C.3	Goddard High Resolution Spectrograph (GHRS)

The GHRS had two 500-element Digicon detectors that provided sensitivity from 1100 Å to 1900 Å (Side 1, solar blind) and 1150 Å to 3200 Å (Side 2); these detectors offered photon noise-limited data if an observing strategy was undertaken to map out photocathode response irregularities with the FP-SPLIT option. Signal-to-noise ratios of 100 or more were routinely achieved, and upwards of 1000 on occasion.

The GHRS modes included a first order grating covering 1100 Å to1900 Å at R ~ 2500 (285 Å bandpass), four first order holographic gratings with very low scattered light covering 1150 Å – 3200 Å at R ~ 25,000 (27 Å – 45 Å bandpass), and cross-dispersed echelles at R ~ 80,000 over 1150 Å – 3200 Å (6 Å – 15 Å bandpass).

The GHRS had two apertures: the 2.0" Large Science Aperture (LSA), and 0.25" Small Science Aperture (SSA); post-COSTAR the aperture projections were reduced to 1.74" and 0.22", respectively. The SSA projected to one resolution element; thus, even pre-COSTAR data taken with this aperture had the designed spectral resolution, albeit at reduced throughput.

Some data were acquired at time resolutions as short as 50 milliseconds in a Rapid Readout mode. Most observations were acquired in accumulation mode, which provided for oversampling, compensation for sensitivity variations along the Digicon array, and simultaneous monitoring of detector backgrounds. Routine observations of the onboard Pt-Ne emission line lamp provided data with well calibrated wavelengths.

The GHRS received about 20% to 25% of the total HST observing time over Cycles 1 through 6, resulting in a large and diverse range of high quality science data. Due to the high signal-to-noise ratio and large dynamic range capabilities in the far ultraviolet, much of these data are unique.

A major reprocessing of the entire GHRS archive (science and non-science data) has been completed by ST-ECF, CADC, and STScI. This effort substantially improves the data quality and homogeneity. For more information about the reprocessed data, see ISR GHRS-92. The science data can be retrieved currently through the HST online interface at MAST.