Abstract

The Far Ultraviolet Spectroscopic Explorer satellite observes light in the far-ultraviolet spectral region, 905-1187 Å, with a high spectral resolution. The instrument consists of four co-aligned prime-focus telescopes and Rowland spectrographs with microchannel plate detectors. Two of the telescope channels use Al : LiF coatings for optimum reflectivity between approximately 1000 and 1187 Å, and the other two channels use SiC coatings for optimized throughput between 905 and 1105 Å. The gratings are holographically ruled to correct largely for astigmatism and to minimize scattered light. The microchannel plate detectors have KBr photocathodes and use photon counting to achieve good quantum efficiency with low background signal. The sensitivity is sufficient to examine reddened lines of sight within the Milky Way and also sufficient to use as active galactic nuclei and QSOs for absorption-line studies of both Milky Way and extragalactic gas clouds. This spectral region contains a number of key scientific diagnostics, including O VI, H I, D I, and the strong electronic transitions of H2 and HD.