for the VIS channel when used for drift monitoring on bright guide stars.

There are similar results for the NUV spot signals with low gain setting. For all low gain operations, we note that the flat fields show large variations in signal from centre to edge of the field, so that flux calibrations will be reliable only for objects in the central part. In addition, low gain is intended for cases where there are bright objects giving high count rates. We can adopt low event thresholds to account for the lower gain, but this allows noise to be included in the accepted data. The result of these considerations is that the calibration plots, shown below, show significant scatter and non-linear response at low signal levels. Nevertheless, they are useful in allowing observations of bright targets, which should be placed at the centre of the field and read with a small subarray.

So far, we have used only signal levels in counts from the images as described, and flux levels as measured in arbitrary units on our photometer, via a beamsplitter. In order to convert signal levels to photons at the photocathode, we need to apply a factor that measures the 