The FUV detector for the cosmic origins spectrograph on the Hubble Space Telescope

The Cosmic Origins Spectrograph (COS) is a high throughput spectrometer that will be placed on the Hubble Space Telescope (HST) during the last servicing mission in the year 2003. COS will be the most sensitive UV spectrograph ever flown aboard HST and will investigate such fundamental issues as the ionization and baryon content of the intergalactic medium and the origin of large-scale structure of the Universe. The driving design goal for COS is to maximize throughput at a moderate spectral resolution of >20,000 using optics with very few reflections and detectors with high quantum efficiency in two bandpass channels: FUV (1150-1775 A) and NUV (1750-3200 A). The COS FUV detector, a windowless microchannel plate (MCP) detector, consists of two segments each 85 mmx10 mm concatenated end to end with a 9 mm gap between them. The design is based on the Far Ultraviolet Spectroscopic Explorer detectors with identical format and front surface radius of curvature that matches the grating focal plane of the spectrograph. However, enhancements have been made in the design and fabrication of the MCPs, the photocathode, the delay line anode and the readout electronics. We discuss these design enhancements and their significance