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The Broad Very Telescopes (VLT) is a whole of 4 Télescope S principal and 4 auxiliaries with the Observatoire of Cerro Paranal, located in the desert of Atacama at the north of the Chile, at an altitude of 2.635 Mètre S. It allows the study of the stars in the wavelengths energy of the Ultraviolet the Infrarouge.

It is a European project of the southern European Observatoire (ESO).

History

The idea of the VLT germinated in 1977, at the time of the conference of the ESO, with Geneva in Suisse:

In 1983, before the official beginning of the project, the search for a site starts.
the December 8th 1987, the Council of the ESO launches officially project VLT.
In 1988, Chile gives the site of Cerro Paranal to the ESO. Its surface is of 725 Kilomètre S square.
In 1990, the ESO officially chosen the site of the Cerro Paranal for the VLT. Work begins one year later.
In 1992, the first mirror primary education is run by the German company Schott .
In 1995, the first dome and the first primary education mirror are finished.
In 1997, the first mirror primary education is installed. One year later, the first telescope is inaugurated and used by the researchers.
In 1999, the second telescope is inaugurated. The president of the Republic of Chile, Eduardo Frei, inaugurates officially the VLT the March 5th.
In 2001, all the principal telescopes are operational.
In 2002, a Franco-German team using the VLT proves the presence of a Black hole in the center of our Galaxie.
In 2004, the instrument AMBER is installed. It makes it possible to recombine 3 of the 4 telescopes of 8m, making VLT the largest telescope of the world (on the collecting surface and to be able of resolution combined).
In 2006 the first light of the last instrument of first generation, CRIRES, took place.

Site

The VLT is on the Cerro Paranal pertaining to the Cordillère of the Coast, in the desert of Atacama in the north of the Chile. The site is at an altitude of 2.635 meters and with 12 Kilometer S of the sea, and with 130 kilometers in the south of Antofagasta.

Google Earth coordinates: 24 37 ' 39.1 S, 70 24 ' 14.81 W.

This site offers many advantages:

a quasi non-existent cloud cover (on average 350 nights ago released per annum), being spread out over a kind of 300 kilometers broad band by 2 000 length
altitude makes it possible to have an atmospheric agitation minimal
the proximity of the sea makes it possible to have a thermal difference between the ground and the minimal atmosphere
the geographical insulation makes it possible not to be disturbed by the human activities, in particular with regard to the lights and the clouds of pollution

It is thus a quasi ideal site to place a telescope there, only the earthquakes caused by the tectonic plate of Nazca can inconvenience the observations. For this reason all the buildings of the VLT are built by respecting paraseismic standards.

Installations

Principal telescopes

There are four principal telescopes (also called UT for Unit Telescope ):

UT1: Antu (the Sun), brought into service in June 1998.
UT2: Kueyen (the the Moon), brought into service in October 1999.
UT3: Melipal (the Cross of the South), brought into service in January 2000.
UT4: Yepun (Venus), brought into service in September 2000.

The names of the telescopes are in language Mapuche, a local dialect.

The diameter of each primary education mirror is of 8,2 meters. In addition to their important size, their characteristic is to be very fine, only 17,6 Centimètre S thickness. This smoothness offers important advantages to the level of the manufacturing costs, because as the mirrors are finer, they are less heavy.

But that causes difficulties during their manufacture and their installation. Even if they are fine, they weigh all the same 23 ton S each one and their weight tends to deform them. To cure it, the ESO developed a system Optique activates , where jacks (150 axial and 64 side) cancel the deformations of the mirrors, to keep an optimal form to them whatever their position.

However this flexibility of the mirrors does not make it possible to deform them quickly and the system of active optics is satisfied to compensate for the deformations of the mirrors had with gravity. Other flexible mirrors, much smaller, called deformable mirrors, make it possible as for them to correct the fast aberrations which had with atmospheric turbulence. It is what is called the adaptive Optique and one finds them in particular in instrument NAOS or systems MACAO of the VLTI.

All these autocorrections make VLT the most powerful telescope of the world.

Instruments

The VLT is able to observe the light in a broad spectrum. For this reason the principal telescopes have several hearths making it possible to install various instruments there:

Technology multi-objects (MOS for Multi Object Spectroscopy or in French Spectroscopy multi-objects ) makes it possible to take the luminous spectrum several objects in only one installation. That makes it possible to improve the effectiveness of the telescope, since that avoids making several installations. As example, VIMOS can measure the distances and the properties of close 1.000 celestial objects in only one observation. Where VIMOS takes its readings in a few hours, it would take several months without technology MOS.

Auxiliary telescopes

The VLT was envisaged to be able to make function the four principal telescopes unit, or in recombination per pairs or triplets. This technique is called the optical interferometry (in opposition to the radio interferometry used by the Radiotélescope S). To supplement the network, one can add a group of mobile telescopes. For this reason four auxiliary telescopes (also called AT for Auxiliary Telescopes ) also form part of the installations. These auxiliary telescopes are reserved for interferometry, contrary to UT. It is thus possible to carry out in parallel observations monotélescope " classiques" on UT, and interferometric observations with at.

Each auxiliary telescope has a mirror of 1,8 measure diameter.

First in January 2004 was installed. Second arrived at the end of 2004. Third arrived fine 2005. The two first were tested together, successfully, in the night of the 2 to the February 3rd 2005. They were given officially to the community of the astronomers, on October 1st, 2005.

The four auxiliary telescopes will be operational at the end 2007.

Operating processes

It was expected that the VLT can function according to three modes :

by using the four telescopes independently;
as a recombinator mode: when the images of the 4 mirrors are combined, one obtains an instrument whose collecting surface is equivalent to that of a telescope of 16,6 meters of diameter, but with the same resolution as each telescope of 8 meters;
in interferometric mode, i.e. while combining by Interferometry up to 3 of the telescopes of the VLT (with instrument AMBER or MIDDAY). One then obtains an instrument with a To be able of resolution equivalent to that of a telescope which can go up to 200 meters in diameter (according to the telescopes used), but with the collecting surface of only one telescope of 8m (at best);

In fact, the second mode was not installed for technical reasons. The main part of the observations is thus carried out according to the first mode. The interferometric mode requires the simultaneous use of two or three UT for only one program of observations. In terms of number report/ratio of observations/time spent, this mode thus costs twice expensive but it allows impossible observations in the first mode (thanks to largest To be able of resolution).

However it is only if one observes with UT that the interferometric mode costs more. Auxiliary Telescopes are them held for the Interférométrie and allow the simultaneous operation of the first and the third mode.

Optical interferometry

Just like the radio interferometry used since many years by the radio telescopes, optical interferometry consists in gathering using computers the catches of sights of several telescopes to do only one of them. This technique makes it possible to create a larger telescope virtually. In the case of the VLT, when optical interferometry is used, the precision are such as one could see a man on the the Moon.

The VLTI (I as “interferometer”) is an extremely complex system in order to gather in a coherent way up to three beams coming from UT or many In a part called the focal laboratory which has various instruments which can observe in various fields wavelengths:

AMBER , recombines three telescopes in the close infra-red, from 1 to 2,4 micrometers
MIDDAY makes it possible as for him to recombine two telescopes in the remote infra-red, from 7 to 14 micrometers
VINCI is an instrument of tests and allows to recombine two telescopes in the close infra-red

Effectiveness

The VLT also inaugurates other methods of observations for the researchers. For those which come on the site, all the visual process is made via interposed screens. But the observations can also be carried out, recorded on support CD/DVD and sent. A permanent personnel is charged to carry out technical maintenance, the observations and the service.

Time became a factor more constraining than the purity of the sky. The massive use of technology multi-objects must precisely make it possible to accelerate the number of catches. The VLT must hold its promises of output to ensure its perenniality. The annual budget of the whole of the site is estimated at 50 million Euro.

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