Alberto Gennai

The VLT Interferometer: a unique instrument for high-resolution astronomy

The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion in this year when the fourth of the 8-m Unit Telescopes will see first light. At the same time, the preparation for first fringes of the VLT Interferometer (VLTI) is advancing rapidly with the goal of having the first fringes with two siderostats within this year. In this article we describe the status of the VLTI and its subsystems, we discuss the planning for first fringes with the different telescopes and instruments. Eventually, we present an outlook for the future of interferometry with Very Large Telescopes.

The VLTI – A Status Report

The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635 m) in Northern Chile is approaching completion. After the four 8-m Unit Telescopes (UT) individually saw first light in the last years, two of them were combined for the first time on October 30, 2001 to form a stellar interferometer, the VLT Interferometer. The remaining two UTs will be integrated into the interferometric array later this year. In this article, we will describe the subsystems of the VLTI and the planning for the following years.

The VLT Interferometer

One of the observing modes available with the ESO Very Large Telescope will be coherent combination of the light received by up to four 8m unit telescopes and several 1.8m auxiliary telescopes. The location of the main telescopes is fixed, while auxiliary telescopes can be moved among some 30 observing stations. The locations of these stations were chosen to augment the (µ, υ) coverage of the unit telescopes as well as to function as an independent interferometric array.

Growing Up — The Completion of the VLTI

The completed VLTI with eight Delay Lines and eight ATs forms the basis for the second generation instrumentation. We describe the events up to first fringes with the test instrument VINCI using the siderostats, and the planning for the immediate future. Multi beam combination for ‘smoother images’ will be briefly discussed as well as artificial guide stars for fringe tracking. New technological developments like fiber optics amplifiers and integrated optics in combination with STJ open the door for a new type of interferometric arrays. Baselines as long as a a few kilometres come into reach. Examples of these second generation interferometers will be given.

FINITO: A Fringe Sensor for VLTI in Two and Three Beam Configuration

ESO and the Astronomical Observatory of Torino (OATo) are collaborating for development of a Fringe Sensor Unit (FSU) for the Very Large Telescope Interferometer (VLTI, see [1] and references therein): FINITO (Fringe-tracking Instrument of Nice and Torino), derived from the concept demonstrated by a prototype from the Observatoire de la Cote d’Azur. It will be used for stabilisation of the Optical Path Difference (OPD) between telescope beams for the first generation scientific interferometers (AMBER, MIDI, VINCI), providing an improvement of up to five magnitudes in limiting sensitivity, thanks to the integration time increased from a few ten milliseconds up to minutes. Accordingly to current planning, FINITO is the only FSU able to support three-beam observations, allowing closure phase measurements in the near and mid IR range.