subwindow (293Hz) readout. Images were accumulated for all these settings, and their relative count rates logged by measuring the event counts per second (not per frame) of the same (brightest) spot. The sub-arrays were chosen to cover a region well-filled with measurable spots. In any images, there are spots covering a wide range (about a factor 5) of signal levels. The spots are small and well-separated, so no crowded field issues arise, and they simulate star images in a sparse field. Having established that there is no wavelength dependence of signal saturation, one wavelength was selected for this work for each channel. In addition, all data were taken at nominal and low gain.

In any one image, about 6-10 spots were measured, covering the full range of signals. The same spots were measured for all four flux settings. The signals were measures using the ‘rimexam’ task within IRAF, using radii selected for the spots for object and surrounding background. The same measurement settings were used for all spots and channels. The measurement radius was 5 pixels, and the sky an annulus between radii 7 and 9 pixels. The signal levels were accumulated in a table for all spots and flux 