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After having established routine science operations for four 8 m single dish telescopes and their first set of instruments at the Paranal Observatory, the next big engineering challenge for ESO has been the VLT Interferometer. Following an intense integration period at Paranal, first fringes were obtained in the course of last year, first with two smaller test siderostats and later with two 8 m VLT telescopes. Even though optical interferometry today may be considered more experimental than single telescope astronomy, we have aimed at developing a system with the same requirements on reliability and operability as for a single VLT telescope. The VLTI control system is responsible for controlling and co-ordinating all devices making up VLTI, where a telescope is just one out of many subsystems. Thus the pure size of the complete system increases the complexity and likelihood of failure. Secondly, some of the new subsystems introduced, in particular the delay lines and the associated fringe-tracking loop, have more demanding requirements in terms of control loop bandwidth, computing power and communication. We have developed an innovative generic multiprocessor controller within the VLT framework to address these requirements. Finally, we have decided to use the VLT science operation model, whereby the observation is driven by observation blocks with minimum human real-time interaction, which implies that VLTI is seen as one machine and not as a set of telescopes and other subsystems by the astronomical instrument. In this paper we describe the as-built architecture of the VLTI control and data flow system, emphasising how new techniques have been incorporated, while at the same time the investments in technology and know-how obtained during the VLT years have been protected. The result has been a faster development cycle, a robustness approaching that of VLT single dish telescopes and a "look and feel" identical to all other ESO observing facilities. We present operation, performance and development cost data to confirm this. Finally we discuss the plans for the coming years, when more and more subsystems will be added in order to explore the full potential of the VLTI.
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