View source for Extremely Large Telescope

{{Short description|Major astronomical facility in Chile}}
{{Use dmy dates|date=September 2019}}
{{for|the general type of observatory|extremely large telescope}}
{{Infobox telescope
| wavelength = optical and near-infrared
| dome = 86 metres diameter, 74 metres height
| mounting = [[Altazimuth mount|Elevation over azimuth]]
}}

The '''Extremely Large Telescope''' ('''ELT''') is an [[Observatory#Astronomical observatories|astronomical observatory]] under construction.<ref name="NYT-20240308">{{cite news |last=Overbye |first=Dennis |date=8 March 2024 |title=Good News and Bad News for Astronomers' Biggest Dream – The National Science Foundation takes a step (just one) toward an "extremely large telescope." |url=https://www.nytimes.com/2024/03/08/science/astronomy-extremely-large-telescope.html |url-status=live |archiveurl=https://archive.today/20240308112703/https://www.nytimes.com/2024/03/08/science/astronomy-extremely-large-telescope.html |archivedate=8 March 2024 |accessdate=8 March 2024 |work=[[The New York Times]] |language=en |authorlink=Dennis Overbye}}</ref> When completed, it will be the world's largest [[Optical telescope|optical]] and [[Infrared#Regions within the infrared|near-infrared]] [[extremely large telescope]]. Part of the [[European Southern Observatory]] (ESO) agency, it is located on top of [[Cerro Armazones]] in the [[Atacama Desert]] of northern [[Chile]].

The design consists of a reflecting telescope with a {{convert|39|m|ft|sigfig=2|abbr=off|adj=mid|-diameter}} [[Segmented mirror|segmented]] [[primary mirror]] and a {{convert|4.2|m|ft|sigfig=2|abbr=on}} diameter secondary mirror. The telescope is equipped with [[adaptive optics]], six [[laser guide star]] units, and various large-scale scientific instruments.<ref name=govert /><ref name=50percent /> The observatory's design will gather 100 million times more light than the human eye, equivalent to about 10 times more light than the largest [[List of largest optical reflecting telescopes|optical telescopes]] in existence as of 2023, with the ability to correct for atmospheric distortion. It has around 250 times the light-gathering area of the [[Hubble Space Telescope]] and, according to the ELT's specifications, will provide images 15 times sharper than those from Hubble.<ref>{{cite web |url=https://elt.eso.org/about/faq/#question_15 |title=Frequently asked questions about the ELT, question 15 |website=European Southern Observatory |access-date=2 May 2025}}</ref>

The project was originally called the '''European Extremely Large Telescope''' ('''E-ELT'''), but the name was shortened in 2017.<ref>{{Cite web |last=information@eso.org |title=Renaming the E-ELT – Statement from ESO's Director General |url=https://www.eso.org/public/announcements/ann17031/ |access-date=9 October 2023 |website=www.eso.org |language=en}}</ref> The ELT is intended to advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, supermassive black holes, the nature of the Universe's dark sector, and to detect water and organic molecules in [[protoplanetary disk]]s around other stars.<ref name="E-ELT Alone" /> As planned in 2011, the facility was expected to take 11 years to construct, from 2014 to 2025.<ref name="eso executive summary">{{cite news |url=https://www.eso.org/sci/facilities/eelt/docs/e-elt_executivesummary.pdf |title= The E-ELT construction proposal |website=European Southern Observatory |access-date=16 January 2011 }}</ref>

On 11 June 2012, the ESO Council approved the ELT programme's plans to begin civil works at the telescope site, with the construction of the telescope itself pending final agreement with governments of some member states.<ref name="Approval" /> Construction work on the ELT site started in June 2014.<ref name="beginning" /> By December 2014, ESO had secured over 90% of the total funding and authorized construction of the telescope to start, estimated to cost around one billion [[euro]]s for the first construction phase.<ref name="spaceref2014" /> The first stone of the telescope was ceremonially laid on 26 May 2017, initiating the construction of the dome's main structure and telescope.<ref>{{cite news|url=https://www.reuters.com/article/us-chile-telescope-idUSKBN18M2JX|title=Construction begins on world's largest telescope in Chilean desert|date=26 May 2017|work=Reuters}}</ref><ref name="eso1419-groundbreaking" /> The telescope passed the halfway point in its development and construction in July 2023, with the expected completion and [[First light (astronomy)|first light]] set for March 2029.<ref name="start2027">{{cite web |title=ELT Timeline |url=https://elt.eso.org/about/timeline/ |access-date=8 March 2024}}</ref><ref name="50percent">{{cite web |title=ESO's Extremely Large Telescope is now half completed |website=European Southern Observatory |url=https://www.eso.org/public/news/eso2310/|access-date=8 March 2024}}</ref><ref name=":0">{{Cite web |last=information@eso.org |title=Telescope first light for ESO’s Extremely Large Telescope now planned for March 2029 |url=https://www.eso.org/public/announcements/ann25001/ |access-date=2025-03-22 |website=www.eso.org |language=en}}</ref>

== History ==
[[File:The ESO Council during their meeting in Garching on 11–12 June 2012.jpg|thumb|left|upright=1.2|ESO Council meets at ESO headquarters in [[Garching]] bei München, Germany, 2012.<ref name="eso1225">{{cite web |title=ESO To Build World's Biggest Eye on the Sky|url=http://www.eso.org/public/news/eso1225/|access-date=13 June 2012 |website=European Southern Observatory}}</ref>]]

On 26 April 2010, the [[European Southern Observatory]] (ESO) Council selected [[Cerro Armazones]], [[Chile]], as the baseline site for the planned ELT.<ref name="eso1018" /> Other sites that were under discussion included Cerro Macon, Salta, in Argentina; [[Roque de los Muchachos Observatory]], on the Canary Islands; and sites in North Africa, Morocco, and Antarctica.<ref name="Finding-a-home" /><ref name="Vernin2011">{{cite journal |author1=Vernin, Jean |author2=Muñoz-Tuñón, Casiana |author3=Sarazin, Marc |author4=Vazquez Ramió, Héctor |author5=Varela, Antonia M. |author6=Trinquet, Hervé |author7=Miguel Delgado, José |author8=Jiménez Fuensalida, Jesús |author9=Reyes, Marcos |author10=Benhida, Abdelmajid |author11=Benkhaldoun, Zouhair |author12=García Lambas, Diego |author13=Hach, Youssef |author14=Lazrek, M. |author15=Lombardi, Gianluca |author16=Navarrete, Julio |author17=Recabarren, Pablo |author18=Renzi, Victor |author19=Sabil, Mohammed |author20=Vrech, Rubén |title=European Extremely Large Telescope Site Characterization I: Overview |url=http://www.iac.es/site-testing/images/documents/pasp_overview.pdf |journal=Publications of the Astronomical Society of the Pacific |date=1 November 2011 |volume=123 |issue=909 |pages=1334–1346 |doi=10.1086/662995 |bibcode=2011PASP..123.1334V |s2cid=120016246 |access-date=22 June 2014 |archive-url=https://web.archive.org/web/20160304030723/http://www.iac.es/site-testing/images/documents/pasp_overview.pdf |archive-date=4 March 2016 }}</ref>

Early designs included a segmented primary mirror with a diameter of {{convert|42|m|ft|sigfig=2|abbr=off}} and an area of about {{convert| 1300 |m2|ft2|sigfig=3|abbr=on}}, with a secondary mirror with a diameter of {{convert| 5.9 |m|ft|sigfig=2|abbr=on}}. However, in 2011 a proposal was put forward to reduce overall size by 13% to 978 m<sup>2</sup>, with a {{convert| 39 |m|ft|sigfig=2|abbr=on}} diameter primary mirror and a {{convert| 4.2 |m|ft|sigfig=2|abbr=on}} diameter secondary mirror.<ref name="govert">{{cite news |author=Schilling |first=Govert |date=14 June 2011 |title=Europe Downscales Monster Telescope to Save Money |agency=Science Insider |url=https://www.science.org/content/article/europe-downscales-monster-telescope-save-money |access-date=29 June 2020}}</ref> This reduced projected costs from 1.275 billion to 1.055 billion euros and should allow the telescope to be finished sooner. The smaller secondary is a particularly important change; {{convert| 4.2 |m|ft|sigfig=2|abbr=on}} places it within the capabilities of multiple manufacturers, and the lighter mirror unit avoids the need for high-strength materials in the secondary mirror support spider.<ref name="constrproposal" />{{rp||page=15}}

[[File:A towering telescope (DJI 0976).jpg|thumb|upright=1.2|ELT construction status on 1 May 2025]]

ESO's Director General commented in a 2011 press release that "With the new E-ELT design we can still satisfy the bold science goals and also ensure that the construction can be completed in only 10–11 years."<ref name=esoann11034>{{cite news |url=http://www.eso.org/public/announcements/ann11034/ |title=ESO Moves One Step Closer to the First Extremely Large Telescope |agency=ESO |date=15 June 2011 |access-date=17 August 2011 }}</ref> The ESO Council endorsed the revised baseline design in June 2011 and expected a construction proposal for approval in December 2011.<ref name=esoann11034 /> Funding was subsequently included in the 2012 budget for initial work to begin in early 2012.<ref name="eso1150">{{cite web |url=http://www.eso.org/public/news/eso1150/ |title=The E-ELT Moves Closer to Reality |work=ESO |date=9 December 2011}}</ref> The project received preliminary approval in June 2012.<ref name=Approval /> ESO approved the start of construction in December 2014, with over 90% funding of the nominal budget secured.<ref name="spaceref2014" />

The design phase of the 5-mirror [[anastigmat]] was fully funded within the ESO budget. With the 2011 changes in the baseline design (such as a reduction in the size of the primary mirror from 42 m to 39 m), in 2017 the construction cost was estimated to be €1.15 billion (including first generation instruments).<ref>{{cite web|url=http://eso.org/public/blog/eso-collaboration-with-industry/|title=ESO's collaboration with industry|work=Xavier Bacons|publisher=ESO|date=22 December 2017|access-date=14 January 2018}}</ref><ref name="E-ELT Revolution">{{cite web |title=ESO – Preparing a Revolution |url=https://www.eso.org/public/teles-instr/elt/elt_rev/ |access-date=24 January 2018 |archive-date=10 April 2019 |archive-url=https://web.archive.org/web/20190410220027/https://www.eso.org/public/teles-instr/elt/elt_rev/ |url-status=dead }}</ref> In 2014, the start of operations was planned for 2024.<ref name="eso1419-groundbreaking" /> Actual construction officially began in early 2017,<ref>{{cite web|url=https://phys.org/news/2017-05-world-super-telescope.html|title=Construction begins on the world's first super telescope|publisher=phys.org}}</ref> and a technical [[First light (astronomy)|first light]] is planned for 2029.<ref name=":0" /><ref name=start2027 /> First scientific observations are planned for December 2030.<ref name=":0" />

== Planning ==
[[Image:Cerro Armazones night-time panorama.jpg|thumb|Cerro Armazones at night (2010)|upright=1.6]]

ESO focused on the current design after a feasibility study concluded the proposed {{convert| 100 |m|ft|sigfig=3|abbr=on}} diameter, [[Overwhelmingly Large Telescope]], would cost €1.5 billion (£1 billion), and be too complex. Both current fabrication technology and road transportation constraints limit single mirrors to being roughly {{convert| 8 |m|ft|sigfig=2|abbr=on}} per piece. The next-largest telescopes currently in use are the [[Keck Telescopes]], the [[Gran Telescopio Canarias]] and the [[Southern African Large Telescope]], which each use small hexagonal mirrors fitted together to make a composite mirror slightly over {{convert| 10 |m|ft|sigfig=2|abbr=on}} across. The ELT uses a similar design, as well as techniques to work around atmospheric distortion of incoming light, known as [[adaptive optics]].<ref name=messenger />

A 40-metre-class mirror will allow the study of the atmospheres of [[extrasolar planet]]s.<ref name=esoscience>{{cite book|title=An Expanded View of the Universe – Science with the European Extremely Large Telescope|publisher=ESO Science Office|url=http://www.eso.org/sci/facilities/eelt/science/doc/eelt_sciencecase.pdf}}</ref> The ELT is the highest priority in the European planning activities for research infrastructures, such as the [[Astronet]] Science Vision and Infrastructure Roadmap and the ESFRI Roadmap.<ref name='E-ELT Europe'>{{cite web |title=ESO – Europe's Window on the Universe |url=http://www.eso.org/public/teles-instr/e-elt/e-elt_euw.html | access-date=15 June 2011}}</ref> The telescope underwent a Phase B study in 2014 that included "contracts with industry to design and manufacture prototypes of key elements like the primary mirror segments, the adaptive fourth mirror or the mechanical structure (...) [and] concept studies for eight instruments".<ref name="astronet-eu.org">{{Citation |author=Astronet |title=The ASTRONET Infrastructure Roadmap: A Strategic Plan for European Astronomy |date=2008 |url=http://www.astronet-eu.org/IMG/pdf/Astronet-Book.pdf |page=43 |editor1=Bode |editor-first=Michael F. |access-date=21 June 2014 |archive-url=https://web.archive.org/web/20150923175621/http://www.astronet-eu.org/IMG/pdf/Astronet-Book.pdf |isbn=978-3-923524-63-1 |archive-date=23 September 2015 |editor2=Cruz |editor2-first=Maria J. |editor3=Molster |editor3-first=Frank J.}}</ref>

== Design ==
[[File:The European Extremely Large Telescope.jpg|thumb|upright=1.2|Schematic view of the ELT]]

The ELT will use a novel design with a total of five mirrors.<ref>{{cite journal|last = Hippler|first = Stefan|title = Adaptive Optics for Extremely Large Telescopes|journal = [[Journal of Astronomical Instrumentation]]|volume = 8|issue = 2|pages = 1950001–322|date = 2019|doi = 10.1142/S2251171719500016|bibcode = 2019JAI.....850001H|arxiv = 1808.02693|s2cid = 119505402}}</ref> The first three mirrors are curved (non-spherical) and form a [[three-mirror anastigmat]] design for excellent image quality over the 10-arcminute field of view (one-third of the width of the full Moon). The fourth and fifth mirrors are (almost) flat, and respectively provide [[adaptive optics]] correction for atmospheric distortions (mirror 4) and tip-tilt correction for image stabilization (mirror 5). The fourth and fifth mirrors also send the light sideways to one of two [[Nasmyth telescope|Nasmyth]] focal stations at either side of the telescope structure, allowing multiple large instruments to be mounted simultaneously.

=== ELT mirror and sensors contracts ===
==== Primary mirror ====

[[File:The optical system of the ELT showing the location of the mirrors.jpg|thumb|upright=2|The optical system of the ELT showing the location of the mirrors<ref>{{cite web|title=Contracts Signed for ELT Mirrors and Sensors|url=https://www.eso.org/public/news/eso1704/|website=www.eso.org|access-date=23 January 2017}}</ref>]]

The {{convert|39|m|ft|adj=on}} primary mirror will be composed of 798 hexagonal segments, each approximately {{convert|1.4|m|ft}} across and with a thickness of {{cvt|50|mm|in}}.<ref>{{cite web|title=ESO – E-ELT Optics|url=https://www.eso.org/sci/facilities/eelt/telescope/mirrors/|website=www.eso.org|access-date=24 January 2017|language=de}}</ref> Two segments will be re-coated and replaced each working day, to keep the mirror always clean and highly reflective.

Edge sensors constantly measure the positions of the primary mirror segments relative to their immediate neighbours. 2394 position [[actuators]] (3 for each segment) use this information to adjust the system, keeping the overall surface shape unchanged against deformations caused by external factors such as wind, gravity, temperature changes and vibrations.<ref>{{cite web|title=Multiple E-ELT Mirror Segments Tested Together for the First Time|url=https://www.eso.org/public/announcements/ann12013/|website=www.eso.org|access-date=24 January 2017}}</ref>

In January 2017,<ref name="contracts-eso">{{cite web|title=Contracts Signed for ELT Mirrors and Sensors|url=https://www.eso.org/public/news/eso1704/|website=www.eso.org|access-date=24 January 2017}}</ref> ESO awarded the contract for the fabrication of the 4608 edge sensors to the FAMES consortium, which is composed of French company Fogale<ref>{{cite web|title=FOGALE nanotech|url=http://www.fogale.fr/|website=www.fogale.fr|access-date=24 January 2017}}</ref> and German company Micro-Epsilon.<ref>{{cite web|title=High precision sensors, measurement devices and systems|url=http://www.micro-epsilon.com/?sLang=en|website=www.micro-epsilon.com|access-date=24 January 2017|language=en}}</ref> These sensors can measure relative positions to an accuracy of a few nanometres, the most accurate ever used in a telescope.

[[File:First ELT Main Mirror Segments Successfully Cast.jpg|left|thumb|upright=1.2|Cast of the first ELT main mirror segments<ref>{{cite web|title=First ELT Main Mirror Segments Successfully Cast|url=https://www.eso.org/public/news/eso1801/|website=www.eso.org|access-date=9 January 2018}}</ref>]]

In May 2017, ESO awarded two additional contracts. One was awarded to the German company [[Schott AG]] who manufactures the blanks of the 798 segments, as well as a maintenance set of 133 additional segments. This maintenance set allows segments to be removed, replaced, and recoated on a rotating basis once the ELT is in operation. The mirror is being cast from the same low-expansion ceramic [[Zerodur]] as the existing [[Very Large Telescope]] mirrors in Chile.

[[File:E-ELT mirror segments under test.jpg|thumb|upright=1.2|Several segments of the primary mirror]]

The other contract was awarded to the French company, Safran Reosc,<ref>{{cite web|title=Safran Reosc|url=https://www.safran-reosc.com/|website=Safran Reosc|access-date=29 May 2017}}</ref> a subsidiary of [[Safran Electronics & Defense]]. They receive the mirror blanks from Schott, and polish one mirror segment per day to meet the 7-year deadline. During this process, each segment is polished until it has no surface irregularity greater than 7.5 nm [[root mean square]]. Afterward, Safran Reosc mounts, tests, and completes all optical testing before delivery. This is the second-largest contract for ELT construction and the third-largest contract ESO has ever signed.

The segment support system units for the primary mirror were designed and are produced by CESA (Spain)<ref>{{cite web|title=Compañía Española de Sistemas Aeronáuticos|url=http://www.cesa.aero/en/|access-date=29 May 2017}}</ref> and [[VDL Groep|VDL]] (the Netherlands). The contracts signed with ESO also include the delivery of detailed and complete instructions and engineering drawings for their production. Additionally, they include the development of the procedures required to integrate the supports with the ELT glass segments; to handle and transport the segment assemblies; and to operate and maintain them.<ref>{{cite web|title=ESO Awards Contracts for E-ELT Primary Mirror Segment Support System Units|url=https://www.eso.org/public/announcements/ann15003/|website=www.eso.org|access-date=29 May 2017}}</ref>

As of July 2023, over 70% of the mirror segment blanks and their supporting structures had been manufactured,<ref name=50percent /> and by early 2024 tens of segments had been polished.<ref name="SPIE2024">{{cite web |url=https://spie.org/astronomical-telescopes-instrumentation/presentation/ESOs-ELT-halfway-through-construction/13094-43 |title=ESO's ELT halfway through construction |author= Roberto Tamai|publisher=SPIE |date=June 19, 2024}}</ref>

==== Secondary mirror ====

[[File:Blank Canvas the ELT’s M2 Mirror Blank.tif|thumb|upright=1.2|ELT's M2 mirror blank<ref>{{cite web |title=World's largest convex mirror blank ready for final touches – The ELT's M2 blank is being shipped to France for fine polishing |url=https://www.eso.org/public/announcements/ann19003/ |access-date=16 January 2019 |website=www.eso.org |language=en}}</ref>]]
Making the secondary mirror is a major challenge as it is highly convex, and aspheric. It is also very large; at {{convert|4.2|m|ft}} in diameter and weighing {{convert|3.5|t|lb}}, it will be the largest secondary mirror ever employed on an optical telescope and the largest convex mirror ever produced.

In January 2017,<ref name="contracts-eso"/> ESO awarded a contract for the mirror blank to [[Schott AG]], who cast it later the same year from [[Zerodur]]. In May 2017,<ref>{{Cite web |date=2017-05-30 |title=ESO signs contracts for the ELT's gigantic primary mirror |url=https://phys.org/news/2017-05-eso-elt-gigantic-primary-mirror.html |url-status=live |archive-url=https://web.archive.org/web/20240614114357/https://phys.org/news/2017-05-eso-elt-gigantic-primary-mirror.html |archive-date=2024-06-14 |access-date=2024-06-14 |website=[[Phys.org]]}}</ref> Schott AG was also awarded the contract for the much larger primary segment of the mirror.

Complex support cells are also necessary to ensure the flexible secondary and tertiary mirrors retain their correct shape and position; these support cells will be provided by [[SENER]].<ref>{{cite web |title=SENER group |url=http://www.engineeringandconstruction.sener/sener-group |url-status=dead |archive-url=https://web.archive.org/web/20170227121133/http://www.engineeringandconstruction.sener/sener-group |archive-date=2017-02-27 |access-date=2017-01-24 |website=[[SENER]] |language=es}}</ref> Like the tertiary mirror, the secondary mirror will be mounted on 32 points, with 14 along its edges and 18 on the back. The entire assembly will be mounted on a hexapod, allowing its position to be aligned every few minutes to sub-micrometer precision. Deformations on the secondary mirror have a much smaller effect on the final image compared to errors on the tertiary, quaternary, or quinary mirrors.<ref name=M2M3 />

The pre-formed glass-ceramic blank of the secondary mirror is being polished and tested by Safran Reosc.<ref>{{cite web|title=ESO Signs Contract to Polish the E-ELT Secondary Mirror – French company Reosc will polish the largest secondary mirror ever built|url=https://www.eso.org/public/announcements/ann16045/|website=www.eso.org|access-date=24 January 2017}}</ref><ref>{{cite web|title=Safran|url=http://www.safran-group.com/|website=Safran|access-date=24 January 2017}}</ref><ref name=50percent /> The mirror will be shaped and polished to a precision of 15 nanometres (15 millionths of a millimetre) over the optical surface.

By early 2024 this mirror was reported to be close to final accuracy.<ref name="SPIE2024"/>

==== Tertiary mirror ====
The {{convert|3.8|m|ft|adj=on}} concave tertiary mirror, also cast from Zerodur, will be an unusual feature of the telescope. Most current large telescopes, including the VLT and the NASA/ESA Hubble Space Telescope, use two curved mirrors to form an image. In these cases, a small, flat tertiary mirror is sometimes introduced to divert the light to a convenient focus. However, in the ELT the tertiary mirror also has a curved surface, as the use of three mirrors delivers a better final image quality over a larger field of view than would be possible with a two-mirror design.<ref name="contracts-eso"/>

Much like the secondary mirror (with which it shares many design characteristics), the tertiary mirror will be slightly deformable to regularly allow deviations to be corrected. Both mirrors will be mounted on 32 points, with 18 on their backside and 14 along their edges.<ref name="M2M3">{{cite web|title=M2 and M3 Mirrors|url=https://elt.eso.org/mirror/M2M3/|website=ESO|access-date=12 July 2023}}</ref>

As of July 2023, the tertiary mirror has been cast and is in polishing.<ref name=50percent />

==== Quaternary mirror ====

The {{convert|2.4|m|ft|adj=on}} quaternary mirror is a flat, {{cvt|2|mm|in|sigfig=1}} thick adaptive mirror. With up to 8,000 actuators, the surface can be readjusted one thousand times per second.<ref>{{cite web|title=ESO – E-ELT Optics|url=https://www.eso.org/sci/facilities/eelt/telescope/mirrors/|website=www.eso.org|access-date=29 May 2017|language=de}}</ref> The deformable mirror will be the largest adaptive mirror ever made,<ref name="contract-ann">{{cite web|title=ESO Awards Contract for E-ELT Adaptive Mirror Design Study|url=https://www.eso.org/public/announcements/ann12032/|website=www.eso.org|access-date=29 May 2017}}</ref> and consists of six component petals, control systems, and voice-coil actuators. The image distortion caused by the turbulence of the Earth's atmosphere can be corrected in real-time, as well as deformations caused by the wind upon the main telescope. The ELT's adaptive optics system will provide an improvement of about a factor of 500 in the resolution compared to the best seeing conditions achieved so far without adaptive optics.<ref name="contract-ann"/>
 
The AdOptica consortium,<ref>{{cite web|title=www.adoptica.it|url=http://www.adoptica.it/|website=www.adoptica.it|access-date=29 May 2017}}</ref> partnered with [[INAF]] (Istituto Nazionale di Astrofisica) as subcontractors, are responsible for the design and manufacture of the quaternary mirror.<ref>{{cite web|title=Contract Signed for Final Design and Construction of Largest Adaptive Mirror Unit in the World|url=http://www.eso.org/public/announcements/ann15045/|website=www.eso.org|access-date=29 May 2017}}</ref> The 6 petals were cast by Schott in Germany and polished by Safran Reosc.<ref>{{cite web|title=ESO Signs Contract for Deformable Shell Mirrors for E-ELT|url=http://www.eso.org/public/announcements/ann15055/|website=www.eso.org|access-date=29 May 2017}}</ref><ref>{{cite web|title=M4 Mirror|url=https://elt.eso.org/mirror/M4/|website=www.eso.org|access-date=12 July 2023}}</ref>

As of July 2023, all six petals are completed and in the process of being integrated into their support structure.<ref name=50percent /> The six laser sources for the adaptive optics system, which will work hand-in-hand with the quaternary mirror, have also been completed and are in testing.

==== Quinary mirror ====

The {{convert|2.7|by|2.2|m|ft|adj=on}} quinary mirror is a tip-tilt mirror used to refine the image using [[adaptive optics]]. The mirror will include a fast tip-tilt system for image stabilization that will compensate perturbations caused by wind, atmospheric turbulence, and the telescope itself before reaching the ELT instruments.<ref>{{cite web|title=M5 Mirror|url=https://elt.eso.org/mirror/M5/ |access-date= 7 July 2021 |website=European Southern Observatory}}</ref>

As of early 2024 the six component petals had been fabricated and are being brazed into a single unit.<ref 
 name="SPIE2024"/>

=== ELT dome and structure ===
==== Mount ====
The telescope mount, called the ''Main Structure'', is an enormous engineering project in its own right.  The main goals are to point the telescope during operation, to minimize optical path deformations (both repeatable and non-repeatable), and to guarantee safety both during normal operations and seismic events.<ref>{{cite journal 
 |title=ELT design status: the most powerful ground telescope
 |last1=Marchiori |first1=Gianpietro |last2=Rampini |first2=Francesco |last3=Ghedin |first3=Leonardo
 |last4=Bressan |first4=Riccardo
 |journal=Ground-based and Airborne Telescopes VII
 |volume=10700
 |pages=611--630
 |year=2018
 |publisher=SPIE }}</ref>
<ref>{{cite journal 
 |title=ELT design progress: The main structure
 |last1=Colussi |first1=Leo |last2=Colussi |first2=Marco |last3=Franceschini |first3=Gianmarco
 |last4=Martinez |first4=Pascal |last5=Kraus |first5=Maximilian
 |journal=Ground-based and Airborne Telescopes VIII
 |volume=11445
 |pages=1007--1030
 |year=2020
 |publisher=SPIE }}</ref>

At the very bottom, on top of the foundation, is a vibration and seismic isolation layer.  This consists of rubber isolation bearings, springs, and a locking system for larger seismic events.  On top of this is the large concrete block (the pier) on which the telescope rests.  This differs from most large telescopes where the isolation is between the pier and the telescope.  This gives an unusually large seismic mass of 20,000 tonnes, which helps with seismic isolation, avoids mount resonances, and limits seismic acceleration imparted to instrumentation on the telescope.

On top of the concrete pier are the azimuth tracks.  On top of them sits the azimuth structure, rotating around a vertical axis and supported by [[Fluid bearing|hydrostatic bearings]].  On top of that sits the altitude structure, which allows the telescope to tip from horizontal to vertical, and is also supported on hydrostatic bearings.  The rotating mass is about 3,700 tonnes, and can track at {{math|±2}} degrees/second in azimuth and {{math|±1}} degrees/second in elevation.

Both azimuth and elevation angles need to be measured and controlled extremely accurately.  Therefore each axis contains an [[Linear encoder#Optical|optical tape readout]] and direct drive electrical motors.  Both systems are redundant for increased availability.

Within the elevation assembly, there is a complex system of active mirror supports, designed to hold the 5 main mirrors in proper shape and alignment, despite environmental influences and changes in orientation.<ref>{{cite web |url=https://www.eso.org/sci/libraries/SPIE2012/8444-69.pdf |title=E-ELT Optomechanics: Overview |author=M. Cayrel}}</ref><ref>{{cite journal |title=ELT optomechanics: construction status
  |last1= Cayrel |first1= Marc |last2= Dierickx |first2= Philippe |last3= Förster |first3= Andreas
  |last4= Derie |first4= Frédéric |last5= Jochum |first5= Liselotte |last6= Pettazzi |first6= Lorenzo
  |last7= Lucuix |first7= Christian |last8= Haupt |first8= Christoph |last9= Müller |first9= Michael
  |last10= Vernet |first10= Elise |last11= Pirard |first11= Jean-François 
  |journal=Ground-based and Airborne Telescopes VII
  |volume=10700
  |pages=351--370
  |year=2018
  |publisher=SPIE}}</ref>  This system also supports the optics needed to launch, and receive, the [[laser guide star]]s that are used to correct for atmospheric distortion using the adaptive optics of mirror M4.

==== Dome ====

[[File:ELT concept.jpg|thumb|upright=1.2|ELT concept]]

The ELT dome has a height of {{convert|80|m|ft}} from the ground and a diameter of {{convert|93|m|ft}},<ref>{{cite web|url=https://elt.eso.org/telescope/dome/ |title=Dome |date=2025|access-date=25 April 2025}}</ref> making it the largest dome ever built for a telescope. The dome has a total mass of around {{convert|6100|t|lb}}, and the telescope mounting and tube structure has a total moving mass of around {{convert|3700|t|lb}}.

For the observing slit, the dome is equipped with a single pair of large sliding doors. This pair of doors allows an opening of {{convert|41|m|ft}}.

ESO signed a contract for its construction,<ref>{{cite web|title=ESO Signs Largest Ever Ground-based Astronomy Contract for ELT Dome and Telescope Structure|url=http://www.eso.org/public/news/eso1617/|website=www.eso.org|access-date=29 May 2017}}</ref> together with the main structure of the telescopes, with the Italian ACe Consortium, consisting of [[Astaldi]] and Cimolai<ref>{{cite web|title=Cimolai – Home Page|url=http://www.cimolai.com/index.php?lang=en|website=www.cimolai.com|access-date=29 May 2017}}</ref> and the nominated subcontractor, Italy's EIE Group.<ref>{{cite web|title=EIE GROUP Home|url=http://www.eie.it|website=www.eie.it|access-date=29 May 2017|language=en}}</ref> The signature ceremony took place on 25 May 2016<ref>{{cite web|title=ESO Signs Largest Ever Ground-based Astronomy Contract for E-ELT Dome and Telescope Structure|url=https://www.eso.org/public/news/eso1617/|website=www.eso.org|access-date=24 January 2017}}</ref> at ESO's Headquarters in Garching bei München, Germany.

The dome is to provide needed protection to the telescope in inclement weather and during the day. A number of concepts for the dome were evaluated. The final design is a nearly hemispherical dome, rotating atop a concrete pier, with curved laterally-opening doors. This is a re-optimisation from the previous design, aimed at reducing cost.<ref>{{cite web|title=E-ELT Phase B Final Design Review|url=https://www.eso.org/sci/facilities/eelt/docs/E-ELT-PhaseB-BoardReport_ExecSummary.pdf|website=www.eso.org|access-date=24 January 2017}}</ref>

One year after signing the contract, and after the laying of the first stone ceremony in May 2017, the site was handed over to ACe, signifying the beginning of the construction of the dome's main structure. As of April 2025, the dome was complete and the doors were in the process of installation.<ref>{{cite web |title=World's largest telescope gets its protective shell (photos) |url=https://www.space.com/the-universe/worlds-largest-telescope-gets-its-protective-shell-photos |author= Samantha Mathewson}}</ref><ref>{{cite web |url=https://www.space.com/space-exploration/tech/incredible-photo-catches-the-sun-rising-behind-the-worlds-largest-telescope |title=Incredible photo catches the sun rising behind the world's largest telescope |date=23 April 2025 |author=Samantha Mathewson}}</ref>

==== Astronomical performance ====
In terms of astronomical performance the dome is required to be able to track about the 1-degree [[zenith]]al avoidance locus as well as preset to a new target within 5 minutes. This requires the dome to be able to accelerate and move at angular speeds of 2 degrees/s (the linear speed is approximately {{cvt|5|km/h|ft/s|disp=or}}).<ref>{{cite web|title=ESO – E-ELT Enclosure|url=https://www.eso.org/sci/facilities/eelt/telescope/dome/|website=www.eso.org|access-date=24 January 2017|language=de}}</ref>

[[File:Size comparison between the E-ELT and other telescope domes.jpg|thumb|upright=3.8|center|Size comparison between the ELT and other telescope domes]]

The dome is designed to allow complete freedom to the telescope so that it can position itself whether it is opened or closed. It will also permit observations from the zenith down to 20 degrees from the horizon.

==== Windscreen ====
With such a large opening, the ELT dome requires the presence of a windscreen to protect the telescope's mirrors (apart from the secondary), from direct exposure to the wind. The design of the windscreen minimises the volume required to house it. Four spherical blades, spanning the width of the opening, can be raised and lowered in front of the telescope aperture to restrict the wind.

==== Ventilation and air-conditioning ====
The dome design ensures that the dome provides sufficient ventilation for the telescope not to be limited by [[Astronomical seeing|dome seeing]]. For this the dome is also equipped with louvers, whereby the windscreen is designed to allow them to fulfill their function.

Computational fluid dynamic simulations and wind tunnel work are being carried out to study the airflow in and around the dome, as well as the effectiveness of the dome and windscreen in protecting the telescope.

Besides being designed for water-tightness, air-tightness is also one of the requirements as it is critical to minimise the air-conditioning load. The air-conditioning of the dome is necessary not only to thermally prepare the telescope for the forthcoming night but also in order to keep the telescope optics clean.

The air-conditioning of the telescope during the day is critical and the current specifications permit the dome to cool the telescope and internal volume by {{cvt|10|C-change}} over 12 hours.

== Science goals ==
[[File:E-ELT Trailer.ogv|thumb|upright=1.2|The official trailer for the ELT showing the preliminary design.]]
The ELT will search for extrasolar planets—planets orbiting other stars. This will include not only the discovery of planets down to Earth-like masses through indirect measurements of the wobbling motion of stars perturbed by the planets that orbit them, but also the direct imaging of larger planets and possibly even the characterisation of their atmospheres.<ref name=brochure>{{cite book|title=E-ELT The European Extremely Large Telescope – The World's Biggest Eye on the Sky (brochure)|publisher=ESO|url=http://www.eso.org/public/products/brochures/e-elt/}}</ref> The telescope will attempt to image Earthlike [[exoplanets]].<ref name=govert />

Furthermore, the ELT's suite of instruments will allow astronomers to probe the earliest stages of the formation of planetary systems and to detect water and organic molecules in protoplanetary discs around stars in the making. Thus, the ELT will answer fundamental questions regarding planet formation and evolution.<ref name="E-ELT Alone">{{cite web |title = ESO – Are We Alone? |url = http://www.eso.org/public/teles-instr/e-elt/e-elt_exo.html |access-date = 15 June 2011 |archive-url = https://web.archive.org/web/20130117081550/http://www.eso.org/public/teles-instr/e-elt/e-elt_exo.html |archive-date = 17 January 2013 }}</ref>

By probing the most distant objects the ELT will provide clues to understanding the formation of the first objects that formed: primordial stars, primordial galaxies and black holes and their relationships. Studies of extreme objects like black holes will benefit from the power of the ELT to gain more insight into time-dependent phenomena linked with the various processes at play around compact objects.<ref name=brochure />

The ELT is designed to make detailed studies of the first galaxies. Observations of these early galaxies with the ELT will give clues that will help understand how these objects form and evolve. In addition, the ELT will be a unique tool for making an inventory of the changing content of the various elements in the Universe with time, and to understand star formation history in galaxies.<ref name="E-ELT First" />

One of the goals of the ELT is the possibility of making a direct measurement of the acceleration of the Universe's expansion. Such a measurement would have a major impact on our understanding of the Universe. The ELT will also search for possible variations in the fundamental physical constants with time. An unambiguous detection of such variations would have far-reaching consequences for our comprehension of the general laws of physics.<ref name="E-ELT First" />

== Instrumentation ==
[[File:The First E-ELT Instruments.jpg|thumb|upright=1.2|The first ELT instruments<ref>{{cite web|title=First Instruments for E-ELT Approved|url=http://www.eso.org/public/announcements/ann15056/|access-date=13 July 2015}}</ref>]]

The telescope will have several science instruments and will be able to switch from one instrument to another within minutes. The telescope and dome will also be able to change positions on the sky and start a new observation in a short time.

Four of its instruments, the first generation, will be available at or shortly after first light, while two others will begin operations later. Throughout its operation other instruments can be installed.<ref>{{Cite web |title=Instruments {{!}} ELT {{!}} ESO |url=https://elt.eso.org/instrument/ |access-date=2022-05-07 |website=elt.eso.org |language=en}}</ref>

The first generation includes four instruments: MICADO, HARMONI and METIS, along with the adaptive optics system MORFEO.

* '''HARMONI''': The High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI) will function as the telescope's workhorse instrument for spectroscopy.<ref name="HARMONI Webpage">{{cite web |title = ESO HARMONI Webpage |url = https://www.eso.org/public/teles-instr/elt/elt-instr/harmoni/ |access-date = 16 April 2019}}</ref>
* '''METIS''': The Mid-infrared ELT Imager and Spectrograph (METIS) will be a mid-infrared imager and spectrograph.<ref name="METIS Webpage">{{cite web |title = ESO METIS Webpage |url = https://www.eso.org/public/teles-instr/elt/elt-instr/metis/ |access-date = 16 April 2019}}</ref>
*'''MICADO''': The Multi-AO (adaptive optics) Imaging Camera for Deep Observations (MICADO) will be the first dedicated imaging camera for the ELT and will work with the Multiconjugate adaptive Optics Relay For ELT Observations, ('''MORFEO''', formerly '''MAORY''').<ref name="MICADO Webpage">{{cite web |title = ESO MICADO Webpage |url = https://www.eso.org/public/teles-instr/elt/elt-instr/micado/ |access-date = 16 April 2019}}</ref><ref name="MORFEO Webpage">{{cite web |title = ESO MORFEO Webpage |url = https://elt.eso.org/instrument/MORFEO/ |access-date = 23 April 2023}}</ref>

The second generation of instruments consists of MOSAIC and ANDES.
* '''MOSAIC''': A proposed multi-object spectrograph which will allow astronomers to trace the growth of galaxies and the distribution of matter from shortly after the Big Bang to the present day.<ref name="MOSAIC Webpage">{{cite web |title = ESO MOSAIC Webpage |url = https://www.eso.org/public/teles-instr/elt/elt-instr/mosaic/ |access-date = 16 April 2019}}</ref>
* '''ANDES''' (formerly '''HIRES'''): The ArmazoNes high Dispersion Echelle Spectrograph will be used to search for indications of life on Earth-like exoplanets, find the first-born stars of the universe, test for possible variations of the fundamental constants of physics, and measure the acceleration of the Universe's expansion.<ref name="HIRES Webpage">{{cite web |title = ESO HIRES Webpage |url = https://www.eso.org/public/teles-instr/elt/elt-instr/hires/ |access-date = 16 April 2019}}</ref>

== Comparison ==
{{comparison optical telescope primary mirrors.svg|400px}}
[[File:E-ELT and VLT vs Colosseum.jpg|thumb|upright=1.6|ELT compared to the [[Very Large Telescope|VLT]] and the [[Colosseum]]]]

One of the [[List of largest optical reflecting telescopes|largest optical telescopes]] operating today is the [[Gran Telescopio Canarias]], with a {{convert|10.4|m|ft|adj=on}} aperture and a light-collecting area of {{cvt|74|m2|ft2}}. Other planned [[extremely large telescope]]s include the [[Giant Magellan Telescope]] with a mirror diameter of {{cvt|25|m|ft}} and area of {{cvt|368|m2|ft2}}, and the [[Thirty Meter Telescope]] with a diameter of {{cvt|30|m|ft}}, and an area of {{cvt|655|m2|ft2}}. Both of these are also targeting the second half of the 2020 decade for completion. These two other telescopes roughly belong to the same next generation of optical ground-based telescopes.<ref name='GMT Overview'>{{cite web | title = GMT Overview – Giant Magellan Telescope | url = http://www.gmto.org/overview.html | access-date = 15 June 2011 | archive-url = https://web.archive.org/web/20110609023406/http://www.gmto.org/overview.html | archive-date = 9 June 2011 | df = dmy-all }}</ref><ref name='About TMT'>{{cite web | title = About TMT – Thirty Meter Telescope | url = http://www.tmt.org/about-tmt | access-date = 15 June 2011 | archive-url = https://web.archive.org/web/20110808172454/http://www.tmt.org/about-tmt | archive-date = 8 August 2011 }}</ref> Each design is much larger than previous telescopes.<ref name=govert />

The size of the ELT has been reduced from its original design. Even with that reduction, the ELT is significantly larger than both other planned extremely large telescopes.<ref name=govert /> It has the aim of observing the universe in greater detail than the [[Hubble Space Telescope]] by taking images 15 times sharper, although it is designed to be complementary to space telescopes, which typically have very limited observing time available.<ref name=esoscience /> The ELT's 4.2-metre [[secondary mirror]] is the same size as the primary mirror on the [[William Herschel Telescope]], the second largest optical telescope in Europe.

{| class="wikitable sortable" style="text-align: center;"
|-
! Name !! Aperture<br />diameter (m) !! Collecting<br />area (m²) !! First light !! Ref
|-
| align=left | '''Extremely Large Telescope (ELT)''' || 39 || 978 || 2029 ||<ref name="start2027" />
|-
| align=left | [[Thirty Meter Telescope]] (TMT) || 30.0 || 655 ||? ||
|-
| align=left | [[Giant Magellan Telescope]] (GMT) || 25.4  || 368 || 2030s ||<ref>{{Cite web |date=12 March 2021 |title=Giant Magellan Telescope – 6th mirror cast |url=https://www.universetoday.com/150506/the-giant-magellan-telescopes-6th-mirror-has-just-been-cast-one-more-to-go/ |access-date=11 July 2023}}</ref>
|-
| align=left | [[Large Binocular Telescope]] (LBT)
|2 x 8.4 (22.8)
|111
|2005
|
|-
| align=left | [[Southern African Large Telescope]] (SALT) || 11.1 × 9.8 || 79 || 2005 ||
|-
| align=left | [[Hobby–Eberly Telescope]] (HET)
|11.1 × 9.8
|79
|1996
|
|-
| align=left | [[Gran Telescopio Canarias]] (GTC) || 10.4 || 74 || 2007 ||
|-
| align=left | [[Keck Telescopes]] || 10.0 || 76 || 1990, 1996 ||
|-
| align=left | [[Very Large Telescope]] (VLT) || 8.2 || 50 (×4) || 1998–2000 ||
|-
| colspan=4 style="font-size: 0.85em; padding: 6px 0 4px 4px;" | Notes: Future first-light dates are provisional and likely to change. 
|}

The ELT under ideal conditions has an angular resolution of 0.005 [[arcsecond]] which corresponds to separating two light sources 1 [[Astronomical unit|AU]] apart from {{convert|200|pc|ly|lk=in|abbr=on}} distance, or two light sources {{cvt|30|cm|in}} apart from roughly {{cvt|12000|km|mi}} distance. At 0.03 arcseconds, the contrast is expected to be 10<sup>8</sup>, sufficient to search for exoplanets.<ref>[https://www.eso.org/sci/libraries/SPIE2010/7735-84.pdf EPICS: direct imagine of exoplanets with the E-ELT]</ref> The [[Naked eye|unaided human eye]] has an angular resolution of 1 arcminute which corresponds to separating two light sources 30&nbsp;cm apart from 1&nbsp;km distance.{{Citation needed|date=October 2024}}
{{Clear}}

== See also ==
{{div col|colwidth=21em}}
* [[Lists of telescopes]]
* [[List of largest optical reflecting telescopes]]
* [[List of optical telescopes]]
* [[Large Binocular Telescope]]
* [[European Solar Telescope]] (planned completion in 2025)
* [[Gran Telescopio Canarias]]
* [[Overwhelmingly Large Telescope]]
* [[List of astronomical observatories]]
* [[List of highest astronomical observatories]]
* Other observatories in Chile:
** [[Llano de Chajnantor Observatory]] 
***[[Atacama Large Millimeter Array]]
** [[Paranal Observatory]]
*** [[Very Large Telescope]]
** [[Cerro Armazones Observatory]]
** [[La Silla Observatory]]
** [[Cerro Tololo Inter-American Observatory]]
{{div col end}}

== References ==
{{reflist
|30em
|refs=

<ref name=beginning>{{cite news
 |url=https://www.independent.co.uk/news/science/european-extremely-large-telescope-to-break-ground-using-dynamite-live-later-today-9548123.html
 |title=European Extremely Large Telescope to break ground (using dynamite) live later today
 |author=James Vincent
 |work=The Independent
 |date=19 June 2014}}</ref>

<ref name="eso1419-groundbreaking">{{cite web
 |url=http://www.eso.org/public/news/eso1419/
 |title=Groundbreaking for the E-ELT
 |work=ESO
 |date=19 June 2014}}</ref>

<ref name="constrproposal">{{citation
 |title=The E-ELT Construction Proposal
 |url=https://www.eso.org/sci/facilities/eelt/docs/e-elt_constrproposal.pdf
 |work=ESO
 |date=2 December 2011
 |access-date=22 June 2014}}</ref>

<ref name=Approval>{{cite news
 |last=Amos |first=Jonathan
 |title=European Extremely Large Telescope given go-ahead
 |url=https://www.bbc.co.uk/news/science-environment-18396853
 |work=BBC News
 |access-date=11 June 2012
 |date=11 June 2012}}</ref>

<ref name="spaceref2014">{{cite web
 |url=http://spaceref.com/astronomy/construction-of-extremely-large-telescope-approved.html
 |title=Construction of Extremely Large Telescope Approved
 |work=Spaceref
 |date=4 December 2014
 |access-date=7 December 2014
 |archive-date=23 August 2020
 |archive-url=https://web.archive.org/web/20200823090057/http://spaceref.com/astronomy/construction-of-extremely-large-telescope-approved.html
 |url-status=dead
 }}</ref>

<ref name=eso1018>{{cite news
 |url=http://www.eso.org/public/news/eso1018/
 |title= E-ELT Site Chosen
 |agency=ESO
 |date=26 April 2010
 |access-date=17 August 2011}}</ref>

<ref name="Finding-a-home">{{cite web
 |title       = E-ELT: Finding a home
 |url       = http://www.eso.org/public/teles-instr/elt/elt_sit/
 |access-date       = 24 January 2018
 |df       = dmy-all
 |archive-url       = https://web.archive.org/web/20190410220026/https://www.eso.org/public/teles-instr/elt/elt_sit/
 |archive-date       = 10 April 2019
 }}</ref>

<ref name="messenger">{{cite journal
 |title=The European Extremely Large Telescope (E-ELT)
 |url=http://www.eso.org/sci/publications/messenger/archive/no.127-mar07/messenger-no127-11-19.pdf
 |journal=The Messenger
 |volume=127
 |date=March 2007
 |issue=127
 |pages=11–19
 |last1=Gilmozzi |first1=Roberto |last2=Spyromilio |first2=Jason
 |bibcode=2007Msngr.127...11G}}</ref>

<ref name="E-ELT First">{{cite web |title = ESO – The First Objects in the Universe |url = http://www.eso.org/public/teles-instr/e-elt/e-elt_fir.html |archive-url=https://web.archive.org/web/20190410220027/https://www.eso.org/public/teles-instr/elt/elt_fir/ |archive-date=2019-04-10 |access-date = 17 August 2011 }}</ref>

<!--
<ref name="eso-mirrors">{{cite web
 |url=http://www.eso.org/sci/facilities/eelt/telescope/mirrors/
 |title=E-ELT OPTICS
 |work=ESO
 |date=27 June 2011
 |access-date= 1 September 2015}}</ref>

<ref name="dome contract ann">{{cite web
 |url=http://www.eso.org/public/news/eso1617/
 |title=ESO Signs Largest Ever Ground-based Astronomy Contract for E-ELT Dome and Telescope Structure
 |work=ESO
 |date=25 May 2016}}</ref>

<ref name="final design doc">{{cite web
 |url=http://www.eso.org/sci/facilities/eelt/docs/E-ELT-PhaseB-BoardReport_ExecSummary.pdf
 |title=E-ELT Phase B Final Design Review
 |work=ESO Science Office
 |date=25 May 2016}}</ref>

<ref name="ukweb">{{cite web
 |url=http://www.eelt.org.uk/industry/dome-structure/
 |title=The UK involvement in the E-ELT}}</ref>

<ref name="idom">{{cite web
 |url=http://www.idom.com/en/project/e-elt-dome-foundations/
 |title=E-ELT DOME & FOUNDATIONS}}</ref>

<ref name="eso-design">{{cite web
 |url=http://www.eso.org/sci/facilities/eelt/telescope/index.html
 |title=E-ELT TELESCOPE DESIGN
 |work=ESO
 |date=23 August 2012
 |access-date=1 September 2015}}</ref>
-->

}}
== External links ==
{{Commons category|Extremely Large Telescope}}
* [http://www.eso.org/public/teles-instr/elt/ ESO Extremely Large Telescope]
* [https://www.eso.org/sci/facilities/eelt/ ESO The Extremely Large Telescope ("E-ELT") Project]
* [http://news.bbc.co.uk/2/hi/science/nature/6176185.stm Final stage for telescope design]
* [https://www.theregister.co.uk/2005/04/11/test_design_elt/ Green light for ELT]
* [http://news.bbc.co.uk/1/hi/sci/tech/4426535.stm Ground Telescope to Super Size]
* [http://www.mpe.mpg.de/ir/micado MICADO]
* [http://metis.strw.leidenuniv.nl/ METIS]
* [https://www2.physics.ox.ac.uk/research/visible-and-infrared-instruments/harmoni HARMONI]
* [http://news.bbc.co.uk/1/hi/sci/tech/5252228.stm Record mirror for Euro telescope BBC Online August 7 2006]
* [https://archive.today/20120910200228/http://www.spaceref.com/news/viewpr.html?pid=21445 ESO Council Gives Green Light to Detailed Study of the European Extremely Large Telescope Spaceref.com] (via [[archive.today]])

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