The next generation of ground-based telescopes will take another giant leap forward on Saturday, with the completion of the third mirror for the upcoming Giant Magellan Telescope (GMT).
The GMT will commence its scientific endeavors at the Las Campanas Observatory in 2020, joining a battery of smaller telescopes in Chile’s Atacama Desert. It will feature a segmented primary mirror composed of seven monolith mirrors forming a single optical surface, and a light-capturing aperture 80 feet in diameter. The GMT will boast 10 times the resolving power of the venerable Hubble Space Telescope. It will collect more light than any telescope in history. However, before the GMT can employ these formidable advantages in scientific objectives, its mammoth mirrors must be completed.
The third such mirror will be cast in a rotating furnace at the University of Arizona’s Seward Observatory Mirror Lab, the only facility on Earth capable of creating such enormous mirrors. As with the other six mirrors that will comprise the GMT’s main optical surface, the third mirror will weigh approximately 20 tons and measure 27 feet in diameter.
The spin-casting process creates the essential paraboloid shape for the mirror; thanks to the spinning motion, a depression forms in the center of the casting medium as its outer edges rise up the walls of the furnace. In the case of the GMT mirrors, about 21 tons of borosilicate glass flow into a prefabricated mold to form the lightweight honeycomb structure of the mirror; this structure will be very stiff and allow the mirror to adjust rapidly to temperature fluctuations at night, preventing distortion of the images obtained. Once it is cast, the third mirror will be polished to an optical surface accuracy within approximately 25 nanometers.
The GMT’s optical systems will be the most sophisticated ever deployed. Light will reflect off the seven mirror segments of the primary mirror, again off the seven smaller secondary mirrors, and finally down through the central primary segment to the imaging cameras. The secondary mirrors are flexible, thanks to hundred of actuators under the surface of each, to counteract atmospheric turbulence and provide unprecedented resolution.
The GMT is a joint endeavor by institutions around the world, including Astronomy Australia Ltd., Australian National University, Carnegie Institution for Science, Harvard University, Korea Astronomy and Space Science Institute, Smithsonian Institution, University of Texas at Austin, Texas A&M University, University of Arizona, and University of Chicago.