An Image Photon Counting System for Optical Astronomy

Publisher Summary This chapter presents an image photon counting system for optical astronomy. The information contained in an optical image can be expressed as the spatial and temporal variation in the number of photons. The problem of detecting and recording such an image is then essentially of counting the number of photons in each image element. The photoelectron events exhibited by a high-gain cascade image intensifier are detected by means of a continuously scanning television camera, acting both as a sensor and buffer store, which is put on-line to a small computer. The framing period of the television camera is set to ensure that the probability of superimposing two or more events in consecutive frames is small. The camera operates with a digital frame scan and an analog line scan. Clock pulses cause a 10-bit counter to accumulate during the scan of each frame. One counter drives a digital-to-analog converter, which in turn provides the reference signal for the frame-scan coil driver. It is observed that the observed modulation is of the level expected for the combination of the photographic resolution pattern and the input lens for the intensifier and does not represent the limitation of the detecting system itself.