Left panel: intensity map of the differential transmission fiber-to-fiber for the V500-grating (known as fiber-flat), corresponding to the night of the 10th of June 2010. The effects of vignetting are clearly visible with a significant drop of the transmission at the edges of the image. Right panel: spatial distribution of the fraction of pixels free of vignetting, when adopting a conservative criterion of 75% of the average transmission in the central fibers. Two fibers at the bottom/right show a transmission below this limit in the whole wavelength range.