The observations - already described in Paper II - were obtained with the La Silla NTT telescope using the Multislit option of EMMI's RILD mode. In this mode, grism # 5 was used, which in combination with a 0.8 arcsec wide slit yields a resolution of 1.3 Å/pixel. The wavelength range is 3600 to 6000 Å in most cases, although the limits vary slightly between individual spectra, as they depend on the position of each slitlet within the mask. A total of 7 masks were produced each including an average of 35 slitlets. The basic reduction steps were described in Paper II including bias correction, flat fielding, wavelength calibration, and aperture extraction. Here we need to discuss some aspects of the wavelength calibration not included in Paper II that are particularly relevant for the determination of radial velocities.
During the observations a technical problem made the Argon lamp too weak to produce reliable identification lines for 2 masks while the strong He I 5875 Å arc line is saturated for several spectra. The problem is worst when both effects appear at the same time as there are no reliable comparison lines redwards He I 5015 Å. For example, this makes useless the He II 5411 Å absorption line for radial velocity measurements in the affected spectra.
The problems with the calibration lamps also altered the normal sequence in which the lamps were taken; the calibration exposures for the last three masks had to be taken at the end of the night. Although there were no telescope presets, the telescope was still tracking so the position angle of the instrument rotator slowly changed and this may introduce systematic errors. We address this issue in the following section.
Copyright ESO 2001