arc lamps (a He, a HgCd and two Ar lamps) which provide a sufficient number of lines over the whole spectral range used (3889–9924Å).

The exact same aperture that was used for the science object was used for the arc frames. The resulting lines were first identified automatically. These identifications were then visually verified, and quite often significantly improved. In most cases, around 20 lines were located in the 3889–9924[Å]{} range, and fitted by a forth order polinomial, with a typical rms accuracy of 1[Å]{} or better. This accuracy is close to that expected from the nominal resolution of the instrument in our configuration. The object spectra were then wavelength calibrated and subsequently rebinned to obtain spectra with a linear wavelength scale.

We also examined the stability of the instrument by repeating the daytime calibrations on different days. No noticeable change was detected. In addition, we verified the wavelength calibration by checking the position of narrow skylines in science exposures – no discrepancy was found within our error estimates.

The wavelength calibration may not be accurate in the range outside the two extreme arc lines identified. As the FORS instruments use a grism, we had to resort to high order 