Photon -counting Reticontm Detector

For applications in faint -object spectroscopy at the Mt. Hopkins Observatory, a photon counting detector has been built using a dual 936 Reticon diode array fibre -optically coupled to a high -gain image intensifier package. The electronic signal processing follows closely the design of Shectman; the diode array is scanned every millisecond, each frame is subtracted from the previous to minimize multiple counts from the tails of the P -20 phosphor decays, and the centroid of each photon event is located to recover much of the resolution of the first image intensifier. For each detected event, the corresponding address in a Nova computer is updated by a direct memory access, thus allowing simultaneous data collection, analysis, and display. Two different intensifier packages have been used, both sandwiching a 25 -mm Gen II microchannel -plate tube fibre -optically between 40 -mm Gen I tubes for the first and third stages. This has made possible a compact intensifier package at the expense of a poor pulseheight distribution. Our first version of the detector has been in use with the 60 -inch telescope at Mt. Hopkins since February 1978. A new version using a custom dual RL1024SF Reticon is now under construction for use on the MultipleMirror Telescope.