B.3 Goddard High Resolution Spectrograph (GHRS)

The GHRS had two, 500-element digicon detectors, which provided sensitivity from 1100 to 1900Å (Side 1-solar blind) and 1150 to 3200Å (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 1100-1900Å at R ~ 2,500 (285Å bandpass), four first order holographic gratings with very low scattered light covering 1150-3200Å at R ~ 25,000 (27-45Å bandpass), and cross-dispersed echelles at R ~80,000 over 1150-3200Å (6-15Å 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 milli-seconds 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 20-25% of the total HST observing time over Cycles 1-6, 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 this data is unique.