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Giant Magellan Telescope (GMT)

Like The Thirty Meter Telescope and the European Extremely Large Telescope, the international Giant Magellan Telescope will have a segmented mirror. But instead of many hundreds of hexagonal pieces, the GMT’s primary consists of just seven 8.4m mirrors – about the largest that can be cast and transported in one piece.

Giant-Magellan-Telescope-300x207 Giant Magellan Telescope (GMT)

Giant Magellan Telescope

The downside of this elegant design is that six of the seven mirrors have to be asymmetric, which makes grinding and polishing a more complicated task. Whereas the TMT is essentially a scaled-up version of the Keck telescopes, the GMT is the logical successor to the twin 6.5m Magellan telescopes operated by the Carnegie Institution for Science at the Las Campanas Observatory in Chile.

In fact, says Las Campanas Observatory director Miguel Roth, “We may eventually build a second Giant Magellan Telescope. The platform that we created on Cerro Las Campanas in March 2012 is large enough to house two similar giant telescopes.” Like the TMT, the GMT has grown into a large international collaboration, with contributions from US universities and institutes, and from organisations in Australia and Korea. The Steward Observatory Mirror Lab of the University of Arizona is developing the telescope’s thin mirrors.

They rely on active optics to maintain their precise parabolic shape. In addition to this, adaptive optics will take care of atmospheric turbulence, so that the giant telescope will have 10 times the angular resolution provided by the Hubble Space Telescope. According to Roth, the GMT will already start producing exciting results as early as 2020, with only four of the seven mirror segments and two powerful spectrographs in place. At that time, it is expected to be the largest optical and nearinfrared telescope in existence.

A few years later, in its full configuration, it will boast a total light-collecting area of 368m2 – 10 times that of each of the current Magellan telescopes. However, by then it will almost certainly have been surpassed by the Thirty Meter Telescope. Equipped with sensitive spectrographs, the GMT will be a prime instrument for carrying out detailed studies of the chemical composition of stars, mass determinations of supermassive black holes, and measurements of stellar wobbles caused by orbiting exoplanets.


  • Mirror size: 22.2m equivalent (seven 8.4m segments)
  • Location: Las Campanas Observatory, Chile (altitude 2,550m)
  • Date of completion: Early scientific operations will start in 2020, with full capability reached in 2024
  • Cost: $1.05 billion (£650 million)
  • Trivia: While the light-collecting power of the GMT’s seven mirrors will be the same as that of a 22.2m telescope, they will provide the angular resolution of a single 24.5m mirror.

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