Quote:
Originally Posted by DarkArts
First off, like Stefan, I'd guess the mask is mis-shapen, mis-centred or at a slight angle.
What does your Airy disk image look like either side of focus? How round are your stars near the edge of the field and is focus/curvature/aberration symmetrical left-right and up-down?
TBH, though I do have a Bahtinov mask for my ED80, I don't use it except sometimes at the start of the night for initial rough setup. I focus by eye these days and before that (when I used a DSLR) I used BYEOS' FWHM measurement.
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I'm not sure about the airy disk side of things. Have not looked at this, but can check it out next clear night.
Here's a PI AbberationInspector of the frame from the first night of imaging (Ha) where I noticed the issue. As mentioned in my OP, although mask focused I was getting very tight stars with Ha, it wasn't until flipping to Sii that I had focus issues. The next night, all three filters had issues focusing with the mask.
Definitely sounds like a replacement mask is worth a shot, even though to my eye this one seems to be in good nick (not bent or anything.)