Thanks Ray - very interesting, and I had read it before.
I don't get this issue with my SBIG STF-8300 - in fact, the field is very evenly illuminated (despite the weird "WTF" sub a few weeks back
). Months ago, I had painted all my black anodised internal Tak adaptors, and redid them all recently with Black 2.0 ultra-flat black paint. No internal reflections to speak of, and I have shone 240 lumens down there, as well as a 500 lumens torch. Didn't see anything to worry about at all and NO light leaks in the system I could perceive, with the STF-8300 anyway. Maybe with this cam?.
The CCD is a Starlight Xpress M26C. Terry Platt has been VERY forthcoming and helpful and he is of the belief it is a damaged matrix on the sensor. During image download and in pure greyscale, the issue is not at all noticeable - nice, even field (hence why I kept going with 5 hrs data!). It was not until I debayered did I shout the profanities (I did try stacking mono, and unfortunately it does show the "donut" after stacking them all). This leads both Terry and myself to the conclusion the red on the bayer matrix is damaged, and red seems the most susceptible to strong UV.
I await Terry waking up again and seeing my bunch of FITS and more testing and see what he says. It is NOT worth my while having the CCD replaced as it will cost more than what I paid for the cam. I will supply more day shots, uncooled shots, cooled shots and a plethora of night shots. Then make a decision - but it's sadly not looking good, which is a shame as the images are otherwise incredible - next to no noise, lovely contrast, big field.