European Extremely Large Telescope

European Extremely Large Telescope
Engineer rendering of the 39-metre European Extremely Large Telescope (E-ELT)
Organisation ESO
Location(s) Cerro Armazones, Chile, near Paranal Observatory
Coordinates 24°35′20″S 70°11′32″W / 24.58889°S 70.19222°W / -24.58889; -70.19222Coordinates: 24°35′20″S 70°11′32″W / 24.58889°S 70.19222°W / -24.58889; -70.19222
Altitude 3,060 m (10,040 ft)[1]
Weather 89% clear fraction[2]
0.67″ median seeing at 500 nm[3]
Wavelength visible, near and mid-infrared
Built start: July 2014 (construction)[4]
first light: by 2024 (expected)[5]
First light 2024
Telescope style Reflector
Diameter 39.3 m (129 ft)
Secondary dia. 4.09 m[6]:124
Tertiary dia. 3.75 m[6]:134
Angular resolution 0.001 to 0.65 arcsec depending on instrument
Collecting area 978 m2
Focal length 34.5 m (f/0.88) primary[6]:94
420–840 m (f/10 – f/20) final
Mounting Nasmyth mount
Enclosure classical[7]

The European Extremely Large Telescope (E-ELT) is an astronomical observatory and the world's largest optical/near-infrared extremely large telescope now under construction. Part of the European Southern Observatory (ESO), it is located on top of Cerro Armazones in the Atacama Desert of northern Chile. The design comprises a reflecting telescope with a 39.3-metre-diameter segmented primary mirror and a 4.2-metre-diameter secondary mirror, and will be supported by adaptive optics, six laser guide star units and multiple large science instruments.[8] The observatory aims to gather 13 times more light than the largest optical telescopes existing today, be able to correct for atmospheric distortions and provide images 16 times sharper than those from the Hubble Space Telescope.[9]

The E-ELT is intended to vastly advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, super-massive black holes, and the nature of the Universe's dark sector, and to detect water and organic molecules in protoplanetary disks around other stars.[10] The facility is expected to take 11 years to construct.[11]

On 11 June 2012, the ESO Council approved the E-ELT programme's plans to begin civil works at the telescope site, with construction of the telescope itself pending final agreement with governments of some member states.[12] Construction work on the E-ELT site started in June 2014.[4] In December 2014 ESO had secured over 90% of the total funding and authorized construction of the telescope to start, which will cost around one billion Euro for the first construction phase.[13] First light is planned for 2024.[5]


On 26 April 2010, the European Southern Observatory (ESO) Council selected Cerro Armazones, Chile, as the baseline site for the planned E-ELT.[14] Other sites that were under discussion included Cerro Macon, Salta, in Argentina; Roque de los Muchachos Observatory, on the Canary Islands; and sites in South Africa, Morocco, and Antarctica.[15][16]

Early designs included a segmented primary mirror with a diameter of 42 metres and area of about 1,300 m2, with a secondary mirror with a diameter of 5.9 m. However, in 2011 a proposal was put forward to reduce its size by 13% to 978 m2, for a 39.3 m diameter primary mirror and a 4.2 m diameter secondary mirror.[8] It 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; 4.2 m 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.[6]:15

ESO Council meets at ESO headquarters in Garching, 2012.[17]

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."[18] The ESO Council endorsed the revised baseline design in June 2011 and expected a construction proposal for approval in December 2011.[18] Funding was subsequently included in the 2012 budget for initial work to begin in early 2012.[19] The project received preliminary approval in June 2012.[12] ESO approved the start of construction in December 2014, with over 90% funding secured.[13]

The design phase of the 5-mirror anastigmat was fully funded within the ESO budget. With the recent changes in the baseline design (such as a reduction in the size of the primary mirror from 42 m to 39.3 m), the construction cost is estimated to be €1.055 billion (including first generation instruments).[20] The start of operations is planned for 2024.[5]


A real night-time panorama of Cerro Armazones, chosen in April 2010.

The ESO focused on the current design after a feasibility study concluded the proposed 100 metres (330 ft) diameter Overwhelmingly Large Telescope would cost €1.5 billion (£1 billion), and be too complex. Current fabrication technology limits single mirrors to being roughly 8 metres (26 ft) in a single 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 hexagonal mirrors fitted together to make a mirror more than 10 metres (33 ft) across. The E-ELT will use a similar design, as well as techniques to work around atmospheric distortion of incoming light, known as adaptive optics.[21]

A 40-metre-class mirror will allow the study of the atmospheres of extrasolar planets.[22] The E-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.[23] The telescope underwent a Phase B study in the past couple of years 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.”[24]


Renderings of the 40-metre-class E-ELT at dusk (left) and from above (right)

The telescope's segmented mirror will be 39.3 metres in diameter and will gather 15 times more light than the largest optical telescopes operating at the time of its development. The telescope has an innovative five-mirror design that includes advanced adaptive optics to correct for the turbulent atmosphere, giving exceptional image quality.[21]

The primary mirror for the 39.3-metre design will be composed of 798 hexagonal segments, each 1.45 metres across and 50 mm thick. A special correcting mirror in the telescope will be supported by more than 6,000 actuators that can distort its shape a thousand times per second.[25] The telescope main structure will weigh about 2,800 tonnes.[26] The Italian ACe Consortium will manufacture, transport, and assemble the E-ELT's frame and protective dome.[27]

Artist’s rendering of the E-ELT based on the detailed construction design for the telescope.
Infographic comparing the size of the E-ELT dome with that of other major ground-based telescopes, either in operation or planned.

Science goals

The E-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.[32] The telescope will attempt to image Earthlike exoplanets, which may be possible.[8]

Furthermore, the E-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 E-ELT will answer fundamental questions regarding planet formation and evolution.[10]

This is the official trailer for the E-ELT. The design for the E-ELT shown here is preliminary.

By probing the most distant objects the E-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 E-ELT to gain more insight into time-dependent phenomena linked with the various processes at play around compact objects.[32]

The E-ELT is designed to make detailed studies of the first galaxies and to follow their evolution through cosmic time. Observations of these early galaxies with the E-ELT will give clues that will help understand how these objects form and evolve. In addition, the E-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.[33]

One of the goals of the E-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 E-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.[33]


"CODEX" redirects here. For the type of book, see Codex.
This video shows engineers adjusting the complex support mechanisms that control the shape and positioning of two of the 798 segments that will form the complete primary mirror of the telescope.
The First E-ELT Instruments.[34]

The telescope will have several science instruments. It will be possible 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 very short time.

Eight different instrument concepts and two post-focal adaptive modules are currently being studied, with the aim that two to three will be ready for first light, with the others becoming available at various points over the following decade.[35] The instruments being studied are:

The two post-focal adaptive optics modules currently being studied are:


Comparison of nominal sizes of primary mirrors of the European Extremely Large Telescope and some notable optical telescopes
E-ELT compared to the VLT and the Colosseum

One of the largest ground-based telescope operating today is the Gran Telescopio Canarias, with a 10.4 m aperture and a light-collecting area of 74 m2. Other planned extremely large telescopes include the 25 m/368 m2 Giant Magellan Telescope and 30 m/655 m2 Thirty Meter Telescope, which are also targeting the beginning of the 2020 decade for completion. These other two telescopes roughly belong to the same next generation of optical ground-based telescopes.[49][50] Each design is much larger than previous telescopes.[8] Even with the descale to 39.3 m the E-ELT is significantly larger than both other planned extremely large telescopes.[8] 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.[22] The E-ELT's 4.2-meter secondary mirror is the same size as the primary mirror on the William Herschel Telescope, the second largest optical telescope in Europe.

Name Aperture
diameter (m)
area (m²)
First light
E-ELT 39.3 978 2024
Thirty Meter Telescope (TMT) 30 655 TBD[51]
Giant Magellan Telescope (GMT) 24.5 368 2025
Southern African Large Telescope (SALT) 11.1 × 9.8 79 2005
Keck Telescopes 10.0 76 1990, 1996
Gran Telescopio Canarias (GTC) 10.4 74 2007
Very Large Telescope (VLT) 8.2   1998-2000
Notes: Future first-light dates are provisional and likely to change.


The images below show artistic renderings of the E-ELT and were produced by ESO.


Comparable instruments

See also


  1. "World's biggest telescope to be located on Cerro Armazones, Chile". Astronomy magazine. 2010-04-28. Retrieved 2011-08-17.
  2. Schöck, M.; Els, S.; Riddle, R.; Skidmore, W.; Travouillon, T.; Blum, R.; Bustos, E.; Chanan, G.; Djorgovski, S. G.; Gillett, P.; Gregory, B.; Nelson, J.; Otárola, A.; Seguel, J.; Vasquez, J.; Walker, A.; Walker, D.; Wang, L. (1 April 2009). "Thirty Meter Telescope Site Testing I: Overview". Publications of the Astronomical Society of the Pacific. 121 (878): 384–395. arXiv:0904.1183Freely accessible. Bibcode:2009PASP..121..384S. doi:10.1086/599287.
  3. "E-ELT Site". ESO website. ESO. Retrieved 25 July 2013. The median seeing is 0.67 arcsec at 500nm with a median coherence time of 3.5 ms.
  4. 1 2 James Vincent (19 June 2014). "European Extremely Large Telescope to break ground (using dynamite) live later today". The Independent.
  5. 1 2 3 "Groundbreaking for the E-ELT". ESO. 19 June 2014.
  6. 1 2 3 4 The E-ELT Construction Proposal (PDF), ESO, 2011-12-02, retrieved 2014-06-22
  7. "E-ELT ENCLOSURE". ESO. 21 June 2011.
  8. 1 2 3 4 5 Govert Schilling (2011-06-14). "Europe Downscales Monster Telescope to Save Money". Science Insider. Retrieved 2011-08-17.
  10. 1 2 "ESO - Are We Alone?". Retrieved 2011-06-15.
  11. "The E-ELT construction proposal" (PDF). ESO. Retrieved 2011-01-16.
  12. 1 2 Amos, Jonathan (11 June 2012). "European Extremely Large Telescope given go-ahead". BBC News. Retrieved 11 June 2012.
  13. 1 2 "Construction of Extremely Large Telescope Approved". Spaceref. 4 December 2014.
  14. "E-ELT Site Chosen". ESO. 2010-04-26. Retrieved 2011-08-17.
  15. "E-ELT: Finding a home". Retrieved 2011-08-17.
  16. Vernin, Jean; Muñoz-Tuñón, Casiana; Sarazin, Marc; Vazquez Ramió, Héctor; Varela, Antonia M.; Trinquet, Hervé; Miguel Delgado, José; Jiménez Fuensalida, Jesús; Reyes, Marcos; Benhida, Abdelmajid; Benkhaldoun, Zouhair; García Lambas, Diego; Hach, Youssef; Lazrek, M.; Lombardi, Gianluca; Navarrete, Julio; Recabarren, Pablo; Renzi, Victor; Sabil, Mohammed; Vrech, Rubén (1 November 2011). "European Extremely Large Telescope Site Characterization I: Overview" (PDF). Publications of the Astronomical Society of the Pacific. 123 (909): 1334–1346. Bibcode:2011PASP..123.1334V. doi:10.1086/662995.
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  22. 1 2 An Expanded View of the Universe – Science with the European Extremely Large Telescope (PDF). ESO Science Office.
  23. "ESO - Europe's Window on the Universe". Retrieved 2011-06-15.
  24. Astronet (2008), Michael F. Bode; Maria J. Cruz; Frank J. Molster, eds., The ASTRONET Infrastructure Roadmap: A Strategic Plan for European Astronomy (PDF), p. 43, ISBN 978-3-923524-63-1, retrieved 2014-06-21
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  26. "E-ELT TELESCOPE DESIGN". ESO. 23 August 2012. Retrieved September 2015. Check date values in: |access-date= (help)
  27. Amos, Jonathan (25 May 2016). "E-ELT: Contract to construct giant telescope". BBC News. Retrieved 2016-05-25.
  28. "ESO Signs Largest Ever Ground-based Astronomy Contract for E-ELT Dome and Telescope Structure". ESO. 25 May 2016.
  29. "E-ELT Phase B Final Design Review" (PDF). ESO Science Office. 25 May 2016.
  30. "The UK involvement in the E-ELT".
  32. 1 2 E-ELT The European Extremely Large Telescope — The World's Biggest Eye on the Sky (brochure). ESO.
  33. 1 2 "ESO - The First Objects in the Universe". Retrieved 2011-08-17.
  34. "First Instruments for E-ELT Approved". Retrieved 13 July 2015.
  35. "E-ELT Instrumentation". Retrieved 2009-10-29.
  36. Pasquini, Luca; et al. (2008). McLean, Ian S; Casali, Mark M, eds. "Proceedings of SPIE—CODEX: the high-resolution visual spectrograph for the E-ELT" (PDF). Ground-based and Airborne Instrumentation for Astronomy II. 7014. SPIE: 70141I–70141I–9. doi:10.1117/12.787936.
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  38. Cuby, Jean-Gabriel; et al. (2010). McLean, Ian S; Ramsay, Suzanne K; Takami, Hideki, eds. "Proceedings of SPIE—EAGLE: a MOAO fed multi-IFU NIR workhorse for E-ELT" (PDF). Ground-based and Airborne Instrumentation for Astronomy III. 7735. SPIE: 77352D–77352D–15. Bibcode:2010SPIE.7735E..80C. doi:10.1117/12.856820. Retrieved 29 November 2012.
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  40. Kasper, Markus E.; et al. (2008). "EPICS: the exoplanet imager for the E-ELT". Adaptive Optics Systems - Proceedings of the SPIE, Volume 7015. SPIE. pp. 70151S–70151S–12. doi:10.1117/12.789047.
  41. Thatte, Niranjan. "HARMONI". University of Oxford. Retrieved 30 November 2012.
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  48. Oliva, E.; Origlia, L. (August 2008). McLean, Ian S; Casali, Mark M, eds. "High-resolution near-IR spectroscopy: from 4m to 40m class telescopes" (PDF). Proceedings of the SPIE. Ground-based and Airborne Instrumentation for Astronomy II. 7014: 70141O–70141O–7. Bibcode:2008SPIE.7014E..56O. doi:10.1117/12.788821. Retrieved 30 November 2012.
  49. "GMT Overview -- Giant Magellan Telescope". Retrieved 2011-06-15.
  50. "About TMT -- Thirty Meter Telescope". Retrieved 2011-06-15.
  51. Stewart, Burnett, Colin M., John (October 14, 2016). "Hawaii Supreme Court voids Thirty Meter Telescope permit". Oahu Publications. West Hawaii Today. Retrieved 19 December 2015.
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