Photo-electric image techniques in astronomy

The underlying scientific principles of light detectors such as the human eye, photographic emulsion or photo-electric device, are explained and their quantum efficiencies compared in terms of signal/noise ratio. Television techniques can be applied to aid astronomical observations and help to overcome, in some measure, the fundamental problems in observing celestial objects through the semi-transparency of the Earth´s atmosphere. The television techniques in the detection of threshold and extremely low light level stellar and planetary images are described. Achievements to date are summarized and the results illustrated by photographs of the Moon and of Mars obtained by the author in 1956. On the latter occasion a low light-level television system was used to record pictures showing the fine surface structure of the planet. The paper concludes with a survey of proposals for the construction of a space orbiting astronomical telescope.