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The peak capacity of a CCD pixel is typically 200,000 electrons, and CCDs are demonstrably linear devices up to within a factor of two of this limit. When the limit is exceeded, the arrival of more photons continues to create further photoelectrons, which then spread out along the column of the CCD.
In addition to saturating the chip, it is possible to saturate the analogue to digital converter. Although the maximum digitised count is 65,535 (2-1), it is advisable to keep the number of counts below about 35,000 to avoid non-linear effects.
Together with readout noise, the saturation level defines the dynamic range of a CCD, crudely, as (saturation level)/(readout noise).