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.

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.

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.

Uh, you lost me there. I might have to read through this thread again...

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.

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.

More later.... Cheers, Doug

Looking forward to it

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