CCD

Abbreviation for charge-coupled device. A CCD is an electronic imaging device widely used in astronomical applications. The CCD consists of semiconducting silicon; when photons of light fall on it, free electrons are released. To preserve the pattern of light falling on the CCD, the photons are collected in a matrix of small picture elements (pixels), which are defined by means of an array of electrodes, called gates, formed on the surface of the CCD. The electric charge in each pixel is then transferred to the ends of the rows by systematically changing the voltages on each gate so that the charge passes along as if on a conveyor belt. Finally, the electrons in each little packet of charge from individual pixels are counted and converted into a form in which the whole image can be stored in a computer or displayed on a television screen. CCDs are particularly useful in astronomy because they have all the qualities required in an astronomical detector: a highly efficient response to light so they can be used to pick up very faint objects, sensitivity over a broad spectrum range, low noise levels and a large dynamic range - that is, they can detect bright and faint objects simultaneously. Furthermore, the output is linear, - the number of electrons collected is in direct proportion to the number of photons received. This means that the image brightness is a direct measure of the real brightness of the object, a property not shared, for example, by photographic emulsions.