I face a problem with calibration of my FIT images.
My CCD camera is SBIG ST2000XM with the following technical specs
Full Well Capacity=45.000 e- unbinned
Gain=0.6e-/ADU unbinned
saturation level=Full Well Capacity/Gain=45.000/0.6=75000 ADU
So in order to take flat frames, I adjust the exposure so the ADUs I have in the background are 75000/3=25000 for each filter.
In the attached image "master_flats" you can see the master flat frames in R,G,B,L filters.
It's obvious that the master B flat frame has more vignetting compared to the other master flat frames.
Because of this vignetting issue, the calibrated light frames are not evenly illuminated.
And as a result of this the RGB frame is not evenly illuminated, you can see the color gradients in the background.
