1997PASP..109..868V

This is the first paper in a series in which we study the kinematical structure and dynamical evolution of OB associations and young stellar clusters. By means of high-resolution high-S/N spectroscopic observations, we aim at obtaining accurate stellar radial and rotational velocities and information on binarity in a selected number of such systems. In the present paper, we discuss the observations and data reduction for a sample of 156 early-type, established or probable, members of the Sco OB2 association. These stars form a subset of a larger sample that was observed by HIPPARCOS satellite. The observations presented here were preformed at the ESO 1.5 m telescope using the ECHELEC echelle CCD spectrograph. The formal S/N obtained is typically 70 at 3800 Åand 300 at 4050 Å. A new data reduction package was developed. For the sake of qualilty control over the final spectra, we critically evaluated the performace of each procedure in the reduction. Emphasis is on techniques for detecting systematic errors in an empirical way and for maximally eliminating them, at least on a differential level. The accuracy of our reduced spectra is limited by intrinsic instrumental imperfections. Local random errors are given and the amplitude of systematic residuals that (may) occur in certain spectral regions is estimated in considerable detail. In anticipation of next papers in this series, we finally show that the accuracy of differential radial velocities (for spectra of the same star) is essentially limited by centering and zero-point uncertainties amounting to greater or equal to 1km/s (rms).