Giant Magellan Telescope

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Giant Magellan Telescope
Giant Magellan Telescope - artist's concept.jpg
Artwork of Giant Magellan Telescope
Alternative names GMT
Observatory Las Campanas Observatory Edit this on Wikidata
Location(s) La Serena, Chile Edit this at Wikidata
Coordinates 29°01′42″S 70°41′09″W / 29.0283°S 70.6858°W / -29.0283; -70.6858Coordinates: 29°01′42″S 70°41′09″W / 29.0283°S 70.6858°W / -29.0283; -70.6858
Altitude 2,516 m (8,255 ft) [1][2]
Wavelength 320 nm (940 THz)-25,000 nm (12 THz)
Built 2015–2025 (2015–2025)[3]
First light 2021 Edit this on Wikidata[3]
Telescope style extremely large telescope
Gregorian telescope
optical telescope Edit this on Wikidata
Diameter 25.448 m (83 ft 5.9 in)[3]
Secondary diameter 3.2 m (10 ft 6 in)
Angular resolution 0.21 second of arc Edit this on Wikidata
Collecting area 368 m2 (3,960 sq ft)
Focal length 18, 202.7 m (59, 665 ft) [4][5]
Mounting altazimuth mount
Giant Magellan Telescope is located in Chile
Giant Magellan Telescope
Location of Giant Magellan Telescope

The Giant Magellan Telescope (GMT) is a ground-based extremely large telescope under construction, planned for completion in 2025.[6] It will consist of seven 8.4 m (27.6 ft) diameter primary segments,[7] that will observe optical and near infrared (320–25000 nm[8]) light, with the resolving power of a 24.5 m (80.4 ft) primary mirror and collecting area equivalent to a 22.0 m (72.2 ft) one,[9] which is about 368 square meters.[10] (7×8.365 m) The telescope is expected to have the resolving power 10 times greater than the Hubble Space Telescope, and will be the largest optical observatory in the world, at the time of its first light. As of December 2015, four mirrors have been cast and the construction of the summit facility has begun.[11][12]

A total of seven primary mirrors are planned, but it will begin operation with four.[13][14] The $1 billion project is US-led in partnership with Australia, Brazil, and Korea, with Chile as the host country.


The location of the telescope is Las Campanas Observatory,[15] which is also the site of the Magellan Telescopes, some 115 km (71 mi) north-northeast of La Serena, Chile, at an altitude of 2,516 m (8,255 ft).[16][17]. The site has been chosen as the new instrument's location because of its outstanding astronomical seeing and clear weather throughout most of the year.[18] Moreover, due to the sparsity of population centers and other favorable geographical conditions, the night sky in most of the surrounding Atacama Desert region is not only free from atmospheric pollution, but in addition it is probably one of the places least affected by light pollution, making the area one of the best spots on Earth for long-term astronomical observation. Major site preparation began with the first blast to level the mountain peak on 23 March 2012. In November 2015, construction was started at the site, with a ground-breaking ceremony.[11][12]


Comparison of nominal sizes of apertures of the Giant Magellan Telescope and some notable optical telescopes

The telescope will use seven of the world's largest mirrors as primary mirror segments, each 8.417 m (27.61 ft) in diameter. These segments will then be arranged with one mirror in the center and the other six arranged symmetrically around it. The challenge is that the outer six mirror segments will be off-axis, and although identical to each other, will not be individually radially symmetrical, necessitating a modification of the usual polishing and testing procedures.[19]

The mirrors are being constructed by the University of Arizona's Steward Observatory Mirror Lab. The casting of the first mirror, in a rotating furnace, was completed on November 3, 2005, but the grinding and polishing were still going on 6½ years later when the second mirror was cast, on 14 January 2012.[20][21] A third segment was cast in August 2013,[13][22] and the fourth in September 2015.[23] The casting of each mirror uses 20 tons of E6 borosilicate glass from the Ohara Corporation of Japan and takes about 12–13 weeks.[24] After being cast, they need to cool for about six months.[13]

Polishing of the first mirror was completed in November 2012.[25] As this was an off-axis segment, a wide array of new optical tests and laboratory infrastructure had to be developed to polish the mirror.

The intention is to build seven identical off-axis mirrors, so that a spare is available to substitute for a segment being recoated, a 1–2 week (per segment) process required every 1–2 years.[26] While the complete telescope will use seven mirrors, it is planned to begin operation with four mirrors.[13]

The primary mirror array as a whole will have a focal ratio (focal length divided by diameter) of f/0.71. For an individual segment – being one third that diameter – this results in a focal ratio of f/2.14.[22] The overall focal ratio of the complete telescope will be f/8 and the optical prescription is an aplanatic Gregorian telescope. Like all modern large telescopes it will make use of adaptive optics.[27]

Scientists expect very high quality images due to the very large aperture and advanced adaptive optics. Image resolution should exceed that of the Hubble Space Telescope.[6]


The Giant Magellan Telescope is one of a new class of telescopes called extremely large telescopes with each design being much larger than previous telescopes.[28] Other planned extremely large telescopes include the Extremely Large Telescope and the Thirty Meter Telescope.[29]

Name Aperture
diameter (m)
area (m²)
First light
Extremely Large Telescope (ELT) 39.3 978 2024
Thirty Meter Telescope (TMT) 30 655 postponed[30]
Giant Magellan Telescope (GMT) 24.5 368 2021
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 dates for first-light are provisional and are likely to change.


The project is US-led in partnership with Australia, Brazil, and Korea, with Chile as the host country.[11] The following organizations are members of the consortium developing the telescope.[31]

The Carnegie Observatories office in Pasadena has an outline of the GMT primary mirror array painted in its parking lot. It is easily visible in satellite imagery at 34°09′21″N 118°08′00″W / 34.15591°N 118.13345°W / 34.15591; -118.13345 (Giant Magellan Telescope outline drawing).

Status of mirrors[edit]

There will be a total of seven primary mirrors: One central mirror and six off-axis segments,[32] The mirrors are made of borosilicate glass and have a honeycomb structure below the mirror surface.[32] An adaptive secondary mirror is also designed for the telescope.[32] A spare off-axis segment is also planned (six off-axis and one primary are used as a primary mirror).[33]

  • Mirror 1, cast in October 2005,[24] completed in August 2012 with polishing completed with a surface accuracy of 19 nanometers, rms.[25]
  • Mirror 2, cast in January 2012.[24]
  • Mirror 3, cast in August 2013.[13][24]
  • Mirror 4, cast in September 2015.[23] This is the central mirror.[14]
  • Mirror 5, in glass inspection phase. Planned for casting in 2017[33]
  • Mirror 6, in early construction phase.
  • Mirror 7, in planning.

See also[edit]


  1. ^
  2. ^
  3. ^ a b c "Giant Magellan Telescope: Super-scope project breaks ground". BBC News. 12 November 2015. 
  4. ^ "Optics" (PDF). Retrieved 24 June 2017. 
  5. ^ "Optics" (PDF). Retrieved 24 June 2017. 
  6. ^ a b Amos, Jonathan (3 June 2015). "Magellan super-scope gets green light for construction". BBC News. Retrieved 2015-06-04. 
  7. ^ "Giant Magellan Telescope Partner Institutions". GMT Consortium. Retrieved 2007-04-03. 
  8. ^ "Giant Magellan Telescope Science Requirements" (PDF). GMT Consortium. p. 11. Retrieved 2008-03-31. 
  9. ^ Maggie McKee (2007-10-04). "Giant telescope in race to become world's largest". New Scientist. Retrieved 2007-10-07. 
  10. ^ "Chapter 6: Optics" (PDF). GMT Conceptual Design Report. GMT Consortium. p. 6-3. Retrieved 2008-04-02. 
  11. ^ a b c Amos, Jonathan (12 November 2015). "Giant Magellan Telescope: Super-scope project breaks ground". BBC News. Retrieved 2015-11-15. 
  12. ^ a b The Giant Magellan Telescope Organization Breaks Ground in Chile
  13. ^ a b c d e Harvard Magazine - Giant Magellan Milestone (2013)
  14. ^ a b Hutchins, Shana K. (September 18, 2015). "Giant Magellan Telescope to Cast Milestone Fourth Mirror". Texas A&M Today. Retrieved 2015-12-17. ‘There is special significance to the fourth mirror. It will be the central unit. Without it, the other mirrors would be much more difficult to bring together to function as a single telescope. Also, our baseline plan starts the operations of the GMT with just these four mirrors, which will all have been cast once this one is complete.’ 
  15. ^ "Giant Magellan telescope site selected". Carnegie Institution. Retrieved 2007-10-05. 
  16. ^ José Terán U.; Daniel H. Neff; Matt Johns (2006-05-29). Conceptual design study of the GMT enclosure (PDF). SPIE 6267: Symposium on Astronomical Telescopes and Instrumentation. Orlando, Florida: SPIE. p. 2. Retrieved 2008-03-31. 
  17. ^ Joanna Thomas-Osip (2007-03-20), "The Seeing and Turbulence Profile at Las Campanas Observatory: GMT Site Testing Progress Report" (PDF), Syposium on Seeing, Kona, Hawaii: AAS, p. 3, retrieved 2008-03-31 
  18. ^ Robinson, Travis (2007-04-03). "Eye on the sky". The Battalion. Archived from the original on 2007-09-29. Retrieved 2007-04-03. 
  19. ^ What is Optical Metrology?, GMTO, archived from the original on 2012-03-28, retrieved 2012-04-08 
  20. ^ Ketelsen, Dean (2012-01-15), GMT polishing at Mirror Lab open house 14 Jan, 2012, retrieved 2012-04-08, While guests toured the facilities, the Lab staff ran both of our polishing machines on current projects, including this view of final polishing on the first GMT segment. 
  21. ^ "Mirror Casting Event for the Giant Magellan Telescope" (Press release). GMTO. 2012-01-09. Archived from the original on 2012-04-11. 
  22. ^ a b Steward Observatory Mirror Lab, Mirror Castings, retrieved 2012-04-08 
  23. ^ a b "Richard F. Caris Mirror Lab Casts Fourth GMT Segment" (Press release). GMTO. September 18, 2015. 
  24. ^ a b c d Mittan, Kyle (2012-01-16). "Steward Observatory casts second mirror for Giant Magellan Telescope". The Daily Wildcat. 
  25. ^ a b "World’s Most Advanced Mirror for Giant Telescope Completed". Australian National University. 2012-11-09. Archived from the original on 2013-03-14. Retrieved 2012-01-14. 
  26. ^ "Telescope Structure" (PDF). GMT Conceptual Design Report. February 2006. p. 7-17 § 7.4.5. Retrieved 2007-10-07. The center segment and cell will not have a spare, thus observations will be interrupted every one or two years for the 1–2 week period required to recoat that mirror. 
  27. ^ "Chapter 2: Overview", GMT Conceptual Design Report, 2006, p. 2-4 § 2.5.1, archived from the original (PDF) on 2012-03-28, retrieved 2012-03-25, GMT is designed from the outset around adaptive optics (AO) with the goal of producing diffraction limited images at 1 μm and longer wavelengths. 
  28. ^ "GMT Overview -- Giant Magellan Telescope". Retrieved 2011-06-15. 
  29. ^ "About TMT -- Thirty Meter Telescope". Retrieved 2011-06-15. 
  30. ^ 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. 
  31. ^ "Giant Magellan Telescope Partner Institutions". GMTO. Archived from the original on 2009-02-20. Retrieved 2009-02-11. 
  32. ^ a b c [1]
  33. ^ a b [2]

External links[edit]