Hi ZWO ASIAir Team,

The AF sequence uses two routines, the first coarse one, jump up 150 steps and comes back down by 30 steps decrements.
Once VCurve is confirmed with focus point estimated, the Fine routine start with a 45 steps jump above estimated focus position, and decrease by 15 steps to then confirm and finalise the Focus position.

On some fast scope, it takes as little as 4 to 6 EAF steps to move the Bahtinov shape from In Focus to aside and out of focus.
Thus 15 steps in AF fine routine is too much, leading to some Focus not being accurate.
Would it be possible in the EAF focuser setting tab, to have a field so user can define the EAF fine routine steps (and number of steps?)?

So for example, if estimated focus is position 1247, Fine sequence as is would jump to 1292, then down to 1277, 1262, 1247, possibly confirm 1247 to have best HFD and correct.
If user could define in EAF:
AF fine steps: 5
EAF would jump to 1292, then decrement down towards 1247 by steps of 5.... 1287, 1282, ...., 1257, 1252, 1247, 1242
Then maybe AF HFD would estimate 1242 to be better focus than 1247.

Would backlash be considered for V1.6 too?
This would help for fine focussing with AF.

    I greatly support this request. on my side, by focusing manually, a difference of 7-8 steps is visible on the result.

    sebriviere So for example, if estimated focus is position 1247, Fine sequence as is would jump to 1292, then down to 1277, 1262, 1247, possibly confirm 1247 to have best HFD and correct.

    The first pass finds the V Curve by using multiple data points (with a least squares fit, the total error is reduced by using multiple points; that is how Gauss helped rediscover the minor planet Ceres, and introducing the mathematical world to the Least Squares method :-).

    The second pass looks for the minima by checking the HFD only for a single sample, and therefore has lots of potential error. For the second pass, ASIAIR should just do a slow slew of the EAF (also starting from the right; this will compensate for the backlash) directly to the position computed from the first pass, instead of searching for the local (noisy) minima. Notice that the current ASIAIR second pass usually stops too early (when noise pushes a sample lower than it actually is, or pushes the next sample higher than it actually is).

    I have already alerted ZWO of this problem when auto focus first appeared on ASIAIR, and they have confirmed a fix (i.e., by not looking for local minima on the second pass), in addition to changing from a parabola least squares fit to a hyperbola fit (which better matches the optics problem at hand). They had sent me some screenshots of the graphs about ten days ago, and the focusing looked quite reasonable.

    Perhaps autofocus could be corrected in the next release.

    I had (up to the point the rains came back) myself been using the Average HFD from ASIAIR's Detect Star tool and feeding the HFD to a program that I have written to do a hyperbola fit to discover the true focus location, and then slewing the EAF there myself. I have found the ASIAIR autofocus to be too inaccurate to use, although you find the FaceBook guys loving it, even though their own screenshots show exactly the focusing error that you have described above!

    Would backlash be considered for V1.6 too?

    EAF Backlash is already taken care of in v1.5, as I noted above.

    Viz., notice that ASIAIR always starts way on the right and moving in from right to left. For the second pass it again starts from the right and move to the left. I.e., any backlash is neutralized by this (i.e., they have the same backlashed offsets :-).

    There is a further bug that I had (more recently) alerted them to, and that is the EAF scale. With different scalings, the EAF movement can be too shallow (or too coarse) to compute an accurate V curve. When I installed the ZWO EAF on a newly acquired belt driven Askar ACL200, the focus plane moves only by 0.44 microns per EAF step (no kidding). As a result, the V curve from the first pass is completely bogus. The data for the V curve should include HFD values that are above 6, preferably 9 or 10 even.

    It is not a universal problem with all belt drives. My WhiteCat51 moves sufficiently (2 microns per EAF step), for example, and it is belt driven. It all depends on the gear ratio and the helical focuser scale and the f-ratio of the telescope/lens. On the other hand, my R&P type focusers all seem to work (both with a ZWO EAF mounted on a FeatherTouch on Borg OTA, and with a ZWO EAF on the Tak FSQ-85's original R&P focuser (4 microns per EAF step)).

    So, ZWO also knows of this problem, and we just need to wait for the next fix. In the meantime -- focus like you did before ASIAIR autofocus came out. If you run MacOS, I can send you my program that computes the V curve from the Average HFD numbers from the ASIAIR Detect Star tool, but I recommend just sitting tight and wait for the real fix.

    Chen

      Favorable to fine steps below 15. My F / D 4.5 requires it 🙂

      • w7ay replied to this.

        w7ay Thanks Chen...
        Hopefully fix will focus within what can be manually expected with bahtinov mask.
        If not, then autorun would be useless with AF...

        • w7ay replied to this.

          Mike_astro Favorable to fine steps below 15. My F / D 4.5 requires it 🙂

          There is really no need measure the HFD at very fine focusing steps.

          The apex of a V curve does not in general fall right at one of the measured HFD points. The coordinate of the apex of the V curve has much higher resolution than the distance between the HFD measurements.

          Chen

          Here is an illustration of how a V-curve can yield a EAF position with a much higher resolution than the resolution of measured HFD values:

          The screen capture is from my own MacOS V-curve program, set to produce minimum mean square match to a hyperbola (white curve). There are 9 measured HFD points (red dots). Each red dot is measured using the ASIAIR Detect Star tool's Average HFD, after stepping the EAF by 200 steps each time.

          Notice that the "final" EAF location (x-axis offset of the vertical hyperbola) is 36241 (white number in the top middle of the window), while the closest red (measured) dots are at EAF locations 36200 and 36400.

          This manual process is not really much slower than the ASIAIR auto focus process. While ASIAIR is capturing the next plate, I will be typing the Detect Star average HFD into my program. So, I am just overlapping time with what ASIAIR would have been doing if it were in auto focus mode. It is just lots more "manual labor," and I have to be awake to do it, while an autofocus process can be done while I am asleep.

          Chen

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