two exposures).

*Data Reduction*
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Data were reduced by our own semi-automatic pipeline built on top of IRAF. In general we followed standard procedures, but had to deviate slightly in several cases to accomodate particularities of the FORS instrument and do a very rigorous book-keeping. In the following sections, we enumerate these changes.

*Bias, Overscan and Trim Correction*
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The FORS CCD’s have in principle 4 read-out modes: high and low gain and one and four amplifier modes. To avoid serious complications, we only used the high gain/one amplifier read-out mode for our spectroscopic observations. This decision resulted in a slightly larger overhead, but this was deemed negligible considering our long integration times, compared to the challenges posed by reducing a 4 amplifier read-out mode spectroscopic observations, where we would have to calibrate the gain of each amplifier very accurately (so we do not introduce artifical features in the spectra).

A sufficient number of full frame bias exposures were taken during each run (typically around 20 per run). These were individually overscan corrected and trimmed. The resulting (bias) frames were averaged with suspect pixels (too high or too low values) filtered out to generate the master bias frame. In each 