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ABSTRACT. 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 10 4 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 echelle mode is more complex in origin, but it also can be accurately removed. Data have been obtained over a wavelength range from below 1100 Â to 3300 Â, at spectral resolutions as high asR = \/A\ = 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 echelle 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 echelle 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.

The Goddard High Resolution Spectrograph (GHRS), currently in Earth orbit on the Hubble Space Telescope (HST), operates in the wavelength range 1150-3200 Â with spectral resolutions (X/SK) of approximately 2X10 3 , 2X10 4 , and 1 X10 5 . The instrument and its development from inception, its current status, the approach to operations, representative results in the major areas of the scientific goals, and prospects for the future are described.

A B S T R A C THigh signal-to-noise ratio spectra of RR Tel obtained at medium resolution with the Goddard High-Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) are used to test available atomic data for the O iv 2s 2 2p 2 P±2s2p 2 4 P multiplet (UV 0.01). The ®ne-structure intervals of the 2s2p 2 4 P term given by Moore (1983) appear to need revision. The¯ux ratios of lines within multiplet UV (0.01), which have a common upper level, depend only on transition probabilities. The observed¯ux ratio of lines from the 4 P 3=2 level differs from that predicted by theory, but this difference cannot be attributed to a blend with a line of S iv]. At the electron densities in the RR Tel nebula, other¯ux ratios give information on the relative electron excitation rates between the 2 P and 4 P ®ne-structure levels. Using the collision strengths calculated by Zhang, Graziani & Pradhan, the rate to the 4 P 5=2 level, relative to the rates to the other J states, appears to be underestimated by ,10 per cent, which is within the expected uncertainty of 20 per cent. We also discuss the S iv 3s 2 3p 2 P±3s3p 2 4 P multiplet.