Very Large Telescope

From Wikipedia, the free encyclopedia

This article is out of date. Future events or plans discussed in the text need to be reconciled with the events that occured after writing.

Very Large Telescope

The four telescopes of the European Southern Observatory Paranal site. The VLTI (Very Large Telescope Interferometer) building is the low structure in front of the telescopes. Image courtesy of the European Southern Observatory.
Organization	European Southern Observatory
Location	Cerro Paranal, Atacama desert, Chile
Coordinates	24°37′33″S, 70°24′11″W
Altitude	2,635 m
Weather	>340 clear nights/year
Webpage	Very Large Telescope
Telescopes
Antu (UT1)	8,2 m reflector
Kueyen (UT2)	8,2 m reflector
Melipal (UT3)	8,2 m reflector
Yepun (UT4)	8,2 m reflector

One of the four telescopes that make up the VLT, named Kueyen. The 8.2 m mirror can be seen below the large horizontal grey beam (as an oval patch of lightness). Image courtesy of the European Southern Observatory.

The Very Large Telescope Project (VLT) consists of a system of four separate optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal telescope, and the Yepun telescope) organized in an array formation. Each telescope has an 8.2 m aperture. The array is complemented by three movable Auxiliary Telescopes (ATs) of 1.8 m aperture. The project is organized by the European Southern Observatory.

VLT is located at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile.

[edit] General information

The VLT consists of a cluster of four large (8.2 meter diameter) telescopes, and an astronomical interferometer (VLTI) which is used to resolve fine features. The interferometer also includes a set of 1.8 meter diameter movable telescopes dedicated to interferometric observations (actually 3, but 5 in the final design). The 8.2 meter telescopes have been named after the names of some astronomical objects in the local Mapuche language: Antu (The Sun), Kueyen (The Moon), Melipal (The Southern Cross), and Yepun (Venus).

The VLT 8.2 meter telescopes can be operated in three modes:

• as a set of 4 independent telescopes (this is the primary mode of operation)
• as a single large incoherent instrument, for extra light-gathering capacity (this mode has now been abandoned, although multiple telescopes are sometimes independently pointed at the same object, either to increase the total light-gathering power, or to provide simultaneous observations with complementary instruments)
• as a single large coherent interferometric instrument (the VLT Interferometer or VLTI), for extra resolution (this is occasionally used, only for observations of relatively bright sources).

The VLTs are equipped with a large set of instruments permitting observations to be performed from the near-UV to the mid-IR (ie a large fraction of the light wavelengths accessible from the surface of the Earth), with the full range of techniques including high-resolution spectroscopy, multi-object spectroscopy, imaging, and high-resolution imaging. In particular, the VLT has several Adaptive optics systems, which at infrared wavelengths correct for the effects of the atmospheric turbulence, providing images almost as sharp as if the telescope was in space. In the near-IR, the Adaptive Optics images of the VLT are up to 3 times sharper than those of the HST, and the spectroscopic resolution is many times better than Hubble. The VLTs are noted for their high level of observing efficiency and automation.

The principle role of the main VLT telescopes is to operate as four independent telescopes. The interferometry (combining light from multiple telescopes) is used about 20% of the time for very high-resolution on bright objects.

Additionally, the four 8.2m telescopes are accompanied by 4 smaller Auxiliary Telescopes of 1.8 m each (2 operational in 2005, the other 2 in 2006), which can be placed on different positions around the four big telescopes in order to provide better interferometric observations.

The VLT is operated by the European Southern Observatory.

In 2005, VLT telescopes produced some of the first infrared images of extrasolar planets GQ Lupi b and 2M1207b.

[edit] Instruments

Instruments on the VLT:

FORS 1

(FOcal Reducer and low dispersion Spectrograph) is a visible light camera and Multi Object Spectrograph with a 6.8 arcminute field of view.

FORS 2

Like FORS 1, but with further multi-object spectroscopy.

ISAAC

(Infrared Spectrometer And Array Camera) is a near infrared imager and spectrograph

UVES

(Ultraviolett and Visual Echelle Spectrograph) is an ultraviolet and visible light spectrograph.

FLAMES

(Fibre Large Area Multi-Element Spectrograph) is a multi-object fibre feed unit for UVES

NACO

NAOS-CONICA, (NAOS meaning Nasmyth Adaptive Optics System and CONICA meaning COude Near Infrared CAmera). Adaptive optics system

Guest focus

Available for guest instruments

[edit] Interferometry and the VLTI

Most interferometry will be done using 1.8 meter Auxiliary Telescopes (ATs), which will be dedicated to full-time interferometric measurements. The first observations using a pair of ATs were conducted in February 2005, and additional ATs are expected to be installed soon. For interferometric observations on the brightest objects, there is little benefit in using 8 meter telescopes rather than 1.8 meter telescopes.

In its interferometric operating mode, the light from the telescopes is reflected off mirrors and directed through tunnels to a central beam combining laboratory. The VLTI is intended to achieve an effective angular resolution of 0.002 arcsecond at a wavelength of 2 µm. This is comparable to the resolution achieved using other arrays such as the Navy Prototype Optical Interferometer and the CHARA array. Using the big telescopes the faintest object the VLTI can observe is magnitude 7 in the near infrared for broadband observations[1], similar to many other near infrared / optical interferometers without fringe tracking². At more challenging mid infrared wavelengths, the VLTI can reach magnitude 4.5, significantly fainter than the Infrared Spatial Interferometer. When fringe tracking is introduced, the limiting magnitude of the VLTI is expected to improve by a factor of almost 1000, reaching a magnitude of about 14. This is similar to what is expected for other fringe tracking interferometers. In spectroscopic mode, the VLTI can currently reach a magnitude of 1.5. The VLTI can work in a fully integrated way, so that interferometric observations are actually quite simple to prepare and execute.

Because of the many mirrors involved in the VLTI system, a significant fraction of the light is lost before reaching the detector. Additionally, the interferometric technique is such that it is very efficient only of objects that are small enough that all their light is concentrated. For instance, an object with a relatively low surface brightness such as the moon cannot be observed, because its light is too diluted. Only targets which are at temperatures of more than 1000 °C have a surface brightness high enough to be observed in the mid-infrared, and objects must be at several thousands of degrees Celsius for near-infrared observations using the VLTI. This includes most of the stars in the solar neighborhood and many extragalactic objects such as bright active galactic nucleii, but this sensitivity limit rules out interferometric observations of most solar-system objects. Although the use of large telescope diameters and adaptive optics correction can improve the sensitivity a small amount, this cannot extend the reach of optical interferometry beyond nearby stars and the brightest active galactic nucleii.

The first two instruments at the VLTI were VINCI (a test instrument used to set-up the system) and MIDI, which only allowed two telescopes to be used at any one time. With the installation of the three-telescope AMBER closure-phase instrument in 2005, the first imaging observations from the VLTI are expected soon. In 2008 the PRIMA instrument will further enhance the imaging capabilities of the VLTI by allowing phase-referenced imaging.

After falling drastically behind schedule and failing to meet some specifications, in December 2004 the VLT Interferometer became the target of a second ESO recovery plan. This involves additional effort concentrated on more rapid improvements to fringe tracking and the performance of the main delay lines. Note that this only applies to the interferometer and not other instruments on Paranal. In 2005, the VLTI was routinely producing observations, although with a brighter limiting magnitude and poorer observing efficiency than expected.

[edit] See also

• List of observatories
• List of optical telescopes
• List of largest optical reflecting telescopes
• List of astronomical interferometers at visible and infrared wavelengths
• Overwhelmingly Large Telescope
• Giant Magellan Telescope

[edit] External links

• Visits to the Paranal Site
• ESO VLT official site for the 8m telescopes.
• ESO VLTI official site for the interferometer (combining the telescopes)
• Delay Lines for the Very Large Telescopes @Dutch Space
• The VLTI Delay Lines @TNO
• The VLTI Delay Line Article and images by Fred Kamphues/Mill House Engineering
• The PRIMA Star Separator
• ASTROVIRTEL Accessing Astronomical Archives as Virtual Telescopes including archives from the VLT
• Telescope to challenge moon doubters -- note description of VLTI capabilities is innaccurate
• Travelogue VLT Visit