I recently photographed M101 on Sunday morning, with my NIKON D750 & Celestron C6-n (750 mm). There was a quarter moon west of the target which washed the images with a lot moon glare, unfortunately I could not shoot on the previous weeks due to cloudy skies. Most of the shoot went well and I was able to take 50 x 2 mins subs of the target, plus 30 Darks, 20 flats and 20 Biases. I did the post processing in Siril (v 1.2.0-beta2) and GIMP (v 2.10.34) but I am having issues with removing the gradient from the stacked image. Attached is the Autostretched image, after doing the photometric color calibration and background extraction, the image has not yet been stretched.. I am not sure how to remove or reduce the bright areas on either side of the image.
I tried gradient tool in GIMP without much success.