Concentric Bands using F6/3 reducer/field flattener after Dyn. background Ex

[bjaffa]

I am trying to capture M31 with f6.3 reducer/flattener (meade). I am using bias, darks, flats, & lights. However after dynamic background extraction/background neutralization/color calibration, I am getting concentric bands of background. I think it is coming from the f6.3 reducer/field flattener.  I am using a cannon DSLR as the camera so I am not combining RGB filtered images.
Has anyone seen this issue before and how did you over come? I need to use the reducer/flattener to capture the full image of M31.

Thanks,

Brent

[wimvb]

You may try this
On first run of dbe, be very carefull where you put samples. Avoid stars and bright areas. Use division as correction method to correct optical deficiencies of your rig. Use subtraction to correct light pollution.
Do a second run of dbe, again with very carefull placements of samples, not necessarily the same samples.
Use many samples, and larger ones if possible.

Hope this helps.

[oldwexi]

Hi Brent!
Are you sure you have saved your MasterFlat and Stacked(!) MasterLight as 32 Bit Fits/XISF File?? !!!
If you did the save of the Master Flat or the stacked MasterLight in 16 bit Tiff
this is a typical artefact of loosing dynamic because of working in the 16 bit room.
Especially with such a difficult(!) high dynamic object and such a reducer which reduces
the light in the corners and borders dramatically.

Gerald.