Thanks Andrew.
I see that Buil comments: “For normal camera use it is mandatory to select the « Even illumination » option (accessible also from the ASCOM interface). If you set this option on, the gradient disappears – in this situation the Atik Horizon camera works properly ” Wayne’s images have a vertical gradient to the background, especially when flatfields are generated, which have short exposure times. So turning off this strange progressive shutter is absolutely essential if photometry is the purpose!