Calibration of the Hubble Space Telescope Faint Object Camera

We will briefly describe the Hubble Space Telescope Faint Object Camera (FOC) and its operation, and the problems and solutions in calibrating it. The FOC is a photon counting imaging system with two different detectors in two different optical relays capable of f/96, f/288, or f/48 mode imaging, the last mode having a long slit spectrograph capability also (Ref. 1). The detectors (shown schematically in Figure 1), by and large the source of calibration difficulties, detect and store photons by using a three stage image intensifier with bialkalii photocathodes coupled with a television tube followed by signal processing electronics to recognize photon events. As photon events are recognized, they are stored in a digital memory which accumulates the image. The detector is capable of imaging sections of the focal plane photocathode in a wide variety of sizes and offsets (referred to as video formats). The largest has dimensions of 512 by 1024 pixels and covers a 44” by 44” area of the sky (f/48 relay).