GHRS Instrument Science Report 029 GHRS Photocathode Granularity Calibration 03/90 Ronald L. Gilliland Abstract: The detector systems for the Goddard High Resolution Spectrograph on HST are photon counting devices, as such they, in principle, should support very high signal-to-noise observations. The limiting factor for most GHRS observations in attaining high S/N will be the characterization of the (2-dimensional) small-scale irregularity of the photocathodes. In this technical note we discuss the physical problem in need of calibration, usual assumptions concerning this calibration, past and current plans, and make a recommendation for a favorable resolution.
