linear fit (for longer slits) or just calculating the average (shorter slits) in each column of each slit, rejecting too high pixels (i.e. the targeted object) and subtracting the result. It is important to note that we did [*not*]{} correct for the very slight curvature of the dispersed spectra on the CCD in this step. With the FORS instruments, this strategy works quite well (as opposed to LRIS on the Keck telescope). Significant sky residuals are only present around the very bright, narrow sky lines – where sky subtration is doomed anyway due to pixel saturation.

This procedure works only for our typical faint objects. Extremely bright objects can illuminate the whole slit, thus making correct sky subtraction impossible. Fortunately, in those (very few) cases identification was still possible due to the extremely high object signal.

*Fringe Removal*
----------------

In some cases (especially in MXU masks), we could take advantage of our dithering strategy to reduce further the effect of fringing and the sky residuals. As neither the fringe pattern nor the sky residuals are significantly affected by the small (spatial) offsets of the telescope, we could, in some cases (with sufficient number of exposures in a given mask) exclude 