Well that certainly fits any definition of "out of focus"! (is that the SCT? Looks maybe out of collimation, as well - but I am *really* not an expert in that realm)
Surely you could see in the focus tab display that you would up with donuts instead of stars (but that doesn't help if you're trying to automate).

When I first started autofocusing I made the mistake of tightening the 'regular' tension adjustment on the focuser, and all kinds of horrible things would happen, mostly related to the stepper motor reporting it had moved but the focuser was still really in the same place (different system, though - I use a Rigelsys NStep).

"Far from clear what the bottom of the curve is" makes me first think to suggest increasing the initial step size. You might (should, really, as long as you are experimenting) also try increasing the exposure time and averaging over 2 or 3 frames - though I still get "bouncy" curves with 2 or 3 frames when the seeing is poor.

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