Each front-end assembly had data from a range of operating gap and MCP gain (voltages) to enable the choice of the optimum values for each. In addition, a number of spatial resolution tests were done on prototype front ends, to characterize the dependence on window voltage and wavelength.

The DFL tests yielded dark frames and pulse-height distributions for detectors at their nominal voltage settings. The event centroiding algorithms built into the flight units are the 3-cross, 3-square, and 5-square subarrays from those described in paper 1. A settable bright-object protection threshold is also built in, which shuts down the high voltage, if it is exceeded in adjacent events in the readout.

The Calgary lab is built about a small, dark vacuum tank, with ports that allow light from lamps with a monochromator. The light flux can be controlled by an aperture, and the level is monitored by a UV-sensitive photometer through a beamsplitter from the main beam. The light can be collimated, but the collimated beam is smaller than the detector windows, and not very uniform. Uniform illumination for flat fields was available using an integrating sphere within the tank, fed by an uncollimated beam.

Molecular contamination is important to 