  Images taken with modern detectors require calibration via flat fielding to
obtain the same flux scale across the whole image. One method for obtaining the
best possible flat fielding accuracy is to derive a photometric model from
dithered stellar observations. A large variety of effects have been taken into
account in such modelling. Recently, Moehler et al. (2010) discovered
systematic variations in available flat frames for the European Southern
Observatory's FORS instrument that change with the orientation of the projected
image on the sky. The effect on photometry is large compared to other
systematic effects that have already been taken into account. In this paper, we
present a correction method for this effect: a generalization of the fitting
procedure of Bramich & Freudling (2012) to include a polynomial representation
of rotating flat fields. We then applied the method to the specific case of
FORS2 photometric observations of a series of standard star fields, and provide
parametrised solutions that can be applied by the users. We found polynomial
coefficients to describe the static and rotating large-scale systematic
flat-field variations across the FORS2 field of view. Applying these
coefficients to FORS2 data, the systematic changes in the flux scale across
FORS2 images can be improved by ~1% to ~2% of the total flux. This represents a
significant improvement in the era of large-scale surveys, which require
homogeneous photometry at the 1% level or better.
