Photometric Nonlinearity of the Faint Object Camera

Abstract

The Faint Object Camera (FOC) of the Hubble Space Telescope builds up an image by counting individual photons. The detector consists of a three-stage image intensifier coupled to a television camera. The video signal from the television camera is processed by a video processing unit (VPU) which attempts to locate the center of the amplified event in order to increment the appropriate memory location. At low light levels essentially all the events are counted, but at higher rates an increasing number are missed by confusion with overlapping events, or by rejection of a cluster of photons as being an ion event.