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Signal levels, noise and dynamic range

The CCD signal charge is converted to a voltage, amplified and digitised; the computer measures the signal as an analogue-digital conversion unit (ADU). The readout of any signal charge from a pixel has an intrinsic associated electrical noise (see Table 2.1).

There is an artificial, but necessary, electrical offset to the measured signal; this ``bias'' has a mean level of 100-200 adu but needs to be measured since it is subject to a small drift (up to 10 adu from night to night).

The CCD integrates all light that falls on it and is not subject to any limitation of photon rate. The recommended maximum signals (for linearity) are 30,000 adu/pixel, although saturation (at 65,535) is permitted without damage. Thus the dynamic range (from max. signal to 3) is about 10000.