Editing Very Large Telescope (section)

== Technical details ==

=== Telescopes ===

Each Unit Telescope is a [[Ritchey-Chretien]] [[Cassegrain telescope]] with a 22-tonne 8.2-metre [[Zerodur]] primary mirror of 14.4-metre focal length, and a 1.1-metre lightweight beryllium secondary mirror. A flat tertiary mirror diverts the light to one of two instruments at the f/15 [[Nasmyth telescope|Nasmyth]] foci on either side, with a system focal length of 120&nbsp;metres,<ref>{{cite web|url=https://www.eso.org/sci/facilities/develop/documents/VLT-SPE-ESO-10000-2723_is1.pdf | title = Requirements for Scientific Instruments of the VLT Unit Telescopes | publisher = ESO | access-date = 18 January 2018}}</ref> or the tertiary tilts aside to allow light through the primary mirror central hole to a third instrument at the Cassegrain focus. This allows switching between any of the three instruments within five minutes, to match observing conditions. Additional mirrors can send the light via tunnels to the central VLTI beam-combiners. The maximum field-of-view (at Nasmyth foci) is around 27&nbsp;arcminutes in diameter, slightly smaller than the full moon, though most instruments view a narrower field.{{Citation needed|date= April 2018}}

Each telescope has an [[Altazimuth mount|alt-azimuth]] mount with total mass around 350&nbsp;tonnes, and uses [[active optics]] with 150 supports on the back of the primary mirror to control the shape of the thin (177&nbsp;mm thick) mirror by computers.<ref>{{cite web |url=https://www.eso.org/sci/facilities/paranal/telescopes/ut/m1unit.html |title=The VLT primary mirrors: mirror production and measured performance
|first1=P. |last1=Dierickx |first2=D. |last2=Enard |first3=R. |last3=Geyl |first4=J. |last4=Paseri |first5=M. |last5=Cayrel |first6=P. |last6=Béraud |publisher=ESO}}</ref>

=== Instruments ===
{{multiple image
| direction         = vertical
| align             = right
| width             = 300
| image1            = VLT Paranal Observatory - Instruments.jpg
| image2            = The SPHERE exoplanet imager for the VLT.jpg
| image3            = KMOS on the Very Large Telescope at the time of first light.jpg
| image4            = Amber-General2-hires.jpg
| image5            = MUSE on Nasmyth side view.jpg
| image6            = Vimos-VLT.jpg
| image7            = X-Shooter (wallpaper).jpg
| image8            = UVES on UT2-KUEYEN.jpg
| image9            = GRAVITY A powerful new probe of black holes.jpg
| image10           = Paranal FORS1.jpg
| caption1          = A diagram showing instruments at VLT
| caption2          = [[Spectro-Polarimetric High-Contrast Exoplanet Research|SPHERE]] is an [[exoplanet]] imager.<ref>{{cite news |title=Exoplanet Imager SPHERE Shipped to Chile |url=http://www.eso.org/public/announcements/ann14013/ |access-date=12 March 2014 |publisher=ESO |date=18 February 2014}}</ref>
| caption3          = [[K-band Multi-Object Spectrograph|KMOS]] on the VLT's ''Antu'' (UT1) at the time of first light in 2012<ref>{{cite press release |title=24-armed Giant to Probe Early Lives of Galaxies |url=http://www.eso.org/public/news/eso1251/ |access-date=12 December 2012 |agency=ESO }}</ref>
| caption4          = The [[Astronomical Multi-Beam Recombiner|AMBER]] instrument before its installation at the VLTI in 2003
| caption5          = [[Multi Unit Spectroscopic Explorer|MUSE]] mounted on VLT ''Yepun'' (UT4)
| caption6          = [[VIMOS]], the Visible Multi Object Spectrograph, at ''Melipal'' (UT3)
| caption7          = X-shooter spectrograph, 2009
| caption8          = UVES [[spectrograph]] (UT2)
| caption9          = GRAVITY ([[Astronomical interferometer|interferometer]])
| caption10         = FORS-1 at the [[cassegrain focus]] (UT2)
}}
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The VLT instrumentation programme is the most ambitious programme ever conceived for a single observatory. It includes large-field imagers, adaptive optics corrected cameras and spectrographs, as well as high-resolution and [[Multi-Object Spectrometer|multi-object]] spectrographs and covers a broad spectral region, from deep ultraviolet (300&nbsp;nm) to mid-infrared (24&nbsp;μm) wavelengths.<ref name="esovlt"/>

{| class="wikitable"
|+ Instruments on the VLT (in 2023)<ref name="vltinstruments">{{cite web |url=http://www.eso.org/sci/facilities/paranal/instruments.html |title=Paranal Observatory Instrumentation | publisher = ESO |access-date=26 December 2023}}</ref>
|-
! UT#
! Telescope name
! Cassegrain-Focus
! Nasmyth-Focus A
! Nasmyth-Focus B
|-
| align=center | 1
| Antu
| FORS2
| 
| [[K-band Multi-Object Spectrograph|KMOS]]
|-
| align=center | 2
| Kueyen
| VISIR
| FLAMES
| UVES
|-
| align=center | 3
| Melipal
| XSHOOTER
| [[Spectro-Polarimetric High-Contrast Exoplanet Research|SPHERE]]
| CRIRES
|-
| align=center | 4
| Yepun
| ERIS
| HAWK-I
| [[Multi Unit Spectroscopic Explorer|MUSE]]
|}

In addition to these, GRAVITY and MATISSE are currently installed in the VLTI lab, along with ESPRESSO fed via fibre-optics (not interferometric). 

;{{anchor|AMBER}} [[Astronomical Multi-Beam Recombiner|AMBER (VLTI)]]
: The astronomical multi-beam recombiner instrument combines three telescopes of the VLT at the same time, dispersing the light in a spectrograph to analyse the composition and shape of the observed object. AMBER is notably the "most-productive interferometric instrument ever".<ref>{{cite web|url=http://apps.jmmc.fr/bibdb/plots/tag_pies.php?catid=4 |title=most-productive interferometric instrument ever |url-status=dead |archive-url=https://web.archive.org/web/20150609062428/http://apps.jmmc.fr/bibdb/plots/tag_pies.php?catid=4 |archive-date=9 June 2015 }}</ref> It has been decommissioned.<ref>{{Cite web |last=information@eso.org |title=AMBER |url=https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/amber/ |access-date=9 August 2022 |website=eso.org |language=en}}</ref> 
;{{anchor|CRIRES}} CRIRES and CRIRES+
: The cryogenic infrared echelle spectrograph is an adaptive optics assisted [[echelle]] spectrograph. It provides a resolving power of up to 100,000 in the infrared spectral range from 1 to 5&nbsp;micrometres.<p> From 2014 to 2020 it underwent a major upgrade to CRIRES+ to provide ten times larger simultaneous wavelength coverage. A new detector focal plane array of three Hawaii 2RG detectors with a 5.3&nbsp;μm cut-off wavelength replaced the existing detectors, a new spectropolarimetric unit is added, and the calibration system is enhanced. One of the scientific objectives of CRIRES+ is in-transit [[spectroscopy]] of exoplanets, which currently provides us with the only means of studying [[exoplanet]]ary atmospheres. Transiting planets are almost always close-in planets that are hot and radiate most of their light in the [[Infrared|infrared (IR)]]. Furthermore, the IR is a spectral region where lines of [[Molecule|molecular]] gases like [[Carbon monoxide|carbon monoxide (CO)]], [[Ammonia|ammonia (NH<sub>3</sub>)]], and [[Methane|methane (CH<sub>4</sub>)]], etc. are expected from the exoplanetary [[atmosphere]]. This important wavelength region is covered by CRIRES+, which will additionally allow tracking multiple [[absorption lines]] simultaneously.<ref>{{Cite web|title=CRIRES+|url=https://www.eso.org/sci/facilities/develop/instruments/crires_up.html|access-date=24 October 2020|publisher=ESO}}</ref></p>
;{{anchor|ERIS}} ERIS
: The enhanced resolution imager and spectrograph is the newest VLT instrument, which started science operation in 2023. It is an adaptive-optics assisted near-infrared imager (with coronagraph option) and integral-field spectrograph. It replaces the former NACO and SINFONI instruments with improved capability.  
;{{anchor|ESPRESSO}} [[ESPRESSO]]
: The echelle spectrograph for rocky exoplanet and stable spectroscopic observations) is a high-resolution, fiber-fed and cross-dispersed echelle spectrograph for the visible wavelength range, capable of operating in 1-UT mode (using one of the four telescopes) and in 4-UT mode (using all four), for the search for rocky extra-solar planets in the habitable zone of their host stars. Its main feature is the spectroscopic stability and the radial-velocity precision. The requirement is to reach 10&nbsp;cm/s, but the aimed goal is to obtain a precision level of few cm/s. ESPRESSO was installed and commissioned at the VLT in 2017–2018.<ref>{{cite web|url=http://espresso.astro.up.pt/ |title=Espresso |publisher=University of Porto |access-date=21 July 2021}}</ref><ref>{{cite web|url=http://www.eso.org/sci/facilities/develop/instruments/espresso.html |title=ESPRESSO |publisher=ESO |access-date=5 October 2015}}</ref>{{Update inline|date=January 2019}}
; {{anchor|FLAMES}} FLAMES
: The fibre large array multi-element spectrograph is a multi-object fibre feed unit for UVES and GIRAFFE, the latter allowing the capability for simultaneously studying hundreds of individual stars in nearby galaxies at moderate spectral resolution in the visible.<ref>{{Cite web |last= |title=FLAMES |url=https://www.eso.org/public/usa/teles-instr/paranal-observatory/vlt/vlt-instr/flames/ |access-date=21 September 2022 |website=www.eso.org |language=en-us}}</ref>
;{{anchor|FORS}} FORS1/FORS2
: The focal reducer and low dispersion spectrograph is a visible light camera and Multi Object [[Spectrograph]] with a 6.8&nbsp;arcminute field of view. FORS2 is an upgraded version over FORS1 and includes further multi-object spectroscopy capabilities. FORS1 was retired in 2009 to make space for X-SHOOTER; FORS2 continues to operate as of 2021.<ref>{{cite web |url=https://www.eso.org/sci/facilities/paranal/instruments/fors.html |title=FORS – FOcal Reducer and low dispersion Spectrograph |publisher=ESO |date=7 September 2014}}</ref>
;{{anchor|GRAVITY}} [[GRAVITY (VLTI)]]
: GRAVITY is an adaptive optics assisted, near-infrared (NIR) instrument for micro-arcsecond precision narrow-angle astrometry and interferometric phase referenced imaging of faint celestial objects. This instrument interferometrically combines NIR light collected by four telescopes at the VLTI.<ref>{{cite web|url=http://www.mpe.mpg.de/ir/gravity |title=GRAVITY |publisher=Max Planck Institute for Extraterrestrial Physics |access-date=5 April 2021}}</ref>
;{{anchor|HAWK-I}} HAWK-I
: The high acuity wide field K-band imager is a near-infrared imager with a relatively large field of view, about 8x8 arcminutes.<ref>{{Cite web |last=Williams |first=Matt |date=7 September 2022 |title=Star Formation in the Center of the Milky Way Started at the Core and Then Worked its way out |url=https://www.universetoday.com/157361/star-formation-in-the-center-of-the-milky-way-started-at-the-core-and-then-worked-its-way-out/ |access-date=8 September 2022 |website=Universe Today |language=en-US}}</ref><ref>{{Cite web |last= |title=HAWK-I |url=https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/hawk-i/ |access-date=8 September 2022 |website=www.eso.org |language=en}}</ref>
;{{anchor|ISAAC}} ISAAC
: The infrared spectrometer and array camera was a near infrared imager and spectrograph; it operated successfully from 2000 to 2013 and was then retired to make way for SPHERE, since most of its capabilities can now be delivered by the newer HAWK-I or KMOS.
;{{anchor|KMOS}} [[K-band Multi-Object Spectrograph|KMOS]]
: KMOS (K-band Multi Object Spectrograph)<ref>{{Cite web |title=ESO – KMOS |url=https://www.eso.org/sci/facilities/paranal/instruments/kmos.html |access-date=8 September 2022 |website=eso.org}}</ref> is a cryogenic near-infrared multi-object spectrometer, observing 24 objects simultaneously, intended primarily for the study of distant galaxies.
;{{anchor|MATISSE}} MATISSE (VLTI)
: The multi aperture mid-infrared spectroscopic experiment is an infrared spectro-interferometer of the [[#Interferometry and the VLTI|VLT-Interferometer]], which potentially combines the beams of all four Unit Telescopes (UTs) and four Auxiliary Telescopes (ATs). The instrument is used for image reconstruction. After 12 years of development It saw its first light at the telescope in Paranal in March 2018.<ref>{{cite web |url=http://www.eso.org/sci/facilities/develop/instruments/matisse.html |title=MATISSE (the Multi AperTure mid-Infrared SpectroScopic Experiment) |publisher=ESO |date=25 September 2014 |access-date=3 July 2015}}</ref><ref>{{Cite journal |url=https://www.eso.org/sci/publications/messenger/archive/no.157-sep14/messenger-no157-5-12.pdf |title=An Overview of the MATISSE Instrument—Science, Concept and Current Status |journal=The Messenger |volume=157 |pages=5 |date=14 September 2014|bibcode=2014Msngr.157....5L |last1=Lopez |first1=B. |last2=Lagarde |first2=S |last3=Jaffe |first3=W. |last4=Petrov |first4=R. |last5=Schöller |first5=M. |last6=Antonelli |first6=P. |last7=Beckmann |first7=U. |last8=Berio |first8=P. |last9=Bettonvil |first9=F. |last10=Glindemann |first10=A. |last11=Gonzalez |first11=J.-C. |last12=Graser |first12=U. |last13=Hofmann |first13=K.-H. |last14=Millour |first14=F. |last15=Robbe-Dubois |first15=S. |last16=Venema |first16=L. |last17=Wolf |first17=S. |last18=Henning |first18=T. |last19=Lanz |first19=T. |last20=Weigelt |first20=G. |last21=Agocs |first21=T. |last22=Bailet |first22=C. |last23=Bresson |first23=Y. |last24=Bristow |first24=P. |last25=Dugué |first25=M. |last26=Heininger |first26=M. |last27=Kroes |first27=G. |last28=Laun |first28=W. |last29=Lehmitz |first29=M. |last30=Neumann |first30=U. |display-authors=6 }}</ref><ref name="eso1808">{{cite web|title=MATISSE Instrument Sees First Light on ESO's Very Large Telescope Interferometer – Most powerful interferometric instrument ever at mid-infrared wavelengths|url=https://www.eso.org/public/news/eso1808/|publisher=ESO|access-date=5 March 2018}}</ref>
;{{anchor|MIDI}} MIDI (VLTI)
: MIDI is an instrument combining two telescopes of the VLT in the mid-infrared, dispersing the light in a spectrograph to analyse the dust composition and shape of the observed object. MIDI is notably the second most-productive interferometric instrument ever (surpassed by [[Astronomical Multi-Beam Recombiner|AMBER]] recently). MIDI was retired in March 2015 to prepare the VLTI for the arrival of GRAVITY and MATISSE.<ref>{{Cite web |last= |first= |date=21 February 2022 |title=Astronomers Uncover Hidden Supermassive Black Hole |url=https://scitechdaily.com/astronomers-uncover-hidden-supermassive-black-hole/ |access-date=15 September 2022 |website=SciTechDaily |language=en-us}}</ref>
;{{anchor|MUSE}} [[Multi Unit Spectroscopic Explorer|MUSE]]
: MUSE is a huge "3-dimensional" spectroscopic explorer which will provide complete visible spectra of all objects contained in "pencil beams" through the Universe.<ref>{{cite web|url=http://www.eso.org/sci/facilities/develop/instruments/muse/ |title=Muse |publisher=ESO |access-date=17 June 2013}}</ref>
;{{anchor|NACO}} NACO
: NAOS-CONICA, NAOS meaning Nasmyth adaptive optics system and CONICA, meaning Coude near infrared camera) is an [[adaptive optics]] facility which produces infrared images as sharp as if taken in space and includes spectroscopic, polarimetric and coronagraphic capabilities.<ref>{{Cite web |last= |title=NACO |url=https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/naco/ |access-date=21 September 2022 |website=eso.org |language=en}}</ref>
; [[PIONIER (VLTI)]] 
: Is an instrument to combine the light of all 8-metre telescopes, allowing to pick up details about 16 times finer than can be seen with one UT.<ref name="eso.org">{{cite web|url=http://www.eso.org/public/announcements/ann11021/ |title=ann11021 – Light from all Four VLT Unit Telescopes Combined for the First Time |publisher=ESO |date=20 April 2011 |access-date=17 June 2013}}</ref>
;{{anchor|SINFONI}} SINFONI
: The spectrograph for integral field observations in the near infrared) was a medium resolution, [[near-infrared]] (1 to 2.5&nbsp;micrometres) integral field spectrograph fed by an adaptive optics module. It operated from 2003, then retired in June 2019 to make space for the future ERIS.<ref name="Paranal – decommissioned instrument">{{cite web |title=Paranal – decommissioned instruments |url=https://www.eso.org/sci/facilities/paranal/decommissioned.html |access-date=21 July 2021}}</ref>
; [[Spectro-Polarimetric High-Contrast Exoplanet Research|SPHERE]]
: The spectro-polarimetric high-contrast exoplanet research, a high-contrast adaptive optics system dedicated to the discovery and study of [[exoplanet]]s.<ref>{{cite web|url=http://www.eso.org/sci/facilities/paranal/instruments/sphere.html |title=Sphere |publisher=ESO |access-date=2 July 2015}}</ref><ref>{{cite press release |url=http://www.eso.org/public/news/eso1417/ |title=First Light for SPHERE Exoplanet Imager |date=4 June 2014 |publisher=ESO |access-date=25 May 2021}}</ref>
;{{anchor|ULTRACAM}} ULTRACAM
: ULTRACAM is a visitor instrument for ultra-high-speed photometry of variable objects. ULTRACAM provides three simultaneous bands of optical photometry.<ref>{{Cite web |last=Kennedy |first=M. R. |date=18 February 2022 |title=Measuring the mass of the black widow PSR J1555-2908 |url=https://academic.oup.com/mnras/article/512/2/3001/6530666 |access-date=15 September 2022 |website=Oxford Academic}}</ref>
;{{anchor|UVES}} UVES
: The ultraviolet and visual echelle spectrograph is a high-resolution [[ultraviolet]] and visible light [[echelle]] spectrograph.<ref>{{Cite web |title=UVES |url=http://www.eso.org/sci/facilities/paranal/instruments/uves.html |access-date=16 September 2022 |website=www.eso.org}}</ref>
; [[Visible Multi Object Spectrograph|VIMOS]]
: The visible multi-object spectrograph delivered visible images and spectra of up to 1,000 galaxies at a time in a 14 × 14&nbsp;arcmin field of view. It was mainly used for several large redshift surveys of distant galaxies, including VVDS, zCOSMOS and VIPERS. It was retired in 2018 to make space for the return of CRIRES+.<ref name="Paranal – decommissioned instrument"/>
;{{anchor|VINCI}} VINCI (VLTI) 
: VINCI was a test instrument combining two telescopes of the VLT. It was the first-light instrument of the VLTI and is no longer in use.<ref>{{Cite web |last= |title=VINCI |url=https://www.eso.org/public/teles-instr/paranal-observatory/vlt/vlt-instr/vinci/ |access-date=16 September 2022 |website=eso.org |language=en}}</ref>
;{{anchor|VISIR}} VISIR
: The VLT spectrometer and imager for the mid-infrared provides diffraction-limited imaging and spectroscopy at a range of resolutions in the 10 and 20 micrometre mid-infrared (MIR) atmospheric windows. VISIR hosts the NEAR science demonstration, where NEAR is new earths in the alpha centauri region.<ref>{{cite web | url=https://www.eso.org/sci/activities/vltsv/nearsd.html | title=ESO – NEAR Science Demonstration }}</ref>
;{{anchor|X-Shooter}} X-Shooter
: X-Shooter is the first second-generation instrument, operating since 2009.<ref>{{Cite journal |last1=Vernet |first1=J. |last2=Dekker |first2=H. |last3=D’Odorico |first3=S. |last4=Kaper |first4=L. |last5=Kjaergaard |first5=P. |last6=Hammer |first6=F. |last7=Randich |first7=S. |last8=Zerbi |first8=F. |last9=Groot |first9=P. J. |last10=Hjorth |first10=J. |last11=Guinouard |first11=I. |last12=Navarro |first12=R. |last13=Adolfse |first13=T. |last14=Albers |first14=P. W. |last15=Amans |first15=J.-P. |date=1 December 2011 |title=X-shooter, the new wide band intermediate resolution spectrograph at the ESO Very Large Telescope |url=https://www.aanda.org/articles/aa/abs/2011/12/aa17752-11/aa17752-11.html |journal=Astronomy & Astrophysics |language=en |volume=536 |pages=A105 |doi=10.1051/0004-6361/201117752 |arxiv=1110.1944 |bibcode=2011A&A...536A.105V |s2cid=218529727 |issn=0004-6361|doi-access=free }}</ref> It is a very wide-band [UV to near infrared] single-object spectrometer designed to explore the properties of rare, unusual or unidentified sources.

{| class="wikitable sortable"
|-
|+ Instrument summary (as of 2019)<ref name="vltinstruments"/>
|-
! Instrument !! Type !! data-sort-type=number|Wavelength range (nm) !! data-sort-type=number|Resolution (arcsec) !! data-sort-type=number|Spectral resolution || data-sort-type="isoDate" |First light !! Unit !! Position 
|-
| ESPRESSO || Spectrometer || 380–780 || 4 || 140000–180000 || data-sort-value="2018-02-01" |27 November 2017 || 1/all || Coude
|-
| FLAMES || Multi-object spectrometer || 370–950 || n/a || 7500–30000 || data-sort-value="2002-08-01" |Aug 2002 || UT2 || Nasmyth A
|-
| FORS2 || Imager/Spectrometer || 330–1100 || 0.125 || 260–1600 || data-sort-value="1999-01-01" |1999 || UT1 || Cassegrain
|-
| GRAVITY || Imager || 2000–2400 || 0.003 || 22, 500, 4500 || data-sort-value="2015-01-01" |2015 || all || Interferometer
|-
| HAWK-I || Near-IR Imager || 900–2500 || 0.106 || || data-sort-value="2006-07-31" |31 July 2006 || UT4 || Nasmyth A
|-
| KMOS || Near-IR Spectrometer || 800–2500 || 0.2 || 1500–5000 || data-sort-value="2012-11-01" |Nov 2012 || UT1 || Nasmyth B
|-
| MUSE || Integral-field Spectrometer || 365–930 || 0.2 || 1700–3400 || data-sort-value="2014-03-01" |Mar 2014 || UT4 || Nasmyth B
|-
| NACO || AO Imager/Spectrometer || 800–2500 || || 400–1100 || data-sort-value="2001-10-01" |Oct 2001 || UT1 || Nasmyth A
|-
| PIONIER || Imager || 1500–2400 || 0.0025 || || data-sort-value="2010-10-01" |Oct 2010 || all || Interferometer
|-
| SINFONI || Near-IR IFU || 1000–2500 || 0.05 || 1500–4000 || data-sort-value="2004-08-01" |Aug 2004 || UT4 || Cassegrain
|-
| SPHERE || AO || 500–2320 || 0.02 || 30–350 || data-sort-value="2014-05-04" |4 May 2014 || UT3 || Nasmyth A
|-
| UVES || UV/Vis Spectrometer ||  300–500, 420–1100 || 0.16 || 80000–110000 ||data-sort-value="1999-09-01" |Sep 1999 || UT2 || Nasmyth B
|-
| VIMOS || Imager/Multislit Spectrometer || 360–1000, 1100–1800 || 0.205 || 200–2500 || data-sort-value="2002-02-26" |26 February 2002 || UT3 || Nasmyth B
|-
| VISIR || Mid-IR Spectrometer || 16500–24500 ||  || || data-sort-value="2004-01-01" |2004 || UT3 || Cassegrain
|-
| X-SHOOTER || UV-NIR Spectrometer || 300–2500 || || 4000–17000 || data-sort-value="2009-03-01" |Mar 2009 || UT2 || Cassegrain
|}

=== Interferometry ===

[[File:Four VLT Unit Telescopes Working as One.jpg|thumb|upright=1.7|left|All four 8.2-metre Unit Telescopes and 1.8-metre Auxiliary Telescopes were connected for the first time on 17 March 2011, becoming the VLT [[Astronomical interferometer|Interferometer]] (VLTI) with six baselines.<ref>{{cite press release |url=http://www.eso.org/public/unitedkingdom/announcements/ann11021/ |title=Light from all Four VLT Unit Telescopes Combined for the First Time |agency=ESO |date=20 April 2011}}</ref>]]

In its [[Astronomical interferometer|interferometric]] operating mode, the light from the telescopes is reflected off mirrors and directed through tunnels to a central beam combining laboratory. In the year 2001, during commissioning, the VLTI successfully measured the angular diameters of four red dwarfs including [[Proxima Centauri]]. During this operation it achieved an angular resolution of ±0.08 milli-arc-seconds (0.388 nano-radians). This is comparable to the resolution achieved using other arrays such as the [[Navy Prototype Optical Interferometer]] and the [[CHARA array]]. Unlike many earlier optical and infrared interferometers, the [[Astronomical Multi-Beam Recombiner]] (AMBER) instrument on VLTI was initially designed to perform coherent integration (which requires signal-to-noise greater than one in each atmospheric coherence time). Using the big telescopes and coherent integration, the faintest object the VLTI can observe is [[apparent magnitude|magnitude]] 7 in the near infrared for broadband observations,<ref>{{cite web|url=http://www.eso.org/instruments/amber/ |title=AMBER – Astronomical Multi-BEam combineR |publisher=ESO |access-date=17 June 2013}}</ref> similar to many [[List of astronomical interferometers at visible and infrared wavelengths|other near infrared / optical interferometers]] without fringe tracking. In 2011, an incoherent integration mode was introduced<ref>{{cite web|url=http://fizeau.oca.eu/spip.php?article189 |archive-url=https://web.archive.org/web/20120326002449/https://fizeau.oca.eu/spip.php?article189 |url-status=dead |archive-date=26 March 2012 |title=AMBER 'blind mode' |publisher=Fizeau.oca.eu |date=1 January 2012 |access-date=17 June 2013}}</ref> called AMBER "blind mode", which is more similar to the observation mode used at earlier interferometer arrays such as COAST, IOTA and CHARA. In this "blind mode", AMBER can observe sources as faint as K=10 in medium spectral resolution. 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. The VLTI has become worldwide the first general user optical/infrared interferometric facility offered with this kind of service to the astronomical community.<ref>{{cite web |author=|url=http://www.eso.org/sci/publications/messenger/archive/no.119-mar05/The-Messenger-119-willkVLTI.html |title=Observing with the ESO VLT Interferometer |publisher=ESO |date=29 June 2006 |access-date=17 June 2013 |url-status=dead |archive-url=https://web.archive.org/web/20121020223822/http://www.eso.org/sci/publications/messenger/archive/no.119-mar05/The-Messenger-119-willkVLTI.html |archive-date=20 October 2012 }}</ref>

[[File:First light for MATISSE interferometric instrument.jpg|thumb|First light for MATISSE interferometric instrument<ref name="eso1808"/>]]

Because of the many mirrors involved in the optical train, about 95% of the light is lost before reaching the instruments at a wavelength of 1&nbsp;μm, 90% at 2&nbsp;μm and 75% at 10&nbsp;μm.<ref>{{Cite tech report|date=2006|number=VLT-ICD-ESO-15000-1826|title=Interface Control Document between VLTI and its instruments|first1=F.|last1=Puech|first2=P.|last2=Gitton}}</ref> This refers to reflection off 32 surfaces including the [[Reflecting telescope#Coudé|Coudé]] train, the star separator, the main delay line, beam compressor and feeding optics. Additionally, the interferometric technique is such that it is very efficient only for objects that are small enough that all their light is concentrated.<!-- Extremely misleading, should be reworded R J Mathar Mar 17 2011 -->

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 {{convert|1000|C|F|sigfig=2}} 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 nucleus|active galactic nuclei]], but this sensitivity limit rules out [[interferometry|interferometric]] observations of most solar-system objects. Although the use of large telescope diameters and [[adaptive optics]] correction can improve the sensitivity, this cannot extend the reach of optical interferometry beyond nearby stars and the brightest [[active galactic nucleus|active galactic nuclei]].

Because the Unit Telescopes are used most of the time independently, they are used in the interferometric mode mostly during bright time (that is, close to full moon). At other times, [[astronomical interferometer|interferometry]] is done using 1.8-metre Auxiliary Telescopes (ATs), which are dedicated to full-time interferometric measurements. The first observations using a pair of ATs were conducted in February 2005, and all the four ATs have now been commissioned. For interferometric observations on the brightest objects, there is little benefit in using 8 meter telescopes rather than 1.8-metre telescopes.

The first two instruments at the VLTI were VINCI (a test instrument used to set up the system, now decommissioned) and MIDI,<ref>{{cite web|url=http://www.eso.org/sci/facilities/paranal/instruments/midi/ |title=Mid-Infrared Interferometric instrument |publisher=ESO |access-date=17 June 2013}}</ref> which only allow 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.

Deployment of the Phase Referenced Imaging and Microarcsecond Astrometry (PRIMA) instrument started 2008 with the aim to allow phase-referenced measurements in either an astrometric two-beam mode or as a fringe-tracker successor to VINCI, operated concurrent with one of the other instruments.<ref>{{cite journal |first1=J. |last1=Sahlmann|first2=S. |last2=Ménardi|first3=R. |last3=Abuter |first4=M. |last4=Accardo |title=The PRIMA fringe sensor unit |journal=Astronomy & Astrophysics |volume=507 |issue=3 |date=2009 |pages=1739–1757 |doi=10.1051/0004-6361/200912271 |bibcode=2009A&A...507.1739S |last5=Mottini |first5=S. |last6=Delplancke |first6=F.|arxiv = 0909.1470 |s2cid=274903}}</ref><ref>{{cite journal |first1=Francoise |last1=Delplancke |journal=New Astronomical Review| volume=52 |issue=2–5 |date=2008 |pages=189–207 |doi=10.1016/j.newar.2008.04.016 |title=The PRIMA facility phase-referenced imaging and micro-arcsecond astrometry |bibcode=2008NewAR..52..199D}}</ref><ref>{{cite book|first1=J.|last2=Abuter|first2=R.|first3=S.|last3=Menardi|title=Optical and Infrared Interferometry II|series=Proceedings of the SPIE |doi=10.1117/12.856896|date=2010|bibcode=2010SPIE.7734E..22S|issue=7734|last1=Sahlmann|last4=Schmid|first4=C.|last5=Di Lieto|first5=N.|last6=Delplancke|first6=F.|last7=Frahm|first7=R.|last8=Gomes|first8=N.|last9=Haguenauer|first9=P.|last10=Lévêque|first10=S.|last11=Morel|first11=S.|last12=Mueller|first12=A.|last13=Phan Duc|first13=T.|last14=Schuhler|first14=N.|last15=Van Belle|first15=G.|chapter=First results from fringe tracking with the PRIMA fringe sensor unit |pages=773422–773422–12|arxiv = 1012.1321|editor1-last=Danchi|editor1-first=William C|editor2-last=Delplancke|editor2-first=Françoise|editor3-last=Rajagopal|editor3-first=Jayadev K|volume=7734 |s2cid=118479949|display-authors=9}}</ref>

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 improvements to fringe tracking and the performance of the main [[Optical cavity#Optical delay lines|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.

{{asof|March 2008}}, the VLTI had already led to the publication of 89 peer-reviewed publications<ref>{{cite web|url=http://archive.eso.org/wdb/wdb/eso/publications/form |title=ESO Telescope Bibliography |publisher=ESO |access-date=17 June 2013}}</ref> and had published a first-ever image of the inner structure of the mysterious [[Eta Carinae]].<ref>{{cite web |date=23 February 2007 |title=eso0706b – The Inner Winds of Eta Carinae |url=http://www.eso.org/public/images/eso0706b/ |access-date=17 June 2013 |publisher=ESO}}</ref> In March 2011, the [[PIONIER (VLTI)|PIONIER]] instrument for the first time simultaneously combined the light of the four Unit Telescopes, potentially making VLTI the biggest optical telescope in the world.<ref name="eso.org"/> However, this attempt was not really a success.<ref name="mos">{{cite web |last=Moskvitch |first=Katia |date=3 February 2012 |title=Four telescope link-up creates world's largest mirror |url=https://www.bbc.co.uk/news/science-environment-16869022 |access-date=17 June 2013 |work=BBC News}}</ref> The first successful attempt was in February 2012, with four telescopes combined into a 130-metre diameter mirror.<ref name=mos/>

In March 2019, [[European Southern Observatory|ESO]] astronomers, employing the [[#Instruments|GRAVITY instrument]] on their Very Large Telescope Interferometer (VLTI), announced the first [[Methods of detecting exoplanets|direct detection]] of an [[exoplanet]], [[HR 8799 e]], using [[Interferometry|optical interferometry]].<ref name="EA-20190327">{{cite news |author=European Southern Observatory |author-link=European Southern Observatory |title=GRAVITY instrument breaks new ground in exoplanet imaging |url=https://www.eurekalert.org/pub_releases/2019-03/e-gib032619.php |date=27 March 2019 |work=[[EurekAlert!]] |access-date=27 March 2019 }}</ref>

{{multiple image
| direction         = horizontal
| align             = center
| width1            = 215
| width2            = 229
| width3            = 215
| width4            = 215
| width5            = 144
| image1            = Moonset over ESO's Very Large Telescope.jpg
| image2            = Wallpaper of Paranal and the Basecamp.jpg
| image3            = Paranal residencia.jpg
| image4            = A Busy Universe.jpg
| image5            = Eso2004a.jpg
| caption1          = Moonset over Cerro Paranal
| caption2          = The [[ESO Hotel|Paranal Residencia]] and Basecamp at 2,400 meters (7,900 ft)
| caption3          = Inside the Paranal Residencia
| caption4          = A wide view of the VLT with its laser in operation
| caption5          = The night sky at ESO's Paranal Observatory around twilight
}}