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How does glass chunk become a large telescope?

    How does glass chunk become a large telescope?
    Issue Time:2017-11-09
      How does glass chunk become  a large telescope?
    With the casting of its fifth giant mirror segment, the world’s largest telescope clears a major milestone toward completion.
    Sporting a flowerlike primary mirror design of six circular segments surrounding a seventh in the center, the Giant Magellan Telescope, or GMT, will give astronomers a one-of-a-kind tool to answer some of the most fundamental questions about the cosmos and our place within, including searching for signatures of biological activity on planets outside our solar system.

    While its mirror segments are in production (including a spare to be rotated into the mix during maintenance), construction of the facility that will house the telescope is underway at Las Campanas Observatory in the Atacama Desert in northern Chile. The GMT is one of only two projected next-generation instruments commonly referred to as extremely large telescopes that will be able to observe the entire southern skies.
    With its unique configuration of seven honeycomb mirror segments, each spanning 27.5 feet, researchers say the GMT’s light-gathering capability will be unprecedented.
    “Now, with the casting of the fifth mirror, the GMT truly starts to take shape as a giant telescope,” says Laird Close, professor of astronomy at the University of Arizona’s Steward Observatory, where the giant mirrors are being manufactured. “We already have the glass for mirror number six. It’s amazing to think that we’re almost done building the telescope in terms of the glass.”

    Paraboloid surface

    At near-infrared wavelengths, the GMT is designed to make images 10 times sharper and 100 times quicker than the Hubble Space Telescope, Close explains. This would become possible through an adaptive optics system, which counteracts the blurring of the images caused by turbulence in the atmosphere along the path of light from astronomical sources to the telescope.

    Like other mirrors produced by the RFC Mirror Lab, the GMT mirrors are made so that their front surface forms a paraboloid—the shape taken on by water in a bucket when the bucket is spun around its axis; the water rises up the walls of the bucket while a depression forms in the center.

    However, the GMT’s unique off-axis design requires the shape of its outer mirror segments to be asymmetric in profile, so that all mirror segments together assemble into one giant paraboloid mirror.
    When the mirror lab started to produce the telescope’s first mirror, fabricating one with those specifications was widely deemed next to impossible. Led by the lab’s founder and director, professor Roger Angel, technicians and engineers perfected the process of producing the world’s largest, yet lightest, telescope mirrors in a process called spin casting.

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