We have used the ARGUS multi-object spectrometer at the CTIO 4 m Blanco telescope to obtain 2756 radial velocity measurements for 1966 individual stars in the globular cluster ω Centauri brighter than blue photographic magnitude of about 16.5. Of these, 1589 stars are cluster members. A comparison with two independent radial velocity studies, carried out by Suntzeff & Kraft and by Mayor et al., demonstrates that the median error of our measurements is below 2 km s −1 for the stars brighter than B-magnitude 15, which constitute the bulk of the sample. The observed velocity dispersion decreases from about 15 km s −1 in the inner few arcmin to about 6 km s −1 at a radius of 25 . The cluster shows significant rotation, with a maximum amplitude of about 6 km s −1 in the radial zone between 6 and 10 . In a companion paper by van de Ven et al., we correct these radial velocities for the perspective rotation caused by the space motion of the cluster, and combine them with the internal proper motions of nearly 8000 cluster members measured by van Leeuwen et al., to construct a detailed dynamical model of ω Centauri and to measure its distance.
We describe the CTIO vidicon camera, its control electronics, and its associated software for data acquisition and reduction. The silicon vidicon and its intensified counterpart, the SIT vidicon, continue to be competitive detectors in astronomy because of their two-dimensional format, ready availability, faint object capability, and their lack of afterglow or counting-rate problems when used on bright objects. However, considerable care in the design of low-noise and suitably stable electronics is required for the full capabilities of the detectors to be realized. Accordingly, we discuss the hardware and software requirements and the optical considerations for the astronomical use of vidicons. We also present in an appendix essential details and circuit diagrams for the hardware that have not been available in the literature.Several examples of tests and observations at the telescope are given to demonstrate the actual performance of the system. With the SIT vidicon the system noise is the equivalent of about two detected photons, so that for most observations photon noise is dominant. Spectra of compact emission-line objects are illustrated as examples of the skysubtraction capability and the resolution of the system. Spectra of a nearby galaxy and a supernova remnant indicate the value of the two-dimensional format of the vidicons.
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