Thanks Marc, yeah I am using the 1100D so it has a shutter, and I usually take my flats at around 0.05 second so it could well be caused by shutter shadow. I will try rotating the camera and see if that fixes it, however I think you're probably on the money with the shutter.
Thanks Bill. these are actually dawn sheet flats, facing west pointing at that dull area in the sky you get just before the sun rises above the horizon. I'll try to take some longer flats and try turning the camera to see if either helps.
Thanks for the help guys.
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