Photographing the Stars Electronically

Abstract

by Dietrick E. Thomsen The invention of the photographic plate made a revolution in astronomy that was perhaps greater than the invention of the telescope. For millennia astronomers had depended on the retina of the human eye as an imaging medium. To inform colleagues, observers would make drawings or write descriptions of what they had seen. At times the drawings and descriptions varied widely from astronomer to astronomer, as the controversy over the canals of Mars illustrates. Photography provided objective, storable images in which, presumably, everyone would see the same things. Coupled with a spectroscope, the camera could make records of spectra that would then be available for examination and measurement at leisure. Photography made modern astronomy and astrophysics possible. But lately astronomy's totalitarian dependence on the photographic plate has been slipping. The science has expanded beyond the range of visible light to ranges of the electromagnetic spectrum where photography is either impossible or not very useful. Now, even in t,he visible range, astronomers are becoming impatient with photography. Those who want to study the faintest known objects in the sky (which are usually the most distant seen) or very faint spectral lines (whether they happen to be in the spectra of distant or nearby objects) are finding that if the camera does not give the lie direct, it can be at least highly imprecise. Astronomers want photographic plates on which the darkness of the image is well enough related to the brightness of the light that printed it to enable them to deduce that brightness. With extended images and spectral lines they want to be able to make profiles of brightness over the extent of the image. It is from work of this sort that the bulk of astrophysical data are determined. But at the faint limits of current astronomy even the best photographic emulsions are proving to be incapable or imprecise. To record fainter light astronomers are turning to electrooptic devices of one sort or another, and especially to vidicon systems, which replace the photographic emulsion with the principle of a television camera. A device of this sort, which its inventors, Thomas B. McCord of Massachusetts Institute of Technology and James A. Westphal of California Institute of Technology, say is much more versatile and sensitive than any previous effort, has recently finished a series of test observations with the 152-centimeter telescope at the Inter-American Observatory at 4 CQ) 56&