Reduction procedure for FORS2 observation of GJ 1214 during a transit event. The vertical axis shows the change in stellar flux over time. Upper panel: observations after initial reduction (flat fielding, bias, wavelength calibration, etc.). At this scaling, all visible lines have a telluric or stellar origin. Horizontal lines show sudden decreases in telluric transmission. Middle panel: observations after division by reference stars, removing temporal changes in telluric transmission. Lower panel: each wavelength channel normalized by the flux in out-of-transit exposures. The results show typical transit light curves with an average transit depth in agreement with the previously determined size of the planet.