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Further to this post, I did some more work when the skies cleared. I eventually got the autofocus to work across all the broadband and narrowband filters. These are the parameters I found which worked:
Binning: 3x3
Step size: 300
Number of sample points: 7
Backlash compensation: 3000
Exposure time: Broadband - 10 secs, Narrowband - 60 secs
The 3x3 Binning, as well as the smaller step size and number of sample points were necessary to get the Narrowband autofocus to work. 9 sample points generally resulted in the 1st sample having a lower HFR than the 2nd point, and the shape non-conformity (i.e. wing) caused the autofocus to fail. I believe this issue is caused by the out of focus stars forming donuts, as is typical for a SCT, which affect the HFR determination, when far enough away from the optimal focus point. 9 sample points and a larger steps size can be used for broadband filters, as this effect seems to be less pronounced at the extremity of the focus range. I think the range of wavelengths passed by a broadband filter results in different diffraction ring sizes and smears the star shape, compared to the more well defined diffraction effect with the narrowband filters.
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