The Very Large Telescope

eso.org | Aug 17th 2011

The world's most advanced visible-light astronomical observatory

The Very Large Telescope array (VLT) is the flagship facility for European ground-based astronomy at the beginning of the third Millennium. It is the world's most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter and four movable 1.8m diameter Auxiliary Telescopes. The telescopes can work together, to form a giant ‘interferometer’, the ESO Very Large Telescope Interferometer, allowing astronomers to see details up to 25 times finer than with the individual telescopes. The light beams are combined in the VLTI using a complex system of mirrors in underground tunnels where the light paths must be kept equal to distances less than 1/1000 mm over a hundred metres. With this kind of precision the VLTI can reconstruct images with an angular resolution of milliarcseconds, equivalent to distinguishing the two headlights of a car at the distance of the Moon.

The 8.2m diameter Unit Telescopes can also be used individually. With one such telescope, images of celestial objects as faint as magnitude 30 can be obtained in a one-hour exposure. This corresponds to seeing objects that are four billion (four thousand million) times fainter than what can be seen with the unaided eye.

The large telescopes are named Antu, Kueyen, Melipal and Yepun. For more information about the meaning of these names, click here.

More detailed background information about the VLT is provided in the VLT Whitebook. The Whitebook was put on-line during the planning and construction phase of the VLT and was kept up-to-date until the VLT First Light event in late May 1998. The Whitebook is based on a large number of documents and articles of varying scope and level. Please note that this document is not further maintained.

Telescopes and Instruments

The VLT instrumentation programme is the most ambitious programme ever conceived for a single observatory. It includes large-field imagers, adaptive optics corrected cameras and spectrographs, as well as high-resolution and multi-object spectrographs and covers a broad spectral region, from deep ultraviolet (300 nm) to mid-infrared (24 µm) wavelengths.

The Unit Telescopes

The 8.2m diameter telescopes are housed in compact, thermally controlled buildings, which rotate synchronously with the telescopes. This design minimises any adverse effects on the observing conditions, for instance from air turbulence in the telescope tube, which might otherwise occur due to variations in the temperature and wind flow. The first of the Unit Telescopes, 'Antu', went into routine scientific operations on 1 April 1999. Today, all four Unit Telescopes and all four Auxiliary Telescopes are operational.

The VLT has made an undisputed impact on observational astronomy. It is the most productive individual ground-based facility, and results from the VLT have led to the publication of an average of more than one peer-reviewed scientific paper per day. VLT contributes greatly to making ESO the most productive ground-based observatory in the world. The VLT has stimulated a new age of discoveries, with several notable scientific firsts, including the first image of an extrasolar planet (eso0428), tracking individual stars moving around the supermassive black hole at the centre of the Milky Way (eso0846), and observing the afterglow of the furthest known Gamma-Ray Burst.

The Auxiliary Telescopes

Although the four 8.2-metre Unit Telescopes can be combined in the VLTI, they are mostly used for individual observations and are only available for interferometric observations for a limited number of nights every year. But four smaller, dedicated 1.8-metre Auxiliary Telescopes (ATs) are available to allow the VLTI to operate every night.

The top part of each AT is a round enclosure, made from two sets of three segments, which open and close. Its job is to protect the delicate 1.8-metre telescope from the desert conditions. The enclosure is supported by the boxy transporter section, which also contains electronics cabinets, liquid cooling systems, air-conditioning units, power supplies, and more. During astronomical observations the enclosure and transporter are mechanically isolated from the telescope, to ensure that no vibrations compromise the data collected.

The transporter section runs on tracks, so the ATs can be moved to 30 different observing locations. As the VLT Interferometer (VLTI) acts rather like a single telescope as large as the group of telescopes combined, changing the positions of the ATs means that the VLTI can be adjusted according to the needs of the observing project.