Very Large Telescope: Encyclopedia - Very Large Telescope

Very Large Telescope

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 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.

Very Large Telescope - General information

The VLT consists of a cluster of four large (8.2 meter diameter) telescopes, and an interferometer (VLTI) which is used to resolve fine features. The interferometer will include a set of 1.8 meter diameter telescopes dedicated to interferometric observations. 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, usually 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 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.

Very Large Telescope - 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 0.001 arcsecond at a wavelength of 1 µm. While this is comparable to the resolution achieved using other arrays such as the Navy Prototype Optical Interferometer and the CHARA array, the collecting power of the big telescopes permits much fainter objects to be observed. Also, the VLTI is working 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 up to speed, routinely producing observations.

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

Adapted from the Wikipedia article "Very Large Telescope", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki