these slits between readouts so that all bright candidates were observed, while faint objects targeted with other slits were observed with a longer integration time. Due to the extremely high mechanical stability of the FORS instruments, this strategy is very safe. As the sensitivity variation between pixels is potentially color dependent, we decided to generate flat-field frames for each mask. This may not be the optimal strategy since for the slits that are in the same position in two masks, we could use more exposures, thus to create a more accurate flat-field. This alternative strategy would be too complex and the resulting data quality improvement is very marginal, consequently we decided [*against*]{} it.

The last major issue is due to the extremely wide spectral coverage used. As our intention was to correct [*only*]{} for the pixel-to-pixel sensitivity variations, we had to generate a normalization image (a combination of the flat-field lamp spectrum and the overall quantum efficiency of the system as a function of wavelength and spatial position). For high resolution (and smaller wavelength coverage) observations, this is often achieved by collapsing in the spatial direction and fitting a function in the dispersion direction. Unfortunately, this technique proved to be 