Data Flow System for the Very Large Telescope Interferometer P. Ballester 1a , A.M. Chavan a , B. Cotton b , V. Coudé du Foresto c , A. Glindemann a , C. Guirao a , W. Jaffe d , P. Kervella a , A. Longinotti a , I. Percheron d , M. Peron a , T. Phan Duc a , B. Pirenne a , P.J. Quinn a , A. Richichi a , M. Schöller a , A. Wicenec a , R. Wilhelm a , M. Wittkowski a , S. Zampieri a a European Southern Observatory, Karl-Schwarzschildstr. 2, D-85748 Garching, Germany; b NRAO, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA; c Meudon Observatory, 5, place Jules Janssen, F-92190 Meudon, France; d Leiden Observatory, Niels Bohr Weg 2, NL-2333 CA Leiden, The Netherlands ABSTRACT The Data Flow System is the VLT end-to-end software system for handling astronomical observations from the initial observation proposal phase through the acquisition, processing and control of the astronomical data. The VLT Data Flow System has been in place since the opening of the first VLT Unit Telescope in 1998. When completed the VLT Interferometer will make it possible to coherently combine up to three beams coming from the four VLT 8.2m telescopes as well as from a set of initially three 1.8m Auxiliary Telescopes, using a Delay Line tunnel and four interferometry instruments. The Data Flow system is now in the process of installation and adaptation for the VLT Interferometer. Observation preparation for a multi-telescope system, handling large data volume of several tens of gigabytes per night are among the new challenges offered by this system. This introduction paper presents the VLTI Data Flow system installed during the initial phase of VLTI commissioning and discusses the first experiences with this system. Observation preparation, data archival, and data pipeline processing are addressed. Keywords: Very Large Telescope, Interferometer, Data Flow System, Data Handling 1. INTRODUCTION The fourth 8.2m telescope of the Very Large Telescope (VLT) Observatory on Cerro Paranal (2635m) in Northern Chile has been commissioned for operations at the end of the year 2000. During the same year, the Paranal observatory has been equipped with optical Delay Lines installed in an underground Delay Line tunnel with optical entries for the beams coming from each Unit Telescope (UT) of diameter 8.2 meter as well as from movable Auxiliary Telescopes (AT) of diameter 1.8 meter. Eight Delay Lines in the scheme have optical entries in the Delay Line tunnel. By combining the light from two or three telescopes taken among the four Unit Telescopes and the three initially available Auxiliary Telescopes, different configurations are reached corresponding to a given baseline vector. With a maximum baseline length of 202 meters, the VLT Interferometer makes it possible to reach high angular resolution of the order of a few milliarcseconds. In March 2001, first fringes 2 have been obtained with the VLT Interferometer using the test instrument VINCI 3 . Three more instruments will be active at the VLT Interferometer, providing capabilities for coherent combination in the mid-infrared wavelength domain with MIDI, up to three near-infrared optical beams with AMBER, and simultaneous interferometric observations of two objects with PRIMA. The Data Flow System 4 is the VLT end-to-end software system for handling astronomical observations from the initial observation proposal phase through the acquisition, processing and control of the astronomical data. The operations model of the VLT allows principal investigators to apply for visitor-mode or service-mode observation programs. In visitor mode, the astronomer is present at the telescope and can adapt the observation program to specific target properties, changing observation conditions, or calibration needs. In service-mode the observatory scientific operators perform the observations and the data are processed and sent to the requesting astronomer. The service mode 1 has been supported at the VLT since the beginning of operations with the first telescope ANTU in 1998. Most observations in 1 pballest@eso.org; phone +49 89 320 06 435; fax +49 89 320 23 62; http://www.eso.org; European Southern Observatory, Karl- Schwarzschildstr. 2, D-85748 Garching, Germany