integer number of pixels. As the objects were sufficiently well sampled (the pixel scale was significantly smaller than the seeing), this step resulted in nearly negligible bluring of the spectra, while preserving the statistical properties of the exposures.

*Extraction*
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Even though for [*sky subtraction*]{} we could safely ignore the slight curvature of dispersed spectra on the CCD, for the extraction of the object signal this is no longer possible. Therefore, we estimated the object position on the detector by collapsing at least 30 columns (more for really faint objects) in the dispersion direction and measuring the object center in the resulting profile. The object position was fitted typically with a second order polynomial as a function of column (wavelength).

Then an aperture [*width*]{} was visually determined. Except in special cases (e.g. blended objects), our aim was to include most of the object signal without adding too much sky (to maximize the signal-to-noise ratio). A one dimensional spectrum was obtained using the ‘optimal extraction’ method of IRAF. This procedure calculates a weighted average in each column, based on both the estimated object profile and photon statistics.

*Wavelength Calibration*
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Wavelength calibration was based on (daytime) arc calibration frames, using four 