Normalized IRDIS master-flat image in H band. The illuminated detector halves are surrounded by the field mask {a}. The masked regions (and all pixels with value < 0.1) are set to 1000 and shown as white in this image. This enhances several clusters of dead and hot pixels (e.g., {b} around [x, y]≈[400, 400]). A time-varying read-out signature of the detector is visible as columns of 64 pixels wide (e.g., {c}).