[Rex]

Hi Guys,
I have been spending some time trying to optimise my system of late, and I have come across something that I would like some advice about. I have attached one of my typical master flats and a flat frame contour plot from the script in PI. I take flats pretty much with every imaging run that I do and I have run this script on several master flats with similar results, so it is definitely caused by something in my system not an anomaly. What could cause the compressing of the contours on the bottom half of my flats, which obviously will also be causing a similar issue in my images. Is it astigmatism of my main mirror? I am pretty sure it is not collimation as that was the first thing I checked. I am open to suggestions as I really am not sure where to start looking. Thanks for the help.