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.
Plugging in 72mm, 420mmFL, 0.85FR, 2.4umPIX and 687 pixels donut size I get approx 8mm.
Best guess is the sealing glass cover on the front of the CCD camera itself
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.