  One of the basic parameters of a CCD camera is its gain, i.e. the number of
detected electrons per output Analogue to Digital Unit (ADU). This is normally
determined by finding the statistical variances from a series of flat-field
exposures with nearly constant levels over substantial areas, and making use of
the fact that photon (Poisson) noise has variance equal to the mean. However,
when a CCD has been installed in a spectroscopic instrument fed by numerous
optical fibres, or with an echelle format, it is no longer possible to obtain
illumination that is constant over large areas. Instead of making do with
selected small areas, it is shown here that the wide variation of signal level
in a spectroscopic `flat-field' can be used to obtain accurate values of the
CCD gain, needing only a matched pair of exposures (that differ in their
realisation of the noise). Once the gain is known, the CCD readout noise (in
electrons) is easily found from a pair of bias frames. Spatial stability of the
image in the two flat-fields is important, although correction of minor shifts
is shown to be possible, at the expense of further analysis.
