The Goddard High-Resolution Spectrograph: In-Orbit Performance

The in-orbit performance of the Goddard High-Resolution Spectrograph onboard the Hubble Space Telescope (HST) is presented. This report covers the pre-COSTAR period, when instrument performance was limited by the effects of spherical aberration of the telescope's primary mirror. The digicon detectors provide a linear response to count rates spanning over six orders of magnitude, ranging from the normal background flux of 0.01 counts diode⁻¹ s⁻¹ to values larger than 10⁴ counts diode⁻¹ s⁻¹. Scattered light from the first-order gratings is small and can be removed by standard background-subtraction techniques. Scattered light in the échelle mode is more complex in origin, but it also can be accurately removed. Data have been obtained over a wavelength range from below 1100 Å to 3300 Å, at spectral resolutions as high as R = λ/Δλ = 90,000. The wavelength scale is influenced by spectrograph temperature, outgassing of the optical bench, and interaction of the magnetic field within the detector with the Earth's magnetic field. Models of these effects lead to a default wavelength scale with an accuracy better than 1 diode, corresponding to 3 km s⁻¹ in the échelle mode. With care, the wavelength scale can be determined to an accuracy of 0.2 diodes. Calibration of the instrument sensitivity functions is tied into the HST flux calibration through observations of spectrophotometric standard stars. The measurements of vignetting and the échelle blaze function provide relative photometric precision to about 5% or better. The effects of fixed-pattern noise have been investigated, and techniques have been devised for recognizing and removing it from the data. The ultimate signal-to-noise ratio achievable with the spectrograph is essentially limited only by counting statistics, and values approaching 1000:1 have been obtained.