Auto Focus Issue (Failing)

I used to get perfect V/U curves when doing my auto focus routine but now for the past while I can’t get my auto focus to work. I have re-configured it 4 times and I either get a graph that shows various spikes or just a straight line and it keeps failing.

Weight doesn’t seem to be the issue as I am using the same equipment when it used to work.Capture.2PNG Capture

Is there anything I can check or anyone can help me with? See below what I am running.

  • ES102 ED Triplet
  • ASI178MM-Cool
  • ZWO EFW Mini

Maybe I need to remove the focuser and check if its flexing?

With 10 step is not ok. Try 50, 80, 100 step.

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Will do. I think at one point I did 40 steps and still couldn’t get the curve.

Will try higher and report back.

THank you.

I got similar results when my auto focuser wasn’t working correctly; i.e. it wasn’t moving the focuser much OR when there’s not enough stars to focus on.

Verify your focuser’s mechanical functioning.

Any update on your focusing?

Hey Kevin.

Unfortunately not, it has been nothing but clouds here the past week. Last night it was clear, set up the scope then it clouded over when it got dark!

As soon as it clears up I will be testing again and reporting back. I removed the EAF from the focuser, checked the tension and screwed it back on through another few holes. I got a feeling it might have been flexing a bit so this should be a good test.

Reason I’m asking is when I had my zwo1600 on I had very smooth v curves. Now I just got a SBig Aluma 8300 and my curves look like what your getting. And my focuser is a night crawler. Wondering if any upgrades to the program were made with respect to the auto focus routine

Oh yeah that is strange. My setup didn’t change. One day I was getting perfect V curves then the next all out the window so I am assuming the focuser became a bit crap over time/movement?

As soon as I test it I will let you know.

OK, having some luck with the focuser now. Getting some nice V-Cuves which is good.
I am having trouble though with the HFR value though. I find it hovers between 2.5 and 2.8 using the red filter (need to test other ones). During the 1x1 6 second frame and focus subs I get 1.05 HFR.

Any idea how to reduce that HFR when I am capturing at 120 second exposures? Maybe it’s seeing conditions, its really hard to tell.

what did you change to get the VCurve?

Recently I was getting results like the very shallow curve you illustrated when previously all was fine. I changed my autofocus binning from 1x1 to 2x2 and the expected V curve came back. I suspect I was getting some sort of USB traffic problem.

Are you still looking at HFR values? If you are trying the ASTAP metric option in Beta release the value is HFD(iameter). Owing I guess to different sensitivity in integrating star flux the measure is a little more that 2x the old HFR value.

You cannot compare exposures with different exposure times.
Longer exposures gives higher HFR because of seeing and possibly bad guiding and simply because the exposure is longer.


This is the latest auto focus I got with my green filter. At point 7 it was at 100% quality with a 1.3 HFR, it then did two more points and quality when down to 92% with 1.4 HFR.

Is there a way to configure it so it can do less validation checks or do I need to decrease exposure time from 6 seconds to lower?

You should increase the step size a little to get highest HFR 3-4x the lowest HFR.
Have you enabled backlash compensation in SGP? (only SGP, not in the driver) If not it can create bad looking V curves

Yep backlash is enabled in SGP.

I followed a calculation to get my step size which resulted it in being 39

Looking at your AF chart above, backlash looks still to be present on the 34222 data point ,after a 30 step move, so I suggest you need to increase your backlash move by at least an extra 60 steps (and/or check that nothing in the focuser set-up has loosened). While backlash is an undesirable characteristic I see no point in being curmudgeonly with the corrective action. It takes moments for the focuser movement to be applied and I imagine settle time is virtually unchanged whether the move is 50 steps or 100.

OK made the modification, still more or less the same though.

I am finding that I am having focusing issues mainly now with my Blue filter. It always gets below 90% and fails. When it does work the HFR value is low but rarely.

Any issue with Blue filters in general?

Achieving a reasonable SNR is vital for quality detection and measurement of stars during AF.

In post 147 of the thread Does SGP talk to Planewave’s PWI AutoFocus? Han says ASTAP uses a default setting of >30.

Blue stars in general are less common (though more luminous) than red, sensors are typically less sensitive to blue light, and blue light is more readily dissipated by atmospheric pollution, all of which I think make detection and measurement more prone to uncertainty/inaccuracy.

For what its worth I am currently using 15s exposures with B filter compared with 5s for L and 8s for R&G.

By way of compensation perhaps, I understand the human eye relies on Luminance rather than colour to assess sharpness in an image so I guess RGB focus accuracy is somewhat less important then L for seeing fine detail.