wrote: 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.
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.
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.
More later.... Cheers, Doug