Flat fields measured with the narrow-band filters at 442.5 nm (upper left), 580.3 nm (upper right), and 734.3 nm (lower left) and the broadband Johnson/Bessel I filter (lower right). The colour scale is not linear and has been optimised to highlight small variations using histogram equalisation. The four dark corners on all images are not dead pixels but are caused by a stray-light suppression component of the payload called field stop.