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.

I presume that comment is directed at my post, because that is the only one that has posted a V-curve.
This is entirely self-built autofocuser on a WO WhiteCat (helical focuser). It is a geared solution driven by a NEMA17 stepper motor and MyFocuserPro using the MoonLite protocol.

The design of the focuser (3D-printed) allowed me to virtually eliminate backlash, so the focuser hits optimal position literally every time.

Thats awesome! On a side not, how are you liking the Ioptron? I want to go with a CEM70 in the future.

removed since it duplicated my post above and I don't seem to be able to delete the post in its entirety.

It took me a little tinkering to get used to, but now it is working very well for me. The problem is that the stepper motor in DEC is underpowered or poorly matched to the board, which results in it missing steps, if there is too much resistance. It is essential to balance the mount perfectly and also not to tighten the tension screws too much. Just enough so that all discernible wiggle in DEC has been eliminated. That way, I can get good performance out of it.

I wish iOptron were offering the option to modify the firmware on the DEC board to allow me to fine tune the stepper to the board. If I could do that, this mount would be perfect in all respects.
I got it primarily because it is light enough so I can carry the setup full assembled from the living room into the backyard. 2 min setup and tear-down time. That was my primary criterion for going with the CEM25P instead of an HEQ5.

So to the extent that the last couple are directed at the OP, please read the thread. It becomes clear that the driver offers no backlash setting (at least not yet), and the very first sentence of my initial post was:

Still, I appreciate the attempt to help!

If the skies ever clear again I'm going to try with a smaller initial step value -- the smallest that will cause a visible change in the focus -- and a bigger multiplier, so that the initial move definitely takes out the backlash.

I also may change the driver a bit, my motor combined with microstepping makes 10 or even 100 steps completely invisible with Kevin's driver out of the box, so I put in a big multiplier to actually get the shaft to move.

What you are describing is the result of excessive backlash. The linear algorithm assumes there is none and that is what the step calculations are based on. The shift in the curve after the V-curve has been established and then the focuser moved out again is a direct measure of the backlash in your system.

I would think that it would not be too difficult for Hy to implement a backlash compensation in his algorithm, once he is done with his current course work.

But from what you are describing, your system needs some serious tightening up.

Also, you may want to play around a little with the tolerance value. You need at least 5%, otherwise the focuser will fail too often. It never fails in my setup at 5% tolerance.

Wait, what? Did you not previously write:

One of these things is not like the other Hy's answer up the stream also seems to indicate that the algorithm is designed to minimize the effects of backlash.

I mean, "excessive" I might certainly buy, since a step size of 100 makes a visible focuser tube displacement but can be nearly eaten by backlash.

So, as nearly as I can tell, the backlash is actually in the motor; there's really nothing I can do about that without replacing it. Although I recognize that geartrain backlash in a commercial motor seems unlikely compared to my laughably crude setup displaying, shall we say, idiosyncrasies...still, when I reverse direction the thing gronks for a couple seconds and only then does the shaft start to turn. A subsequent move in the same axis makes the gronking and turning simultaneous.

When I ordered the timing pulley they sent me one with a bore about triple the size of what I'd asked for; coming up with a crude bushing for that left me with a setup that certainly might be susceptible to backlash, although as I noted that doesn't appear to be happening. I've ordered another, we'll see what arrives in a week or two on the slow boat from China.

Let me rephrase: The linear focuser inherently compensates for backlash, as it moves so far out, then back in, that it eats up all the backlash in the process.

HOWEVER: While establishing the V-curve, the Linear algorithm determines the calculated minimum on the assumption that there is no backlash. It moves out accordingly and then performs a calculated move towards that minimum. Any backlash will displace the second inward curve by that amount. In addition, if you excessive backlash, it is theoretically possible that the outward movement does not extend beyond the calculated minimum, which will send your focusing attempt off into Nirwana.

That's what I wanted to convey in fewer words and where I obviously failed miserably.

When you look at the curve I posted, it is obvious that there is essentially zero backlash, as the secondary inward curve nearly perfectly overlaps the primary V-curve.

If you have a geared stepper and a gearbox with a lot of play, that will be difficult to fix. I have opted against using a stepper with a gearbox and I am using direct drive microstepping instead. I was worried about steps being missed, but it seems to be working surprisingly well. No major problem with missed steps.

Jo

Right. I agree (mostly--just quibbling about nirvana with what Jo has said.

To rephrase, there is no explicit backlash compensation in Linear, but rather the algorithm tries to set itself up in a situation without backlash.
It does this by first moving far "out", then moving back "in" by quite a bit, in hopes that the inward motion eats up all the backlash.
It then tries to sample a v-curve, moving "in" by step size, sampling the HFR, moving in again ... eventually find where the bottom of that curve is
(by moving past the minimum and noticing the HFR has started to increase). It really doesn't care what the absolution position is, just the value of
the HFR at the bottom of the V. It then performs its 2nd pass, whose goal is to find the minimum again and stop there (at whatever
absolution position, doesn't matter). It does that 2nd pass by again moving back out quite a bit, and again reversing to inward, moving quite a bit to
again hopefully eat up all the backlash. Hopefully, the position it finds itself in at that point is outside the minimum HFR position.
It then performs its 2nd inward sweep, iterating the following: moving in my 1/2 step size, measuring an HFR, and deciding if its done.
It's done if the measured HFR is close enough to the minimum HFR from the 1st pass (and its no longer decreasing).
It may decide that something went wrong, and instead of going off to nirvana, repeating the backward/forward motion.
It always terminates after 30 steps.

It's not perfect, but can often overcome backlash.