C.2	Faint Object Spectrograph (FOS)

The FOS performed low and moderate resolution spectroscopy (R ~ 250 and 1300) in the wavelength range from 1150 Å to 8500 Å. A variety of apertures of different sizes and shapes were available to optimize throughput and spectral or spatial resolution. Ultraviolet linear and circular spectropolarimetric capabilities were also available.

The low resolution mode had two gratings and a prism, and the R = 1300 mode had six gratings to cover the entire spectral range. The photon counting detectors consisted of two 512-element Digicons, one which operated from 1150 Å to 5500 Å (FOS/BLUE), and the other from 1620 Å to 8500 Å (FOS/RED).

Most FOS data were acquired in accumulation and rapid-readout modes; periodic and image modes were used infrequently. Time resolutions as short as 30 msec were feasible. The electron image was magnetically stepped through a programmed pattern during the observations which provided for oversampling, compensation for sensitivity variations along the Digicon array, sky measures and/or measurement of orthogonally polarized spectra. Normally, data were read out in intervals that were short compared to the exposure time.

The FOS received about 20% to 25% of the total HST observing time over Cycles 1 to 6, studying a large and diverse range of science topics. Due to the polarimetric and large dynamic range capabilities, a substantial fraction of these data is and will remain unique.

A major reprocessing of the entire FOS archive has been completed at the Space Telescope-European Coordinating Facility. This new calibration process has substantially improved the data quality and homogeneity of the FOS data. The reprocessed data have been incorporated into the HST Archive and are available through MAST.