The active area of the CCD has 320 pixels parallel to the direction of
the echelle dispersion, spanning a linear dimension of 9.6 mm
(equivalent to an angle of 18'20'' in the sky), and 256
pixels in the cross dispersion direction (spanning 7.68 mm
14' 40''). Each pixel is square (3''.45). When the 18 keV electrons collide with the CCD, they each
generate about 2500 secondary electrons within the silicon layer, and
these secondaries are collected by the charge wells, in much the same
manner as are the electrons that are created by direct
photoillumination. The CCD has two frame registers: one is the active
area that is being bombarded and the other is a storage area of the same
size (and covered by a metal mask) that holds the previous frame's
pattern to be read out.
The large signal gain created by the avalanche of electrons in the CCD
allows the detector to be used as a photon counting device, since each
event stands out as a bright spot with a charge equivalent to 15 times
the rms noise in each pixel. This in turn means that with no great
effort one can achieve a signal-to-noise ratio that is as good as the
limit determined by the statistics of detected photons, rather than by
the readout noise of the CCD or fluctuations in the dark
current. However, this statement does
not apply if one is coadding many independent readouts with very few
events per frame, with no explicit detection of the events as they
arrive. See Jenkins, et al. (1988)
for the mathematical details.