Chunks of Glass, Made Into a Mirror, Will Probe Stars
At the UA, tools designed to tackle tomorrow's challenges, such as scanning alien planets for signs of life, begin as chunks of glistening glass tucked inside a cardboard box. And now five mirror segments have been cast for the Giant Magellan Telescope.

By Daniel Stolte, University Communications
Nov. 2, 2017

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The GMT mirror 5 mold filled with 38,580 pounds of Ohara E6 low expansion glass, ready for the lid of the furnace to be placed.
The GMT mirror 5 mold filled with 38,580 pounds of Ohara E6 low expansion glass, ready for the lid of the furnace to be placed. (Photo: Giant Magellan Telescope – GMTO Corp.)

With the casting of its fifth giant mirror segment at the Richard F. Caris Mirror Lab at the University of Arizona, 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 provide astronomers with 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 being produced (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, 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," said Laird Close, professor of astronomy at the UA'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."

Close explained that at near-infrared wavelengths, the GMT is designed to make images 10 times sharper and 100 times quicker than the Hubble Space Telescope. 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. 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.

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. At the time when the mirror lab, which is managed by the UA's Steward Observatory, embarked to produce the first mirror for the GMT, fabricating a mirror with those specifications was widely deemed next to impossible. Led by the lab's founder and director, UA Regents' 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. 

Four semi trucks delivered the raw material that would become GMT5 to the mirror lab underneath the University's football stadium. Workers unloaded 68 transport crates containing 1,632 cardboard boxes, each holding 25 pounds of pristine blocks of custom-manufactured glass from the Ohara Corp. of Japan. Over the weeks that followed, each chunk was handled and inspected to make sure no foreign matter had made it into the irregularly shaped, brilliantly translucent blocks.

On Oct. 23, technicians spent the better part of a workday lying on their bellies on a platform, reaching down and carefully placing each glass chunk inside the furnace's base, creating a sparkling pond of jagged glass in the process.  

Once the glass was in place and the working platform lifted away, a crane lowered the lid — fire-engine red, as is the rest of the giant oven — and mirror lab staff prepared the furnace for the casting process. Over much of the next two months, the furnace will spin at just under five revolutions per minute, while a technological marvel forms inside.

Heating up the glass takes about a week, according to Jeff Kingsley, associate director of Steward Observatory, who expects the furnace to reach its peak temperature of 1,165 degrees Celsius, or 2,129 degrees Fahrenheit, on Nov. 4. Three days later, his team will bring the temperature down to 700 degrees Celsius, or 1,290 degrees Fahrenheit, slow the rotation speed and begin the annealing process, which will extend to the end of December.

"We then go to a period of very, very slow cool-down to room temperature," Kingsley said. "At the end of January, we'll open the lid, take the sides off the furnace, remove the band and tub walls, and reveal the casting." 

The unique fabrication process resulting in a lightweight honeycomb glass structure is the key to the success of mirrors created at the mirror lab. Not only are these mirrors so light they could float in water, but they are very stiff and quickly adjust to changes in nighttime air temperature, each resulting in sharper images.

Every mirror that came out of the mirror lab was cast in this furnace, which was built in 1984 and is the only one of its kind. The rotation ensures that the glass starts out close to the desired parabolic shape. A total of 270 heaters inside the furnace ensure the glass is heated evenly throughout.

Set to begin science operations in 2023, the GMT will take advantage of the clear dark skies of the Atacama Desert, where almost no water vapor in the atmosphere impedes the view into the night.

At 360 square meters, or about 3,900 square feet, the GMT's seven mirrors combine to form an area just a bit smaller than a basketball court.

The telescope 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.

Astronomers looking to take the next giant leap with the crop of giant telescopes are eagerly awaiting the completion of the GMT. 

One of the most interesting things that the GMT could do, according to Close, is to find life on another world, such as the recently discovered, rocky planet orbiting our closest neighboring star, Proxima Centauri. 

"Imaging the planet next to its star is like looking at two dimes side by side from a hundred miles away," Close said, "but with GMT and adaptive optics, you could easily resolve the two."

In such a scenario, scientists would equip the giant telescope with a coronagraph, a device that blocks the light from the star so as to make it possible to detect the light from the planet, which is millions of times fainter. With suitable instruments that can identify the different light signatures of chemical elements and compounds, they could then scan the planet for so-called bio signatures such as the oxygen in Earth's atmosphere.

"Casting the mirrors for the Giant Magellan Telescope is a huge undertaking, and we are very proud of the UA’s leading role creating this new resource for scientific discovery," said UA President Robert C. Robbins. "The GMT partnership and the Richard F. Caris Mirror Lab are outstanding examples of how we can tackle complex challenges with innovative solutions. The University of Arizona has such an amazing tradition of excellence in space exploration, and I have been constantly impressed by the things our faculty, staff and students in astronomy and space sciences can accomplish.” 

The UA’s Richard F. Caris Mirror Lab is the only facility in the world capable of manufacturing the seven giant mirrors that combined will form the 25-meter (82-foot) diameter primary mirror of the Giant Magellan Telescope. The GMT Organization's contracts with the UA to fabricate the mirrors are anticipated to total $200 million, providing a major economic impact on jobs and expenditures in Arizona.

Coordinated through the GMTO, the partners in the GMT project are Astronomy Australia Ltd., the Australian National University, the Carnegie Institution for Science, Harvard University, the Korea Astronomy and Space Science Institute, São Paulo Research Foundation (FAPESP), the Smithsonian Institution, Texas A&M University,  the University of Arizona, the University of Chicago and the University of Texas, Austin. 

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Daniel Stolte

University Communications