mmarcos I did Flat shots only with the flattener and the circle does not come out. I don't know what it is about the tube that gives that reflex effect.
If the problem is not there when you remove the filter, it sounds like a flat field problem, caused by vignetting from the filter. A mounted 2" filter barely gives you 45mm to 46mm of clear aperture, and you have a large sensor. However, it should still be correctable by proper flat frames.
Just to be sure, take your flats at the same temperature as your light frames, and don't let the focuser slip from when you were taking light frames. Check your bias to make sure the intensity is not bottoming out, if you are unsure, give it much more bias -- that will reduce the dynamic range, but the newer CMOS cameras have plenty of dynamic range to start with.
Try changing the peak ADU when you are taking flat frames; one usually aims for a little under 32000 (half of max ADU).
Go above or below half max ADU depending on the symmetry of the flat field histogram -- if the tail of the response curve is shallower on the low (left) side, increase the flat field exposure -- experiment all the way up to an average ADU of 40000 to see if you can get the non-monotonic flat field to go away. The quantization of the vignetting (i.e., the stepped rings instead of smooth vignetting) appears to indicate that your flat frames may be taken at too low an ADU).
But also experiment with average ADU of the flat frames down to 20000 or so. Any change in the result will be further clues.
All that said, you may be able to find a better placement of the filter tray to reduce the vignetting to start with. If the exit pupil of the flattener is smaller than 44mm diameter (the diagonal of your sensor), you can reduce the vignetting by placing the filter drawer as close as possible to the flattener. If the flattener is larger than 44mm, place the drawer as close as possible to the sensor. The common man's rule of thumb is to place the filters as close as possible to the sensor, but that only applies with simple prime focus, or with flatteners/reducers that are larger than the sensor.
You should also check if your filter is AR-coated on both surfaces. If not, the direction that you mount the filter tray will also matter (which side faces camera). I.e., try to reduce the work that flat frames need to fix.
Again, first check the average ADU from your flat frames; the quantization seems to indicate you don't have enough dynamic range for the flat frames, but hard to tell from a single final image that you had posted.
Chen
