with high gain (1.46 e$^-$/count, 5.15 e$^-$ read-out noise) and normal read-out speed using one read-out port only. This configuration yields a dispersion of 1.2 Å/pixel. As can be seen from Table \[tab-obs\], the seeing was usually better than the slit width, resulting in a seeing-dependent resolution of the spectra.

[psfile=0505\_2.ps hscale=45 vscale=45 hoffset=-10 voffset=-70 angle=0]{}

  ------- ------------------------- -------- --------- -------------- ----------
  setup        start of observation   seeing   airmass                
                                                         illumination   distance
  a         11/01/2002 05:38:52.722      091     1.319          0.053 
            11/01/2002 06:37:32.265      061     1.319          0.050 
            15/01/2002 05:13:58.529      073     1.322          0.027 
  b         20/01/2002 04:50:50.581      077     1.322          0.350 
            08/02/2002 02:26:30.563      082     1.390          0.161 
            10/02/2002 02:46:38.787      068     1.354          0.046 
  c         09/02/2002 04:02:45.640      067     1.314          0.090 
            09/02/2002 04:46:30.947      050     1.318          0.088 
  ------- ------------------------- -------- --------- -------------- ----------

For each night dome flat fields with two different illumination patterns and CdHeHg wavelength calibration spectra were observed. As part of the standard calibration we were also provided with masterbias frames for our data. The masterbias showed no evidence for hot pixels and was smoothed with a 30$\times$30 box filter to keep any possible large scale variations, but erase noise. The flat fields were averaged for each night and bias-corrected by subtracting the smoothed masterbias of that night. From the flat 