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The third major factor in CCD performance for astronomers is saturation level: how large a charge signal can be held by a pixel. The peak capacity is typically 200,000 electrons, and CCDs are demonstrably linear devices up to within a factor of two of this limit. The arrival of further photons continues to create photoelectrons, which then tend to spread in either direction along the column. Together with readout noise, the saturation level defines the dynamic range of a CCD, crudely, as (saturation-level)/(readout noise). This ratio is 200,000/50 or 4,000 for RCA CCDs, and 40,000 for GEC CCDs, the latter corresponding to 11.5 mag. Of course this assumes working down to the 1 level, something which astronomers would never do. In practice, however, dynamic ranges as large as 8 mag have been used.

Fri Jun 10 17:31:56 BST 1994