Editing Very Large Telescope (section)

=== 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
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| width1            = 215
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| 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
}}