When I take flat frames, I end up with images as the attached one. Unfortunately, the circles are also pretty much present in the final images when I processed them with Pixinsight. I cleaned all devices (whereever I could reach them) properly. This didn't change anything. I pulled out the single components one after the other (my first idea was the filterdrawer, but that didn't change anything) and the problem is still present.
If you use flat frames as part of your image calibration process, these dust bunnies should disappear. If they don't, there is something wrong with the image calibration process.
If you don't use flat frames, imo, you really should start using them.
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