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E-ELT: The World's Largest Telescope


E-ELT: World's largest telescope in 2024

Scheduled for "first light" in 2024 in Chile’s Atacama Desert at 10,040 ft., the European Extremely Large Telescope (E-ELT) will then be the world’s largest telescope with a light-collecting surface 39.3 meters (128 ft.) wide.  At a estimated cost of $1.35 billion and a main structure weighing 2,800 tons, its mirror will be composed of 798 hexagonal segments (below, left) 1.44 meters wide and 2" thick. An earlier E-ELT design had a 42 m. primary mirror of 984 hexagonal segments with a 5.9 m. secondary mirror (#2 above); but in 2011 for cost and technological constraints the primary mirror was reduced to 39.3 m. by removing the outermost ring of hexagonal mirror segments. This in turn needed a smaller 4.2 m. diameter secondary mirror, and together these changes reduced projected costs by ¼ billion dollars allowing the telescope to be finished earlier. Unfortunately there are serious questions as to whether or not either size is sufficient for identifying and obtaining information from earth-like planets in the habitable zone around other stars. Obtaining this data is the "holy grail" of the E-EST, and had indeed been within reach of the now rejected 100-meter Overwhelmingly Large Telescope OWL (largest background circle below, right). Note that the largest telescope in the world from 1948 to 1974, the 200" Hale Telescope on Mt. Palomar, is represented by the small blue circle at about 10:30 just inside the largest OWL circle in the diagram below.

In the telescope's light path is a special deformable correcting mirror (#4 in the picture above) supported by over 6000 actuators with the capacity to deform its shape 1000 times a second. This deformation is initiated by eight lasers surrounding the primary mirror (shielded by long yellow tubes above) that create synthetic “guide stars” by fluorescing sodium atoms in the upper mesosphere (60 miles up) in the light path of the astronomical object the telescope is viewing. Since atmospheric distortion of the primary object the telescope is viewing occurs at the same time and over the same light path as the atmospheric distortion of the guide star, it can be digitally subtracted from the primary object's image by computer-controlled deformation of the special mirror, thus cancelling out the atmospheric distortion in the primary image. This fabulous modern procedure now installed on all very large telescopes is known as adaptive optics.

E-ELT mirror segments   Largest Telescope Reflecting surfaces compared

4 of E-ELT's 798 hexagonal mirror segments, ©ESO


Comparison of Largest Telescope Reflecting surfaces

Also in the works are the Thirty Meter Telescope (TMT) below left composed of 492 1.44-meter-wide hexagonal segments that will achieve first light from Hawaii's Mauna Kea in 2022; and the Giant Magellan Telescope (GMT) below right 20 km. from the E-ELT, with seven 27.6-foot-wide (8.4 m) mirrors in one light-collecting surface 80 feet (24 m) across, scheduled for first light in Chile's Atacama Desert in 2021. As these behemoths require more than a decade to plan, fund and build, these will probably be the three largest telescopes in the world for the next 20 years. The race to see which will be first is on!

TMT: Thirt Meter telescope in Arizona   GMT: Giant Magellan Telescope Canary Islands

Thirty Meter Telescope in Arizona: 30 meters, first light 2022


The Giant Magellan Telescope in Chile: 24 meters, first light 2021





Currently (as of January 2015) the world's largest telescope is either the 2x8.4 m. Large Binocular Telescope (LBT: below, left) in Arizona or the 10.4 m. Gran Telescopio Canarias (GTC: below, right) in the Canary Islands. The reason for this ambiguity is that although the GTC (composed of 36 segmented mirrors creating a reflecting surface 10.4 meters across) has the largest single reflecting surface of any telescope now in existence—heretofore the final arbiter on telescope size—there is more light gathering and resolving power in the LBT's two 8.4-meter mirrors, that together provide the light gathering power of an 11.8m/465" mirror and the resolving power of a 22.8m/900" mirror. All five telescopes pictured here, the two current largest below and the three larger ones still to be built, use adaptive optics to increase a ground-based telescope's resolution. To this end some of their segmented or secondary mirrors will be rapidly deformed thousands of time per second to compensate for the turbulence in the earth's atmosphere.

LBT: Large Binocular Telescope, Arizona   GTC: Gran Telescopio Canarias, world's largest

Large Binocular Telescope (LBT) in Arizona


Gran Telescopio Canarias (GTC) in the Canary Islands





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© Carl Woebcke: World's Largest Telescope: the European Extremely Large Telescope E-ELT, 1991-2017. All rights reserved.