The attached image is an auto stretched L image, 120 sec taken tonight at 20:05. Note the strange dark areas around the edges of the image.
What is it and how can I fix/get rid of it?
More info:
* It happens for RGB as well.
* Going to 300 sec makes no real difference.
* Does NOT happen for Ha (300 sec).
* I think the images improve the further into the night and I'll be testing that again soon. I've switched to getting a few Ha images as I write this up.
* From memory, I think this affected SII and OIII as well - don't have those filters in right now to test though.
I know I've had this sort of problem for a while but haven't worked it out. My only real guess is that it's something to do with Light Pollution, but I would have thought it would be washed out over the whole image, just like the centre bits.
Camera is the Sbig ST-8XE. (Btw, it's not vignetting.)
Ideas?