larger than the 40mm detector windows), and the detector window was placed directly in front of the aperture at a distance of 1-2 mm. Flat fields were measured for 5x5 Square and 3x3 Square centroiding algorithms, in photon counting mode, and at high and low voltage for each. Aside from photon statistics, at a particular voltage there would not be any expected broad-scale (i.e. larger than 1 pixel) difference between 5x5 or 3x3 algorithms; the important differences should be seen at sub-pixel resolution where fixed-pattern-noise effects are present. There is an expected broad-scale difference between high and low MCP gain (voltage) however, due to areal variations in MCP gain, as can be seen in Fig 1. At low gain, individual photon events on the CMOS are smaller than a readout pixel, so that the centroiding fixed pattern is extreme. We describe this, and its correction, in the following section. The large-scale flat field images are very uniform for nominal gain, but are significantly peaked in the centre at low gain.

Fixed pattern noise
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There are several factors that affect the accuracy of the sub-pixel part of the centroid, as calculated. The first is because the sampling gradation of the 