Editing Very Large Telescope (section)

== General information ==
[[File:Paranal Platform After Sunset (ESO).jpg|thumb|upright=1.4|VLT's four Unit Telescopes]]

The VLT consists of an arrangement of four large, {{convert|8.2|m|ft|adj=on}} diameter telescopes (called Unit Telescopes or UTs) with optical elements that can combine them into an [[astronomical interferometer]] (VLTI), which is used to resolve small objects. The interferometer also includes a set of four {{convert|1.8|m|ft|adj=on}} diameter movable telescopes dedicated to interferometric observations. The first of the UTs started operating in May 1998 and was offered to the astronomical community on 1 April 1999. The other telescopes became operational in 1999 and 2000, enabling multi-telescope VLT capability. Four 1.8-metre Auxiliary Telescopes (ATs) have been added to the VLTI to make it available when the UTs are being used for other projects. These ATs were installed and became operational between 2004 and 2007.<ref name="esovlt" />

The VLT's 8.2-meter telescopes were originally designed to operate in three modes:<ref>{{cite web |url=http://www.eso.org/sci/publications/messenger/archive/no.130-dec07/messenger-no130-48.pdf |title=Science with the VLT in the ELT Era |access-date=17 June 2013 |url-status=dead |archive-url=https://web.archive.org/web/20120309233722/http://www.eso.org/sci/publications/messenger/archive/no.130-dec07/messenger-no130-48.pdf |archive-date=9 March 2012 }}</ref>
* as a set of four independent telescopes (this is the primary mode of operation).
* as a single large [[Coherence (physics)|coherent]] [[astronomical interferometer|interferometric instrument]] (the VLT Interferometer or VLTI), for extra resolution. This mode is used for observations of relatively bright sources with a small angular extent.
* as a single large incoherent instrument, for extra light-gathering capacity. The instrumentation required to obtain a combined incoherent focus was not originally built. In 2009, new instrumentation proposals were put forward to potentially make that observing mode available.<ref>{{Cite book |doi = 10.1007/978-1-4020-9190-2_68 |chapter = ESPRESSO: A High Resolution Spectrograph for the Combined Coudé Focus of the VLT |url=http://www.eso.org/~jliske/pub/espresso_garching.pdf |title = Science with the VLT in the ELT Era|volume = 9 |pages = 395–399|series = Astrophysics and Space Science Proceedings|year = 2009|last1 = Pasquini |first1 = Luca |display-authors = etal |isbn = 978-1-4020-9189-6 |citeseerx = 10.1.1.218.6892 |archive-url=https://web.archive.org/web/20150609085211/http://www.eso.org/~jliske/pub/espresso_garching.pdf |archive-date=9 June 2015 |url-status=dead |bibcode = 2009ASSP....9..395P }}</ref> 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.

=== Unit telescopes ===
{{multiple image
| direction         = vertical
| align             = right
| width             = 220
| image3            = VLT_Unit_Telescope.jpg
| image1            = The VLT´s Laser Guide Star.jpg
| image2            = Getting the VLT Ready for Even Sharper Images.jpg
| caption3          = Unit telescope undergoing inspection<ref>{{Cite web|url=https://www.eso.org/public/images/potw2014a/|title=A very large eye exam|website=eso.org}}</ref>
| caption1          = Laser used for [[adaptive optics]]. It excites sodium atoms in the atmosphere and creates a [[laser guide star]].
| caption2          = Upgrading ''Yepun'' (UT4) with the "Adaptive Optics Facility" in 2012<ref>{{cite news|title=Getting the VLT Ready for Even Sharper Images|url=http://www.eso.org/public/images/potw1220a/|access-date=14 May 2012|newspaper=ESO Picture of the Week}}</ref>
}}

The UTs are equipped with a large set of instruments permitting observations to be performed from the near-ultraviolet to the mid-infrared (i.e. a large fraction of the [[Infrared astronomy#Astronomers' Infrared Spectrum|light wavelengths accessible from the surface of the Earth]]), with the full range of techniques including high-resolution spectroscopy, [[Multi-Object Spectrometer|multi-object spectroscopy]], imaging, and high-resolution imaging. In particular, the VLT has several [[adaptive optics]] systems, which correct for the effects of atmospheric turbulence, providing images almost as sharp as if the telescope were in space. In the near-infrared, the adaptive optics images of the VLT are up to three times sharper than those of the [[Hubble Space Telescope]], and the spectroscopic resolution is many times better than Hubble. The VLTs are noted for their high level of observing efficiency and automation.

The primary mirrors of the UTs are {{convert|8.2|m|ft|1}} in diameter but, in practice, the pupil of the telescopes is defined by their secondary mirrors, effectively reducing the usable diameter to {{convert|8.0|m|ft|1}} at the [[Nasmyth focus]] and {{convert|8.1|m|ft|1}} at the [[Cassegrain focus]].<ref>{{cite web |title=Very Large Telescope: Requirements for Scientific Instruments on the VLT Unit Telescopes |url=https://www.eso.org/sci/facilities/develop/documents/VLT-SPE-ESO-10000-2723_is1.pdf |website=www.eso.org |access-date=4 January 2024}}</ref>

The 8.2-metre-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.<ref name="vlthandout" />

[[File:The SPHERE instrument attached to the VLT.jpg|left|thumb|The [[Spectro-Polarimetric High-Contrast Exoplanet Research|SPHERE]] instrument attached to the VLT Unit Telescope 3<ref>{{cite news|title=The Strange Case of the Missing Dwarf|url=http://www.eso.org/public/news/eso1506/|access-date=27 February 2015|work=ESO Press Release|agency=European Southern Observatory}}</ref>]]

The principal role of the main VLT telescopes is to operate as four independent telescopes. The interferometry (combining light from multiple telescopes) is used about 20 percent of the time for very high-resolution on bright objects, for example, on [[Betelgeuse]]. This mode allows astronomers to see details up to 25 times finer than with individual telescopes. The light beams are combined in the [[#Interferometry and the VLTI|VLTI]] using a complex system of mirrors in tunnels where the light paths must be kept equal within differences of less than 1&nbsp;μm over a light path of a hundred metres. With this kind of precision, the VLTI can reconstruct images with an angular resolution of milliarcseconds.<ref name="esovlt" />

==== Mapuche names for the Unit Telescopes ====
[[File:Eso-paranal-16.jpg|thumb|left|The interior of ''Antu'' (UT1), meaning "sun" in the [[Mapuche]] language]]

It had long been ESO's intention to provide "real" names to the four VLT Unit Telescopes, to replace the original technical designations of UT1 to UT4. In March 1999, at the time of the Paranal inauguration, four meaningful names of objects in the sky in the [[Mapuche]] language were chosen. These indigenous people live mostly south of Santiago de Chile.

An essay contest was arranged in this connection among schoolchildren of the Chilean II Region of which [[Antofagasta]] is the capital to write about the implications of these names. It drew many entries dealing with the cultural heritage of ESO's host country.

The winning essay was submitted by 17-year-old Jorssy Albanez Castilla from Chuquicamata near the city of [[Calama, Chile|Calama]]. She received the prize, an amateur telescope, during the inauguration of the Paranal site.<ref>{{cite news |date=6 March 1999 |title=VLT Unit Telescopes Named at Paranal Inauguration |url=http://www.eso.org/public/news/eso9921/ |publisher=ESO | access-date=4 May 2011}}</ref>

{{anchor|Antu|Kueyen|Melipal|Yepun}}Unit Telescopes 1–4 are since known as ''Antu'' (Sun), ''Kueyen'' (Moon), ''Melipal'' ([[Southern Cross]]), and ''Yepun'' (Evening Star), respectively.<ref>{{cite web |url=http://www.eso.org/public/teles-instr/vlt/vlt-names.html |title=Names of VLT Unit Telescopes |access-date=4 May 2011}}</ref> Originally there was some confusion as to whether ''Yepun'' actually stands for the evening star Venus, because a Spanish-Mapuche dictionary from the 1940s wrongly translated ''Yepun'' as "Sirius".<ref>{{cite web |url=http://www.eso.org/public/teles-instr/vlt/yepun.html |title=On the Meaning of "YEPUN" |access-date=4 May 2011}}</ref>

=== Auxiliary telescopes ===
[[File:From the Residencia to the Milky Way.jpg|thumb|Auxiliary Telescope, the Residencia and the heart of the [[Milky Way]]<ref>{{cite web|title=From the Residencia to the Milky Way|url=https://www.eso.org/public/images/potw1732a/|website=eso.org|access-date=7 August 2017}}</ref>]]

Although the four 8.2-metre Unit Telescopes can be combined in the [[#Interferometry|VLTI]], their observation time is spent mostly on individual observations, and are used for [[Astronomical interferometer|interferometric]] observations for a limited number of nights every year. However, the four smaller 1.8-metre ATs are available and dedicated to interferometry to allow the VLTI to operate every night.<ref name="vlthandout" />

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 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.<ref name="esovlt" />

The transporter section runs on tracks, so the ATs can be moved to 30 different observing locations. As the 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.<ref name="esovlt" /> The reconfigurable nature of the VLTI is similar to that of the [[Very Large Array]].