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Is the pattern something you've never seen before? Are you doing flat calibration? CMOS sensors tend to have more fixed pattern noise, so you do need to be careful about calibration and dither (did you check that the amount of dither looked adequate in the StarAlignment output?) Longer subs or higher gain might help too.
Careful noise reduction with one of the Multiscale processes (I like MultiscaleLinearTransformation) should help and will allow you to target some of the bigger scale noise. Use a mask that protects everything apart from the background and after NR you should be able to move the HT black point to the right without clipping. You can also use an "S" curve with CurvesTransformation to darken the background further.
Cheers,
Rick.
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