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The CCD signal charge is converted to a voltage, then amplified and digitised to 16 bits. The computer measures the signal as an analogue-to-digital conversion unit (ADU). In the present configuration,
1 ADU = 0.95 electrons.
The readout of any signal charge from a pixel has an intrinsic signal independent electrical noise introduced by the output circuitry of the CCD (readout noise). for the current CCD:
Readout noise sigma = 7 adu = 7 electrons r.m.s.
The measurement of readout noise on an erase (bias) frame, that is a frame of zero exposure, is a useful test of whether the CCD camera is working as expected. Photom/stats should reveal a mean background level of 123 adu with a sigma of 7.
The current CCD has a potential well charge of 65535 electrons. Linearity is good up to 40000 electrons with no problems. Exposures are therefore okay up to 40000 adu.