One way to produce good flat-fields are observations of
the sky during twilight (recommended).
For each image scale and wavelength used
in your observations, record a series of images
at a constant integration time,
while the brightness of the sky is increasing (dawn) or
decreasing (sunset). The root mean square in every pixel will give you
its relative efficiency. Normalization of the resulting rms-frame
(median or average) will return the flat-field.
When recording flat-fields with the Fabry-Perot
etalons, it will be necessary to observe the sky about 1 hour
before sunset or 1 hour after sunrise. ESO staff will help you to
setup a batch file for automatic frame recording.