core sector communique

Join Mirror Casting Event for the Giant Magellan Telescope

Washington, D.C.—On January 14, 2012, the second 8.4-meter (27.6 ft) diameter mirror for the Giant Magellan Telescope (GMT) will be cast inside a rotating furnace at the University of Arizona’s Steward Observatory Mirror Lab (SOML) underneath the campus football stadium. The Mirror Lab will host a special event to highlight the milestone of creating the optics for the Giant Magellan Telescope. The Carnegie Institution is a founding member of the GMTO partnership.*

Members of the media are invited to visit the Mirror Lab on Saturday morning, January 14, 2012, between 9:00 and 11:00 a.m. MST, to see the liquid glass as it is spun cast in a rotating oven at a temperature of 1170 degrees C (2140° F). This casting marks another major step in the construction of the Giant Magellan Telescope. There will be opportunities to interview leading scientists and engineers involved in the project.

This event is supported by the University of Arizona’s Steward Observatory and College of Science and by the GMTO Corporation (GMTC), a nonprofit entity with project offices based in Pasadena, California.*

The GMT features an innovative design using seven mirrors, each 8.4 meters in diameter, arranged in a floral pattern to form a single mirror 24.5 meters (80 feet) in diameter. It will bring starlight to a common focus via a set of adaptive secondary mirrors configured in a similar seven-fold pattern.

“In this design the outer six mirrors are off-axis paraboloids and represent the greatest optics challenge ever undertaken in astronomical optics by a large factor” said Roger Angel, Director of the Steward Observatory Mirror Lab.

The GMT will allow astronomers to answer some of the most pressing questions about the cosmos including the detection, imaging, and characterization of planets orbiting other stars, the nature of dark matter and dark energy, the physics of black holes, and how stars and galaxies evolved during the earliest phases of the universe.

Patrick McCarthy, GMT project director and staff astronomer with the Carnegie Observatories, remarked, “This second GMT casting is going forward now because the primary optics are on the critical path for the project and because the polishing of the first off-axis 8.4-meter GMT mirror is very close to completion, with an optical surface accuracy within about 25 nanometers, or about one-thousandth the thickness of a human hair.”

Like other mirrors produced by the Mirror Lab, the GMT mirrors are designed to be spun cast, thereby achieving the basic front surface in the shape of a paraboloid. A paraboloid is 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. Precision grinding and polishing of the surface is then undertaken to create the figure of an off-axis parabloid.

“The novel technology developed at the Mirror Lab is creating a whole new generation of large telescopes with unsurpassed image sharpness and light collecting power,” said Wendy Freedman, Director of the Carnegie Observatories and Chair of the GMTO Board. “Themirrors in the twin Magellan Telescopes at our Las Campanas Observatory site are performing superbly and led to our adoption of this technology for the GMT.”

Some 21 tons of borosilicate glass, made by the Ohara Corporation, flow into a pre-assembled mold to create a lightweight honeycomb glass structure that is very stiff and quickly adjusts to changes in nighttime air temperature, each resulting in sharper images. The Mirror Lab has already produced the world's four largest astronomical mirrors, each 8.4 meters in diameter. Two are in operation in the Large Binocular Telescope (LBT)—currently the largest telescope in the world, one is for the Large Synoptic Survey Telescope (LSST), and the fourth is the first off-axis mirror for GMT. The Mirror Lab has also produced five 6.5-meter mirrors, two of which are in the twin Magellan telescopes at Las Campanas Observatory in Chile. 

“Astronomical discovery has always been paced by the power of available telescopes and imaging technology. The GMT allows another major step forward in both sensitivity and image sharpness” said Peter Strittmatter, Director of Steward Observatory. “In fact the GMT will be able to acquire images 10 times sharper than the Hubble Space Telescope and will provide a powerful complement not only to NASA’s 6.5-meter James Webb Space Telescope (JWST) but also to the Atacama Large Millimeter Array (ALMA) and the Large Synoptic Survey Telescope (LSST), both located in the southern hemisphere.”

The GMT is set to begin science operations in 2020 at Carnegie’s Las Campanas Observatory, exploiting the clear dark skies of the Atacama Desert in northern Chile.

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*The GMTO manages the GMT Project on behalf of its international partners, namely: Astronomy Australia Ltd., the Australian National University, the Carnegie Institution for Science, Harvard University, the Korea Astronomy and Space Science Institute, the Smithsonian Institution, Texas A&M University, the University of Arizona, the University of Chicago, and the University of Texas at Austin.