TV systems and other detectors
 

Television sensors of various kinds have also found application as detectors. The many advantages that they have include a high quantum efficiency, broad spectral coverage, a large dynamic range, no reciprocity failure for long exposures and the data are in a convenient form for processing by digital computer. As data collectors, they have been superseded by CCD technology but they still remain in use on telescopes for field identification and guidance systems.
One detector system that had great success over a couple of decades was the Image Photon Counting System (IPCS) designed to detect single photoelectrons and assign their positions of origin from within a two-dimensional image, so allowing a picture to be built up. The complete apparatus used a television camera to look at the phosphor screen of a multistage image intensifier. The intensifier gave an amplification sufficient for a single photoelectron to be detected above the readout noise of the television camera. The frame rate of the camera was set to be sufficiently high so that the probability of more than one photoelectron arriving from a resolved picture element during a single frame is extremely low and the decay time of the phosphor was sufficiently short so that the same photoelectron was not counted twice in consecutive frames. A computer controlled the system and the photoelectron events were fed into memory locations which opened and closed in synchronization with the position of the scanning television raster. The IPCS was developed by Boksenberg and had particular success as a detector for the spectrograph on the Anglo-Australian Telescope recording the weak spectra of faint quasars.