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The charge-transfer process is essentially noise-free, and almost all the noise contributed to the signal by the CCD is from the output stage, the voltage detector. This readout noise is from the combined on-chip amplifier and the effective capacitance at the front of this capacitor over which the individual signal charges develop their voltages. For example, the electrode structure of (front-illuminated) GEC chips results in a much smaller effective capacitance than (back-illuminated) RCA chips. Q = CV; therefore given similar amplifier noise, the same charge signal develops a much higher voltage on the GEC chip, which thus has exceptionally low readout noise of some 5 electrons per pixel, compared with 50 electrons per pixel for the RCA chip. (The efficiency of the RCA chip is higher than the uncoated GEC chip, particularly in the blue; the interplay of efficiency and readout noise in terms of signal-to-noise ratio is considered below.) There are various tricks to minimize the readout noise, one of which is very slow clocking to allow long integration of the voltage generated by each charge signal. However, 1/f noise from the output amplifier provides a limit to how slowly the sampling can be done.

Fri Jun 10 17:31:56 BST 1994