SGP and focuser backlash

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KathyNS Canada
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SGP and focuser backlash

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Post by KathyNS »


I was having trouble getting good autofocus in SGP. I tightened up the focus motor gearing as much as I dared, but was still getting a lot of out-of focus frames. A new tool (SGP Autofocus Log Viewer) let me look at focuser performance during a session, and this is what I saw:
Focus Runs with Backlash.png
Temperature with Backlash.png
As you can see, there is a "wing" on the right side of every focus run, and the temperature vs. focus position graph is all over the place. I finally guessed that the "wing" on the right side of the focus graph was caused by backlash. The motor has just driven the focuser a considerable distance out of focus, and is about to come back to try to find the best focus. That first sample would be very vulnerable to backlash because of the change in direction.

That "wing" skews the linear fit of the right side of the graph, producing an intersection (supposedly the best focus) that is too far to the left.

So I learned about SGP focuser backlash compensation and applied an amount equal to my step size. Here are the results from my fist run with the new settings:
Focus Runs without Backlash.png
Temperature without Backlash.png
As you can see, I had a much larger temperature drop in this session, but the "wing" on the focus graph is gone.
Image
DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,Oiii,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier); Helena Observatory (H2O) Astrobin
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