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A CCD is a silicon based semiconductor arranged as an array of photosensitive elements, each one of which generates photoelectrons and stores them (to use the now familiar figure of speech) as a small `bucket' of charge. Each pixel is typically 20 to 30 m square. CCDs in use as astronomical detectors for the past five years have about 200,000 pixels, usually arranged in a format of about 500 rows by 400 columns, and the active area is thus about 1 square centimetre.
When requested, the elements form a bucket brigade; each row of charges is passed from element to element, a process which is known as clocking, down the columns and horizontally along the final row. The value in each pixel is measured in turn and recorded digitally. To ensure that only positive numbers result from this analogue to digital conversion process a fixed offset known as the bias level is introduced. The charge-transfer process is essentially noise-free and almost all of the noise contributed to the signal by the CCD is from the output stage, where the charge content of each bucket is measured. This is called the readout noise.
The dark current of CCDs means that they must be cooled to cryogenic temperatures for use as astronomical detectors. At room temperatures, the dark signal is such that most CCDs would saturate in < 1 second. Cooling via temperature-controlled liquid-nitrogen dewars or by closed-cycle refrigeration provides a 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. The optimum temperature is about 150 K.