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Dark current decrees that CCDs must be cooled to cryogenic temperatures for use as astronomical detectors. The dark signal arises from electrons thermally generated in the substrate close to the collection area. At room temperatures, the dark signal is such that left to their own devices most CCDs would saturate in < 1 second. Not only would astronomical integrations be hopelessly short, but the electrons generated from astronomical objects would be overwhelmed by the numbers of dark-signal electrons by orders of magnitude. The dark signal is fortunately very temperature-dependent, following the exponential diode-law. Cooling via temperature-controlled liquid-nitrogen dewars or by closed- cycle refrigeration provides the crucial reduction in dark current. The mobility of electrons is somewhat impaired by cooling, so that a compromise is required to maintain adequate charge-transfer efficiency; this compromise occurs at a temperature of 150 .
Careful temperature control of the CCD has a second benefit in that it stabilizes the slightly temperature-sensitive efficiencies of the pixels. Individual pixel responses can be calibrated accurately, and this results in improved imaging and in excellent photoelectric (flux measurement) properties for the device.