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The preservation of resolution with wavelength is also remarkable, given
that the photon colour defines the depth in the substrate at which the
photoelectrons are generated. The active region of the substrate is 5 to
10 thick, a dimension which is not small in comparison with the pixel
size of 20 to 30
. In front-illuminated (thick) chips, there is some
slight degradation of resolution with wavelength due to lateral spreading
of photoelectrons before collection, but the effect is small enough to be
very difficult to detect in astronomical applications. A second cause of
resolution-loss is imperfect charge-transfer efficiency. In clocking out,
a small proportion of charge is left behind at each transfer, this
residual adding itself to the next charge packet following through the
array. The effect is minimized by buried-channel architecture, for which
the residual charge from each transfer is < 10 times the signal.
However, it is clearly visible at low signal levels for most CCDs
used by astronomers.