The CCD used on SuperCOSMOS is a Loral-Fairchild CCD181. It is a linear device with a variable usable length. On SuperCOSMOS 2048 data pixels are readout. Each pixel is m square, and the length of the original SuperCOSMOS CCD was measured in the laboratory as having its nominal length to an accuracy of . On readout the accumulated charge in each pixel is transferred into one of two shift registers which are located on either side of the light-sensitive portion of the device. One shift register is used for even-numbered pixels, the other for odd-numbered pixels.
The charge transfer time is s, which is negligible compared with the integration time of 8ms, so the readout process results in essentially no dead-time during the scanning. The shift registers are continually clocked-out during an integration - a process which takes . As this time is less than the integration time, the shift registers are empty when each integration is readout, except for 4ms of accumulated dark current in each pixel.
The shift registers themselves have been coated with an opaque paint in order to prevent them responding to incident light during scanning.
The advantage of this system compared with, say, a 2-D device, is that there is no dead time associated with the readout, and hence the table can be operated in a continuous drift-scan mode. In addition, linear devices are cheap, they can be made cosmetically perfect (i.e. no dead pixels) and they have greater well depths, leading to somewhat higher dynamic range. The time taken to scan a plate is almost independent of the number of pixels in the imaging detector, since the ultimate limit on the time needed is determined by the rate at which the CCD output can be digitised by the A-D converter. The A-D installed in the system digitises to 16-bit accuracy at a maximum rate of s per pixel. To decrease the scan time further, a more complex system containing more A-D would be required, or an A-D with a faster clock speed would be required. Hence the choice of a linear CCD does not limit the time needed to scan a plate.