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INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

Any helpful hints to achieve more stable HFR values for the focuser?

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After some work, I got my Esatto focuser to work on the C11 Edge, with the auto guider, filter wheel and ASI1600MM - all within the required 146mm back focus.

But now I am struggling to get EKOS to autofocus. I am imaging through a C11 Edge at 2800mm focal length f/10. The HFR bounces around too much to allow EKOS to find a good focus. I tried 1 second exposures, 2 second, 5 and even 10. I tried averaging the HFR values 2 and 3 times. Regardless, I would see HFR values like 310, 345, 320, 300, 360 etc…. This made determining the exact best focus impossible.

I never had this issue with my small 349mm f.4.9 WOStar71. The HFR values were consistent between exposures. AutoFocus was a breeze and reliable.
I don't know what the best settings to help stabilize HFR values at this high focal length. I tried many different combinations, from using gradient, centroid, and SEP to changing the tolerance settings. Nothing seemed to fix the issue that my star HFR values jump around too much between equal exposures.

Any ideas I can try?

Peter
3 years 2 months ago #65408

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Are you using the Focus Module option to capture multiple images per focus iteration and average them? If you are getting inconsistent star images or measurements then averaging can reduce the error.
3 years 2 months ago #65413

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There is of course a gigantic difference in focal length between 349 and 2000 mm of about 5.7 times. That difference accounts for the big difference in HFR stability since all seeing effects (but also guiding errors and other issues) will be magnified 5.7x. You will need to take that into account when focusing (and, again, also when guiding). There is nothing you can do about that from earth and it is a struggle that everyone has when using a large focal distance.

In general seeing averages out over 2 to 3 seconds so I'd say 3 to 5 seconds per focus image would be sufficient. Averaging several images, as you already do, should certainly help as well.

Do you really see the same jumpipness in the HFR values when you take, say, 1 sec images and don't average as when you take, say, 5 sec images and average 3 or more images? I'd expect the latter to show more consistent values though it very much depends on the local conditions how much more consistent the values would be. You may need to try this over several to many nights to be sure which method on average works better (which, by the way, is part of what's usually done when professional astronomers are selecting a new location to put a telescope).

HTH, Wouter
3 years 2 months ago #65422

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Are you using the full frame, or just a single star?
I’d try full frame with SEP, and also make sure stellarsolver is doing a good job detecting stars.
3 years 2 months ago #65450

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As Hy suggests, use full field and SEP. In addition, try binning. It stabilized the focus for my ASI183, and at 0.28"/px even a 3x3 bin should still give you a clear curve...
3 years 2 months ago #65479

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I still have trouble finding the best settings to control the Esatto focuser. I tried LINEAR tonight, and got a nice V-curve but EKOS wouldn't stop focusing inward well after a curve was seen and the optimum focus calculated. I'm not sure what setting would make it stop focusing.

Last edit: 3 years 2 months ago by Peter Kennett.
3 years 2 months ago #66586
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That should be the "Tolerance" setting in the "Process" tab.
However, judging from your screen shot, the process had still been in the first stage (at full size steps). It will never stop there. It tries to locate the minimum at the initial step size resolution, only then starts a second scan at half that step size, and there it will stop if the value can be confirmed within the tolerance.
With LINEAR you should also always try to start (very) close to the proper focus.
3 years 2 months ago #66598

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Thanks DerPit.

You say "It tries to locate the minimum at the initial step size resolution, only then starts a second scan at half that step size". Why didn't it stop after it located the minimum with these initial 5000 step sizes?
I can see the yellow marker that shows the minimum at 219000. Why did it continue focusing inward, getting worse and worse before I stopped it myself?
3 years 2 months ago #66610

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