To measure the illumination gradient over the FOV, four pixel-windows have been defined on the master lamp flat product, each one 100x100 large, and with a distance of about 100 pixel from the edge of the chip with centers at

• x=150,y=512 (left)
• x=850,y=512 (right)
• x=512,y=150 (bottom)
• x=512,y=850 (top)

Based on these four windows one flux gradient in x and one in y is calculated. The flux gradients are normalized to the size of the FOV (Delta x = 1024 pixel). E.g. xgradient = -0.2 means, that the counts are 20% less on the right rim than on the left rim of the detector.

In the read mode DET.NCORRS.NAME=FowlerNsamp, flats are contamined resulting in a different y-gradient, with respect to lamp flats acquired with DET.NCORRS.NAME=Double_RdRstRd. The gradient monitors a symptom, not a cause.