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ESO's two FOcal Reducer and low dispersion Spectrographs (FORS) are theprimary optical imaging instruments for the VLT. They are not direct-imaginginstruments, as there are several optical elements in the light path. Inparticular, both instruments are attached to a field rotator. Obtaining trulyphotometric data with such instruments present a significant challenge. In thispaper, we investigate in detail twilight flats taken with the FORS instruments.We find that a large fraction of the structure seen in these flatfields rotateswith the field rotator.  We discuss in detail the methods we use to determine the cause of thiseffect. The effect was tracked down to be caused by the Linear AtmosphericDispersion Corrector (LADC). The results are thus of special interest fordesigners of instruments with LADCs and developers of calibration plans andpipelines for such instruments. The methods described here to find and correctit, however, are of interest also for other instruments using a field rotator.  If not properly corrected, this structure in the flatfield may degrade thephotometric accuracy of imaging observations taken with the FORS instruments byadding a systematic error of up to 4% for broad band filters. We discussseveral strategies to obtain photometric images in the presence of rotatingflatfield pattern.

Correction of Field Rotator-Induced Flat-Field Systematics - A Case Study Using Archived VLT-FORS Data