The fixed-pattern noise is computed in the following way: The
difference of two 100x100 sub-frames of the same master flat field
image is determined. These two sub-frames are are shifted with respect
to each other by 10 pixels in X- and Y-direction. The population
standard deviation of the difference frame, divided by the square root
of 2, minus the photon noise (geometrically subtracted) yields the
fixed-pattern noise, which is then divided by the median of the
respective region.

More information can be found on
the <a href="http://www.eso.org/observing/dfo/quality/FORS2/qc/scrflat_qc1.html">plot tutorial page</a>