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.