Charge-coupled devices

Abstract

In classical photoelectric photometry all the light falling through a diaphragm is integrated. When the image of a double star or part of a stellar cluster lies inside the diaphragm all contributions are automatically added. At the same time the sky background is also included in every measurement, making necessary separate observations of the background. Consequently the objects have to be measured sequentially. But there exist two-dimensional detectors which allow one to obtain images of several objects in a single exposure. If those images can be accurately calibrated in terms of irradiance—relatively or absolutely—then a much better use of telescope time will result. The first attempts with photographic emulsions gave poor results in that respect. The photographic plate can store a tremendous amount of spatial information but with a very poor photometric accuracy (see Chapter 14). In addition, the photographic emulsion cannot reliably record large magnitude differences and has such a low quantum efficiency that long exposures are generally required.