Giant Magellan Telescope

The Giant Magellan Telescope (GMT) is a ground-based extremely large telescope under construction, planned for completion in 2025. It will consist of seven 8.4 m (27.6 ft) diameter primary segments, that will observe optical and near infrared (320–25000 nm) 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, which is about 368 square meters. 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. The Giant Magellan Telescope (GMT) is a ground-based extremely large telescope under construction, planned for completion in 2025. It will consist of seven 8.4 m (27.6 ft) diameter primary segments, that will observe optical and near infrared (320–25000 nm) 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, which is about 368 square meters. 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. A total of seven primary mirrors are planned, but it will begin operation with four. The $1 billion project is US-led in partnership with Australia, Brazil, and South Korea, with Chile as the host country. The location of the telescope is Las Campanas Observatory, which is also the site of the Magellan Telescopes, some 115 km (71 mi) north-northeast of La Serena, Chile and 180 km (112 mi) south of Copiapó, Chile, at an altitude of 2,516 m (8,255 ft). 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. 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. The excavation for the foundations was completed in early 2019. 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. The mirrors are being constructed by the University of Arizona's Steward Observatory Richard F. Caris 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.A third segment was cast in August 2013, and the fourth in September 2015.The casting of each mirror uses 20 tons of E6 borosilicate glass from the Ohara Corporation of Japan and takes about 12–13 weeks.After being cast, they need to cool for about six months. Polishing of the first mirror was completed in November 2012.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.While the complete telescope will use seven mirrors, it is planned to begin operation with four mirrors. 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.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.