Histogram of the percentage of recovered flux, for a set of a 3000 simulated lines randomly offset with respect to the center of the slit (solid line) and for the serendipitous lines (dotted line). The vertical lines show the median for the two distributions, around 55% of the recovered flux. The small inset shows the recovered flux as a function of the offsets for the simulated lines, for the 0.7, 0.9 and 1.2 arcsec FWHM lines (respectively in red, black and blue).