<title>Instrumental artifacts: stray and scattered light in the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope</title>

Abstract

The scientific goals of the Goddard High Resolution Spectrograph (GHRS) require data of very high integrity, with high S/N, spectral resolution, wavelength calibration and photometric accuracy. The instrument was designed to have optical, structural, thermal and detector performance capable of delivering nearly photon limited ultraviolet spectra. Many important characteristics such as geometrical formats, sensitivity functions, resolving power and linearity were predicted from design specifications, and were tested and verified. Instrumental properties which may limit the ultimate performance, such as stray light, distortions and geometrical instabilities were anticipated but not precisely modeled. A careful test program, comprising several distinct pre- and post-launch phases identified, characterized and in several cases eliminated potentially troublesome aspects. In the spirit of 'lessons learned' this paper describes a number of optical and detector related artifacts, how they were identified and measured, and either eliminated by reworking the hardware, or calibrated and compensated for during data reduction. Builders of future instruments should be aware of the types of anomalies we encountered so they may be avoided in the design phases, or well calibrated in the testing phase. We will discuss aspects of the testing programs, both technical and programmatic, which contributed to the successful commissioning of the GHRS and its current return of high quality spectroscopic data.