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Images taken with modern detectors require calibration via flat fielding toobtain the same flux scale across the whole image. One method for obtaining thebest possible flat fielding accuracy is to derive a photometric model fromdithered stellar observations. A large variety of effects have been taken intoaccount in such modelling. Recently, Moehler et al. (2010) discoveredsystematic variations in available flat frames for the European SouthernObservatory's FORS instrument that change with the orientation of the projectedimage on the sky. The effect on photometry is large compared to othersystematic effects that have already been taken into account. In this paper, wepresent a correction method for this effect: a generalization of the fittingprocedure of Bramich & Freudling (2012) to include a polynomial representationof rotating flat fields. We then applied the method to the specific case ofFORS2 photometric observations of a series of standard star fields, and provideparametrised solutions that can be applied by the users. We found polynomialcoefficients to describe the static and rotating large-scale systematicflat-field variations across the FORS2 field of view. Applying thesecoefficients to FORS2 data, the systematic changes in the flux scale acrossFORS2 images can be improved by ~1% to ~2% of the total flux. This represents asignificant improvement in the era of large-scale surveys, which requirehomogeneous photometry at the 1% level or better.

Removal of systematics in photometric measurements: static and rotating illumination corrections in FORS2@VLT data