impractical for us. The main problem was that we were unable to find an ansatz function that could reproduce the very sharp cutoffs at both ends (due to either the order separation filter or the natural cut-off of the CCD detector) without introducing artifical structure on intermediate scale. An additional complication was that the internal flat-field lamps did not illuminate the slits homogenously – there is a slight gradient in the spatial direction. Therefore, after a slight smoothing of the flat-field exposures, we created the normalization image by a linear or (for very long slits) a second order polynomial fit in the spatial direction. Each flat-field exposure was divided by this normalization frame, thus creating a ‘true’ flat-field frame, which only contains pixel-to-pixel sensitivity variations. In regions, where the signal was too low, the flat-field was artificially set to one (to avoid the introduction of too high photon noise).

After these steps, the individual, normalized flat-field frames were merged, eliminating the effect of light reflection on the slit edges. Both science and wavelength calibration frames for a given mask were divided by the resulting master flat-field frame.

*Sky Subtraction*
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The sky background was estimated in each column by a 