I suspect my Optolong Oiii 3nm filter has uneven coatings (or whatever the reason) causing a dark region in the image. I replaced it with an Antlia Oiii and the flats are perfect. Does that prove my theory, or is there something else I need to do? My intention is to have Optolong replace it. Anyone else having, or have had issues with this same filter? Here is a typical flat:
📷 SH2-261_CAL_FILES_FLAT_ZWO ASI6200MM Pro_2026-01-20_19-59-33__O_-9.40_0.76s_0008.jpg
The first thing I would suspect would be the filter wheel not centering or some other obstruction causing that sharp cutoff on the left side.
Tony Gondola · Feb 21, 2026, 05:01 PM
The first thing I would suspect would be the filter wheel not centering or some other obstruction causing that sharp cutoff on the left side.
Replaced the filter and fixed the problem. Checked the filter installation prior to replacing, made sure it was threaded all the way in, etc. Don’t believe the filter wheel was the issue. Also, it was the only filter out of seven that did this. The right side of the image is the problem.
Good news then as it’s fixed. I guess it will remain a mystery as to the actual cause.