and the NUV and VIS share the focal plane of a second telescope, via a beamsplitter. The 38cm telescopes are designed to deliver images of 1" resolution, but the VIS images will be degraded somewhat by passing through the beamsplitter. Each detector will have filters or gratings on a wheel in the converging beam. All channels will operate simultaneously and have the same (29’) field of view. The UV channels are expected to operate mainly as photon-counters, although they all are capable of ‘integrate’ observations by summing the raw CMOS readout frames. The visible channel is intended mainly for monitoring spacecraft drift by centroiding on a star image using integrate mode, but may also be useful for multi-wavelength observations of variable objects, when used in fast-read photon-counting mode.

The system functional description was given in paper 1. Individual ‘front ends’ - the windows, MCP, taper, and CMOS readout - were assembled after selection of individual components for engineering, flight, and spares units. Considerations in the selections were the window-MCP gap, which affects spatial resolution, photocathode uniformity and QE, and the cosmetic cleanliness of the optical fibre tapers. The engineering unit had a NUV photocathode and larger than optimal window gap. 