  ESO's two FOcal Reducer and low dispersion Spectrographs (FORS) are the
primary imaging cameras for the VLT. Since they are not direct-imaging cameras,
the accuracy of photometry which can routinely be obtained is limited by
significant sky concentration and other effects.
  Photometric standard observations are routinely obtained by ESO, and nightly
zero points are computed mainly for the purpose of monitoring the instrument
performance. The accuracy of these zero points is about 10%.
  Recently, we have started a program to investigate, if and how percent-level
absolute photometric accuracy with FORS can be achieved. The main results of
this project are presented in this paper. We first discuss the quality of the
flatfields and how it can be improved. We then use data with improved
flat-fielding to investigate the usefulness of Stetson standard fields for FORS
calibration and the accuracy which can be achieved. The main findings of the
FORS Absolute Photometry Project program are as follows. There are significant
differences between the sky flats and the true photometric response of the
instrument which partially depend on the rotator angle. A second order
correction to the sky flat significantly improves the relative photometry
within the field. Percent level photometric accuracy can be achieved with
FORS1. To achieve this accuracy, observers need to invest some of the assigned
science time for imaging of photometric standard fields in addition to the
routine nightly photometric calibration.
