Hello all,

I put together an autofocuser from a stepper motor, timing belt and pulley, and a bracket to mount it on top of my refractor. I'm using the Waveshare Motor HAT on my Pi 4 running StellarMate OS (1.5.1, I think).

The trouble is that the system has about 100 steps' worth of backlash when it reverses, so the accounting can get all screwed up in the Focus Module. There is a Backlash text-entry box in the Mechanics tab of the Focus Module but it's disabled.

I'm using Kevin Ross's indi_wmh_focuser module, building from source so I can modify that if I need to set something in the driver itself. Just wondered if there was an easy fix before I dive into the code.

TIA!

Not sure, but my hope when I implemented the Linear Focus scheme, which always sweeps from out to in, was to be able to overcome
backlash issues as you've described by mostly avoiding changes of direction (and trying to compensate a bit when the changes are required).
Obviously there's a limit to that, but have you tried "Linear Focus"? Please let me know how it goes, e.g. send a log if it does fail.

Hy

Linear focus is the way to go. Nails perfect focus every time and inherently compensates for backlash.

Jo

Hmm. I tried Linear last night and it was *less* successful than either iterative or polynomial! It wandered all over and failed to converge at all. Maybe I need to fiddle with the parameters. Any suggestions, given that I have right around 90 steps of BL? I just tested it again and it's quite consistent, in both directions: reversal soaks up just barely under 100 steps and after that it just trucks right along.

Can you post the V-curve?
This is what it should look like. Look at the lower right. The yellow dot shows the final focus position the focuser has arrived at. What is your step size?

Sorry, I didn't save a screenshot. But the successive points were piling up on each other, as if they'd slid past the center. I'm using an initial step size of 100, which is about the size of the backlash.

How do you get that display? I blush to say I've no idea how to elicit that tab in Ekos.

I have been using polynomial and I am pretty satisfied with it. I just started to adjust for backlash and my focus has gotten better. I will have to try and go back to linear. I think I have a bit more backlash that I need to compensate for, however, it seems to be getting better.

OP check your backlash settings. I didn't measure my backlash, but I did start to adjust the total and I am evaluating how well it is focusing over the course of the next few weeks. I will see what works best for me. I suppose I could measure, but I don't trust my eye to see the movement lol.

As he says, the BL box is greyed out, likely meaning that the driver he uses doesn't support it.

@fewayne:

• the nice image is Hys new Analyze tab. You might have to use recent/nightly builds to get that
• Take a look at the settings. You want 'SEP' star detection, and 'Linear' method with 'full field'. Then, under 'Mechanics', look at 'Initial Step Size' and 'Out Step Multiple'. The algorithm will first do STEP sized steps, then half of that. So it should be small enough that it's in the order of your desired accuracy. I use 20. The step multiplier will give you two things: STEP times multiplier should be large enough that the star gets unsharp, but still small enough that you are within the clear slope of the V curve. In addition, it will do the same overhead as backlash compensation. I.e., use 20 and 5 -> initial step is 100. So it will move 200 out, then 100 in, then continue to move inwards in steps of 20. So for your case, that step should be larger than your backlash. Keep those three points in mind, then it should work nicely...

That sounds great. Unfortunately my Ekos doesn't provide "Out step multiple". I've got one that I built from source awhile back so I could try the Bahtinov focus metric. (Hopefully that's obviated by the autofocuser!) Like many folks, I'm pretty much a late adopter when it comes to Ekos updates, not wanting to risk a working system for a given bell or whistle. Probably worth it here. Is that in the latest release, or do I have to build from source, do you happen to know? [EDITED TO ADD:] OK, overnight I got a ton of updates and I see that it's in the release. Thanks! I've got the analysis tab too, woo!

SEP star detection did not work at all well for me. I got much, much more reliable results with "Centroid".

> So for your case, that step should be larger than your backlash.

Just to be clear, "that step" means the initial step size * multiplier, right? So in your example I should ensure that my backlash is smaller than 5 * 20 = 100. Which I'm pretty sure it is, but only just barely.

I probably should have been taking better notes in "debug equipment" mode Friday night rather than "get it focused and acquire some data", but it was such a nice night... ;-}

So I tried a few combinations of initial step size and multiplier after updating to the latest KStars release. Mostly I went with 20 and 5. Sometimes Linear achieved good focus; other times it goofed. I neglected to run the logs (sorry) and didn't get a screenshot since I was using my iPad most of the night. But what often seemed to happen was that the software would go way outward to clear backlash, then progress inward for a while, leaving a nice half of a V until it had started climbing the other side. Back outward -- not quite as far -- then inward again. But the dots for the second round wouldn't overlay the original slope, rather they'd extend a sort of shelf out over the deeper part of the curve. I hypothesize that the software's idea of the X coordinate (focuser position) is just dramatically wrong, so its mechanics get messed up. The outward and initial inward moves are much larger than step size * multiplier, which is probably as intended.

And then sometimes it would nail it right at the bottom of the curve on the second or third try. Sometimes Iterative would work, or even Polynomial.

What focuser do you have? On my moonlight CRL I see some backlash, but your focuser plots look dead on (no backlash at all). Looks excellent!

What are your current settings for the focus module?

Have you tried adjusting the backlash setting? Just because the plot looks good does not mean they have ZERO backlash. You set the backlash accordingly and your plot should like theirs. A focuser with improper backlash settings will show readings all over like a scatter plot.

If anyone else has anything to add or has to correct me please do so, however, that is how I understand the focus module and the plot.