case we verified that the bias frame does not change significantly from night to night within a run.

A slight complication was posed by our spectrophotometric standard observations. These frames were also using one amplifier/high gain, but (to save some time) only 500 rows were read out (centered on the standard star). Since ESO does not provide an under/overscan region for windowed frames, we took a sufficient number (typically 10) of bias frames in this configuration. Naturally (lacking under/overscan region) these frames were [*not*]{} overscan corrected, nor trimmed. Instead, they were averaged to create a master bias frame, which [*did*]{} include the artificially introduced bias level. We checked the individual frames and confirmed that the variation of this artifical bias level is negligible for these very high S/N frames.

After creating the full and windowed bias frames, all object and calibration (flat and arc) frames were overscan corrected and trimmed (except the windowed frames) and zero subtracted.

At this point we applied a shift in the dispersion direction, based on the slit position, to bring (very crudely; within 10 pixels or 50Å) the observations on a similar wavelength scale. We also inserted gaps in the spatial direction between the neighbouring 