Quote:
Originally Posted by The_bluester
I was going to add (Though I don't focus with a mask and never dived in to them enough to know what Stefan wrote about them) was that I would trust measured HFR more than induced diffraction spikes. Smallest stars = best focus.
Very few auto focus routines use a mask, they almost exclusively measure the HFR of a star or multiple stars in the field to determine the best focus point. The only focus assist I have seen use a mask is in APT (Astro Photography Tool) but when I was using APT I didn't have a mask, so I didn't test that function.
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Thanks, yes, indeed I trust HFR more than the mask in this case, as the results both to my eye and to software analysis have resulted in clearly smaller stars. It's just bizarre as I don't have the same issue on other telescopes.
Quote:
Originally Posted by Stefan Buda
Sounds like you have a centring issue and that may explain the focusing error.
Do you have a picture of your mask attached to the scope? I'd like to know what's going on because I never experienced this problem.
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I've never been too fussed with making the mask perfectly centred on the dew shield before as it's never been an issue. It's something I tried to eliminate it as the cause here, but with no discernable difference. Unfortunately I don't have any photos of the mask on the telescope, but I do have some ASIAir screenshots of supposed focus with the mask which was proven by star analysis to not be perfectly focused. I have zoomed in on the star in these screenshots.
These were all using the Sii filter. The three Sii screenshots were each captured on three different nights. I have tried different exposure times and binning for the focusing shots with the mask, exposure ranging from 3 to 15s. It hasn't made much difference. Obviously given NB, the signal getting through the mask is quite weak; these were focused on Rigil Kent. Also attached (for reference only) is the same mask, on the same star, with the Lum filter - I did not image in Lum so do not know if this was actually correctly focused, or not. A much stronger signal comes through on Lum.
Maybe you can see something in these that I am missing?