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
Altitude	2,635 m
Weather	>340 clear nights/year
Webpage	Paranal observatory
Telescopes
Antu	8,2 m reflector
Kueyen	8,2 m reflector
Melipal	8,2 m reflector
Yepun	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 a 8.2 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.

General information

The VLT consists of a cluster of four large (8 meter diameter) telescopes, and an interferometer (VLTI) which will be used to resolve fine features. The interferometer will include a set of 1.8 meter diameter telescopes dedicated to interferometric observations. The 8 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 meter telescopes can be operated in three modes:

• as a set of 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)
• as a single large coherent interferometric instrument (the VLT Interferometer or VLTI), for extra resolution (this is occasionally used, usually for observations of relatively bright sources)

The principle role of the VLT is to operate as four independent telescopes, and the interferometry (combining light from multiple telescopes) is a minor secondary role. Most interferometry will be done using 1.8 meter Auxilliary 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 many interferometric observations there is little benefit in using 8 meter telescopes rather than 1.8 meter telescopes.

In its full interferometric operating mode, the four telescopes provide the same light gathering ability (neglecting light losses from the throughput of the instrument) as a single 16 m telescope, making it theoretically the largest optical telescope in the world (the interferometric technique used at the VLTI is not suitable for observing very faint sources, so it actually has poorer sensitivity than single telescopes). The light from the telescopes is reflected off mirrors and directed through tunnels to a central beam combining laboratory. The VLT is intended to achieve an effective angular resolution of 0.001 arcsecond at a wavelength of 1 µm, comparable to that achieved using other arrays such as the Navy Prototype Optical Interferometer and the CHARA Array .

The poor brightness sensitivity of interferometric observations mean that they could not be used for imaging objects with low surface brightness such as the moon. 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 for near-infrared observations using the VLTI. This sensitivity limit rules out interferometric observations of most solar-system objects apart from the Sun. All the stars visible to the naked eye have temperatures of several thousand degrees, and have large enough surface brightnesses and angular sizes to be observed by the VLTI. 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 VLT is operated by the European Southern Observatory.

See also

• List of observatories
• List of optical telescopes
• List of astronomical interferometers at visible and infrared wavelengths
• Overwhelmingly Large Telescope
• Giant Magellan Telescope
• Wikipedia Project: Telescopes

External links

• ESO VLT official site for the 8m telescopes.
• ESO VLTI official site for the interferometer (combining the telescopes)
• ASTROVIRTEL Accessing Astronomical Archives as Virtual Telescopes including archives from the VLT
• Telescope to challenge moon doubters -- note description of VLTI is innaccurate