Thank you very much for your answer John.

With my previous focusing routines, I did not need to know the exact value of the backlash, I just have to overestimate it. Indeed, since they draw an outwards V-curve, and since the backlash correction affects only inwards movements, it acts exactly as the overscan for Linear 1-pass. Since I could make repeatable focus for years with the same hardware, I am confident on the fact that the actual backlash is lower than 15 (which corresponds to 0.15mm).

Now to answer your thoughts:

1) I fully agree, indeed before I started the procedure I was pretty sure to find a trend by setting a overestimated value of backlash. Your videos explains the point. What is puzzling is with that settings I was able to have repeatable focus (ie ekos finds the same solution) up to seven times in a row.
2) I used overscan = 40, then 15. IMO the actual backlash is lower than that, but it is worth trying in a next session.
3) I have checked for any loose screw in the Baader Crayford or in Robofocus… up to now I have not observed so huge drifts with this hardware, in more than ten years. Indeed, few years ago I made hundreds of inwards/outwards move, with the OT pointing at zenith and an heavy Sbig camera attached, the slip is about 1 tick per in/out move, no more. Of course it is worth checking again. But if the slip is greater than the CFZ, the HFR at ekos solution would be poorer that the one of the points around the minimum of the V-curve, I did not observed that.

Here is the manual ( robofocus.com/wp-content/uploads/2021/09...us-User-Manual-1.pdf . It is not very verbose about how the backlash is handled. BTW I did not find a way to select in Indi the direction of the backlash, while it is possible in the original driver.

Again, the most annoying thing is the erratic behaviour, sometimes it is stable, sometimes there is trend. A systematic error would have been easier to chase. As a result, the utility to measure the focus offsets between filters is messed up.

Thanks,
Philippe

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