Giant Magellan Telescope

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Giant Magellan Telescope
Giant Magellan Telescope - artist's concept.jpg
Artwork of Giant Magellan Telescope
Organisation GMT Consortium
Location(s) Las Campanas Observatory, Chile
Coordinates 29°01.7′S 70°41.15′W / 29.0283°S 70.68583°W / -29.0283; -70.68583Coordinates: 29°01.7′S 70°41.15′W / 29.0283°S 70.68583°W / -29.0283; -70.68583
Altitude 2,516 m (8,255 ft)[1][2]
Wavelength optical and near infrared (320–25000 nm[3])
Built Under construction
First light 2021;[4] completion: 2025[4]
Telescope style Gregorian telescope
Diameter 25.448 metres (1,002 in)[5] (7×8.365 m)
Secondary dia. 3.2 metres (130 in)[5] (7×1.063 m)
Angular resolution 0.21–0.3″ at 500 nm[3]
Collecting area 368 m²[5]
Focal length 18.000 m (M1)
202.745 m (M1+M2)[5]
Mounting altazimuth
Website http://www.gmto.org/

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] 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,[8] (which is about 368 square meters).[5] 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.[4][9]

A total of seven primary mirrors are planned, but it will begin operation with four.[10][11] The $1 billion project is US-led in partnership with Australia, Brazil, and Korea, with Chile as the host country.

Site[edit]

The location of the telescope is Las Campanas Observatory,[12] which is also the site of the Magellan Telescopes, some 115 km (71 mi) north-northeast of La Serena, Chile. 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.[13] 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, kicked off with a ground-breaking ceremony.[4][9]

Mirrors[edit]

Comparison of nominal sizes of primary mirrors of the Giant Magellan Telescope and some notable optical telescopes (click for detail)

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.[14]

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.[15][16] A third segment was cast in August 2013,[10][17] and the fourth in September 2015.[18] The casting of each mirror uses 20 tons of E6 borosilicate glass from the Ohara Corporation of Japan and takes about 12–13 weeks.[19] After being cast, they need to cool for about six months.[10]

Polishing of the first mirror was completed in November 2012.[20] 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.[21] Although designed for seven mirrors, it can also begin operation with only four completed mirrors.[10]

Although the primary mirror as a whole has a focal ratio (focal length divided by diameter) of f/0.71, the individual segments, being one third that diameter, have a focal ratio of f/2.14.[17] The overall focal ratio is f/8 and the optical prescription is an aplanatic Gregorian telescope. The telescope will make use of adaptive optics.[22]

Scientists expect very high quality images of the Universe thanks to 25 metres aperture and most advanced adaptive optics to correct atmosphere distortion. Image sharpness or quality should exceed the Hubble Space Telescope.[6]

Comparison[edit]

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.[23][24] Each design is much larger than previous telescopes.[25] Even with the descale to 39.3 m the E-ELT is significantly larger than both other planned extremely large telescopes.[25] 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.[26] 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)
Collecting
area (m²)
First light
European Extremely Large Telescope (E-ELT) 39.3 978 2024
Thirty Meter Telescope (TMT) 30 655 2024
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: dates for first-light are provisional and are likely to change.

Organizations[edit]

The project is US-led in partnership with Australia, Brazil, and Korea, with Chile as the host country.[4] The following organizations are members of the consortium developing the telescope.[27]

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 eight primary mirrors: One central mirror and seven off-axis segments, of which six will be in service at any time.

  • Mirror 1, cast in October 2005,[19] with polishing completed with a surface accuracy of 19 nanometers, rms.[20]
  • Mirror 2, cast in January 2012.[19]
  • Mirror 3, cast in August 2013.[10][19]
  • Mirror 4, cast in September 2015.[18] This is the central mirror.[11]
  • Mirror 5, in glass inspection phase.
  • Mirror 6, in early construction phase.
  • Mirror 7, in planning.
  • Mirror 8, in planning.

See also[edit]

References[edit]

  1. ^ 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. 
  2. ^ 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 
  3. ^ a b "Giant Magellan Telescope Science Requirements" (PDF). GMT Consortium. p. 11. Retrieved 2008-03-31. 
  4. ^ a b c d e Amos, Jonathan (12 November 2015). "Giant Magellan Telescope: Super-scope project breaks ground". BBC News. Retrieved 2015-11-15. 
  5. ^ a b c d e "Chapter 6: Optics" (PDF). GMT Conceptual Design Report. GMT Consortium. p. 6-3. Retrieved 2008-04-02. 
  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. ^ Maggie McKee (2007-10-04). "Giant telescope in race to become world's largest". New Scientist. Retrieved 2007-10-07. 
  9. ^ a b The Giant Magellan Telescope Organization Breaks Ground in Chile
  10. ^ a b c d e Harvard Magazine - Giant Magellan Milestone (2013)
  11. ^ 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.’ 
  12. ^ "Giant Magellan telescope site selected". Carnegie Institution. Retrieved 2007-10-05. 
  13. ^ Robinson, Travis (2007-04-03). "Eye on the sky". The Battalion. Retrieved 2007-04-03. 
  14. ^ What is Optical Metrology?, GMTO, retrieved 2012-04-08 
  15. ^ 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. 
  16. ^ "Mirror Casting Event for the Giant Magellan Telescope" (Press release). GMTO. 2012-01-09. 
  17. ^ a b Steward Observatory Mirror Lab, Mirror Castings, retrieved 2012-04-08 
  18. ^ a b "Richard F. Caris Mirror Lab Casts Fourth GMT Segment" (Press release). GMTO. September 18, 2015. 
  19. ^ a b c d Mittan, Kyle (2012-01-16). "Steward Observatory casts second mirror for Giant Magellan Telescope". The Daily Wildcat. 
  20. ^ a b "World’s Most Advanced Mirror for Giant Telescope Completed". Australian National University. 2012-11-09. Retrieved 2012-01-14. 
  21. ^ "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. 
  22. ^ "Chapter 2: Overview" (PDF), GMT Conceptual Design Report, 2006, p. 2-4 § 2.5.1, 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. 
  23. ^ "GMT Overview -- Giant Magellan Telescope". Retrieved 2011-06-15. 
  24. ^ "About TMT -- Thirty Meter Telescope". Retrieved 2011-06-15. 
  25. ^ a b Govert Schilling (2011-06-14). "Europe Downscales Monster Telescope to Save Money". Science Insider. Retrieved 2011-08-17. 
  26. ^ An Expanded View of the Universe – Science with the European Extremely Large Telescope (PDF). ESO Science Office. 
  27. ^ "Giant Magellan Telescope Partner Institutions". GMTO. Retrieved 2009-02-11. 

External links[edit]