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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.

Fri Jun 10 17:31:56 BST 1994