[peter_4059 (Peter)]

I've recently been playing with some of my images taken at astrofest to try to understand why the flats are apparently overcorrecting causing light corners. I've tried a lot of different combinations of settings in Deep Sky Stacker as well as other calibration software including CCD Stack, Nebulosity and Pixinsight but get the same result. I've finally come across a work around to correct the issue but would like to understand what is causing it. I suspect it is something to do with the QHY8 being a one shot colour camera and the colour of the light in my light box but would be interested to know if others have seen the same thing and if so whether they found a way to resolve it?

I've found a good way to see if this is happening is to use the threshold function in PS after opening the stacked image. I've included a screen shot to show how this looks.