FORS2 Rotating Flat Field Systematics Fixed -- Recent Exchange of FORS   LADC Prisms Improves the Long-known Flat-fielding Problem
  For many years the FORS2 instrument has suffered from artefacts in the flat fields caused by surface inhomogeneities on the longitudinal atmospheric dispersion corrector (LADC) that affected high-precision photometric and spectroscopic measurements. Recently, the FORS LADC prisms were exchanged, and our analysis of a large number of flat fields shows that this exchange has resulted in a significant decrease in the level of small-scale artefacts. 
Henri M.J. Boffin, Sabine Moehler, and Wolfram Freudling
astro-ph.IM