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The World's Largest Telescope


  • The GTC Observatory building stands only 6 meters shorter than the Statue of Liberty.
In 1987, the Instituto de Astrofísica de Canarias (IAC) proposed the construction of a large, next generation telescope capable of detecting exo-planetary systems, exploring dark matter, expanding our scrutiny of black holes and gaining insight about the material created in the Big Bang's aftermath. In 1994, Grantecan S.A. was established as a separate corporation whose charter was to oversee the design and construction of what was envisioned to be the largest telescope on Earth. Known variously as the Grantecan, the Great Telescope of the Canaries or simply as the GTC, the telescope is a joint venture between Spain, Mexico, and the University of Florida (USA). Spain maintains a ninety percent ownership and oversight is provided by the IAC.

Construction commenced in 1998 on a site at the Observatorio del Roque de los Muchachos that's just up the mountain from the building where I was overnighting. Therefore, because my room faced the summit of the island, the enclosure housing the world's largest telescope dominated the view through my window. Due to the absence of familiar nearby objects, it's somewhat difficult to grasp the building's size- it's huge! Standing at the height of a 14-story office building, the telescope's dome would prevail over the skyline of most cities. My excitement immediately started to elevate when I saw the GTC out my window because I knew that David had arranged for a private insider tour during our three day observing run!
  • The GTC's optical tube assembly is gigantic- 85-feet (26m.) from the floor to the tip of the secondary assembly and 45-feet (13.6m.) from side to side.


Constructed at a price that's estimated to approach USD$200 million, the GTC stands 85 feet from the bottom of its altazimuth mount to the tip of its zenith facing secondary mirror and its optical truss assembly spans almost 45 feet!. Even though I have studied the telescope in countless photographs and viewed it live with the Observatory's 24X7 web-cam, I was unprepared for the scale of the instrument when I walked through the door into the room where this beast resides- it's gigantic! It reminded me of my boyhood visit to the Vehicle Assembly Building at the Kennedy Space Center when the Apollo 12 launch vehicle stood fully assembled. Of course, even though a Saturn V moon rocket stands over twice as tall as, what is now, the world's largest telescope, I was nonetheless overwhelmed by the GTC's complexity and dimensions!
  • A flea's perspective- the author stands at the GTC's base.
  • Photo credit: Julio Carballo Bello


The great telescope's first light took place on July 13, 2007 but on the day of my visit, the scope was still a few months from regular operations- several more segments of the Primary mirror were yet to be installed and tested. When fully assembled, the GTC will possess a Primary mirror that measures 34 feet across- over twice the diameter of the venerable Hale Telescope on Mt. Palomar and over a foot wider than either of the twin Keck instruments in Hawaii!

The Primary's light gathering surface is composed of 36 independently controlled hexagonal shaped mirrors each weighing about half a ton, including the weight of their control rigs. When the 36 hexagonal segments are united, the Primary mirror will weigh-in at around sixteen tons! Despite this mass, each segment aligns to within three millimeters of those adjacent regardless of the telescope's pointing angle. Working in concert, they form a single, precision optical unit that will intensify starlight four million times brighter than seen by the unaided human eye.

The mirror's ability to resolve separate, closely spaced objects is staggering- for example, its 409 inch diameter would enable a Canary Island observer to distinguish car headlights half a world away in Australia! The thickness of each mirror segment is only slightly over three inches yet each has a polishing deviation no greater than 15 nanometers, or 3,000 times finer than a human hair- if each mirror segment were the size of Spain, the tallest mountain would only be about one millimeter tall!
  • Temporarily stored in a protected holding room, primary mirror segments await mounting- each is approximately 6-feet in diameter.


The mirror segments are composed of Zerodur, a glass ceramic that possesses an incredibly small coefficient of expansion- which means it doesn't change shape when subjected to wide temperature ranges. This is extremely important for astronomical telescopes because they can't be used for serious research until their optics have reached the same temperature as the surrounding air each night.

The optical acuity of the GTC is critical if it's to perform its mission of peering back to a time that's close to the Universe's origin. Therefore, adaptive optic sensors will be used with each of the mirror panels to keep them perfectly aligned. This will also compensate for atmospheric turbulence and remove the shimmering effect that makes stars twinkle- in essence, the column of air along the telescope's view will appear motionless regardless of its actual speed or agitation. Adaptive optics technology, originally developed for military spy satellites, was de-classified at the end of the cold war and is now in common use at the world's leading telescopes. More »

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