The glass is made in Japan. Although the final product will look toward the future, the first steps call on glassmaking techniques of centuries past.

Craftsmen fill large handmade clay pots with one and a half tons of silica and boron oxide powders. The sandy mix goes into a great furnace, where it is heated for more than two days at temperatures as high as 1,500 degrees Celsius (2,732 Fahrenheit), melting the raw material until the 800-liter pots are brimming with molten glass. Over the course of 13 days, the material cools as the crystal-clear glass hardens uniformly, forming a structure that is resistant to heat and pressure.

The clay pots are then shattered with a sledgehammer, revealing the transparent monoliths within. Each slab weighs about a ton. The Ohara E6 glass is perhaps the purest optical glass in the world, known for its resistance to thermal expansion.

A slab of E6 glass. Ohara Group

The Japanese glass is cut into smaller chunks that weigh four to five kilograms (9 to 11 lbs.) each, which are then shipped to Tucson, Arizona. Here, technicians will use the glass to cast gargantuan mirrors, each one spanning 8.4 meters in diameter—that's 27 feet from edge to edge—and weighing 17 tons. In time, seven of these segments will combine to form an enormous primary telescope mirror spanning 24.5 meters (80 feet) across.

The justification for all this globetrotting, painstaking glass work is the Giant Magellan Telescope, which will become the largest optical telescope in the world. When complete in the mid-2020s, GMT will perch on the summit of Chile's Cerro Las Campanas at 8,200 feet, in a 22-story-tall building that is already under construction. Here, in the high altitudes of the Atacama Desert, the ultra-dry air offers some of the clearest views of the night sky anywhere on the planet.

The giant telescope will have 10 times the resolving power of the Hubble Space Telescope, revealing distant galaxies, the birth of stars, and the compositions of exoplanet atmospheres—a key field of research in the search for extraterrestrial life.

Artist rendering of the Giant Magellan Telescope. GMTO

The beginning of telescope operations, known as first light, is planned for 2023 and will use just four mirrors. The first observations with all seven mirrors will follow in 2025. As for today: The pots are filled in Japan, the furnace is lit in Arizona, the ground is broken in Chile, and the Giant Magellan Telescope is under way.

Polishing Potato Chips

"We had to make the first one before they let us make the second," says Dae Wook Kim, "because they didn't think it was possible to make such a potato chip."

Kim, an assistant professor at the College of Optical Sciences at the University of Arizona, has taken me deep into the bowels of telescope technology. We're standing in the Richard F. Caris Mirror Laboratory, a big bunker beneath the university's 55,000-seat football stadium. Here, below the gridiron of the Arizona Wildcats, is where the largest telescope mirrors in the world come together.

The Mirror Lab has "possibly the only people who can make large single mirrors anymore," says Patrick McCarthy, vice president for operations of the Giant Magellan Telescope Organization (GMTO). Arizona's lab has made mirrors for some of the most powerful telescopes on the planet, including the twin 6.5-meter mirrors for the Magellan Telescopes in Chile, the 8.4-meter ones for Arizona's Large Binocular Telescope (LBT), and the 8.4-meter mirror for the Large Synoptic Survey Telescope (LSST), an ambitious project to take 1,000 pairs of exposures of the night sky with a 3.8-gigapixel digital camera, the largest in the world.

The glass for the 5th GMT mirror segment in the University of Arizona’s revolving furnace for spin-casting giant telescope mirrors, housed underneath the university football stadium. GMTO

At the moment, though, Kim is wrestling with his potato chip problem. The central mirror of the GMT will be a perfectly parabolic concave structure. The outer six segments, however, need to be shaped differently to work as a single primary mirror once aligned together. The six will circle the central mirror like flower petals, and each one needs to reflect the light it collects to the very center of the aperture, which means they must be ground to a potato chip shape.

The Mirror Lab houses a giant rotating furnace to cast the slabs of glass, equally enormous grinding and polishing machines, and a measuring tower to scan the mirrors with lasers. The whole process starts with the chunks of glass from Japan. Technicians fill the industrial merry-go-round furnace by hand. For the fifth mirror, they laid out precisely 17,481 kg (38,539 lbs.) of Ohara E6 glass.

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The furnace begins to spin as the heat is cranked up to 700 C (1,292 F). The machine spins faster and grows hotter over the course of weeks, until the large rotating furnace hits 4.9 rpm and 1,165 C (2,129 F), hot enough to melt the glass blocks entirely.

The lab's secret is spin-casting mirrors in a rotating furnace over a mold to form a honeycomb structure, which makes their creations much lighter and sturdier than a mirror would be with no structural cells. The molten material pools around a mold of individual cells to create the honeycomb on the back, while a smooth surface of uninterrupted glass forms on the front.

The furnace then slows to 1/2 rpm for a three-month cooling process. The slow cooling of the glass allows it to anneal, relieving internal stresses and improving durability. Once the mirror cools and hardens, a year or perhaps longer of grinding and polishing can begin.

The rotating furnace at the Richard F. Caris Mirror Lab, loaded with Ohara E6 glass and about to start the casting process. GMTO

Despite all this telescope wizardry, the technicians at Arizona's Mirror Lab didn't know for sure if they'd be able to make something so precise as the potato chip. Grinding the shallow bowls into the saddle shape required for the outer GMT mirrors had never been attempted. "We don't get there in one shot," says Hubert M. "Buddy" Martin, an associate research professor of optical sciences at the University of Arizona.

The mirrors are measured with lasers in a 28-meter-tall (92 feet) test tower. The tower is structurally isolated from the rest of the stadium. Three meters of solid concrete form its foundation, and pneumatic isolators prevent any vibrations from affecting the precision laser interferometer.



The test tower for measuring large telescope mirrors at the Mirror Lab. GMTO

"We're pretty much immune to vibrations in the building, football stadium, traffic outside, helicopters coming into the hospital," says Martin. "If we weren't isolated from these vibrations, they would wreak havoc on the measurements."



The test tower beams lasers down onto the mirror and collects the rebounding light with a series of sensors. The system creates a contour map of the glass surface. "You can think of it as a sheet of light designed to be the perfect mirror surface," Martin says. "When the light comes back you can see irregularities."

The test tower data feeds into computers that control an industrial grinder and a polisher. The grinder does quick work using 80-grit diamond sandpaper, a noisy process that requires hearing protection. The 35-inch-thick mirror is shaved down from 20 tons to more like 17.



Polishing, however, is a more delicate process. The Mirror Lab worked for a year and a half to polish the GMT's first mirror, moving the gargantuan slab of glass back and forth between the test tower and the polishing machine some 50 times. While the grinder works with a precision of about 75 micrometers, the polishing machine refines the glassy surface to perfection within 25 nanometers, less than a millionth of an inch.

Dr. Dae Wook Kim giving a tour of the Mirror Lab with mirror segment 2 under the test tower. GMTO

"If you made this mirror the entire size of the United States, the Rocky Mountains would be a coin width up," Kim says, standing before the enormous polishing machine. A rotating arm mounted to a control rig waxes a small part of the mirror, like a robotic gearhead meticulously polishing its '63 Sting Ray.



The polisher uses off-the-shelf Silly Putty that the Mirror Lab buys in 5-pound blocks. The MacGyver trick was Kim's idea, and it's an example of the inventive solutions you need in a one-of-a-kind mirror-casting lab. Despite all their advanced gadgets, the lab technicians use fishing wire to remove pads glued to the mirrors, and the furnace is loaded by hand.

Moving GMT mirror segment 4 out of the furnace with glue-on pads. GMTO

The first GMT mirror is now complete. It sits in a climate-controlled storage container on a trailer in Tucson, waiting to begin its journey to Chile. With one mirror in the basket, the Mirror Lab has been given the green light to cast more. There are four GMT mirrors in various stages of completion at the Arizona lab. The team hopes to make the potato chips faster now that the process has been proven.

At the Mirror Lab, you'll hear the technicians joke that they don't actually make mirrors. "We just make really huge pieces of glass," an employee said. The final step of the process—which will take place at the telescope's construction site in Chile—is coating the mirror blanks with a thin layer of aluminum to actually make them reflective.

The Giant Magellan Telescope inches toward first light, when these mirrors will turn to the heavens for the first time. "We want to take a picture of the universe when it was just being born," Kim says.



The Planet Detective

GMTO

When it all comes together in the 2020s, the GMT will be able to resolve an object the size of a dime at 60 miles away, which is an order of magnitude more resolving power than Hubble. Such viewing power opens a universe of possibilities.



"Our favorite science use for GMT is looking at planets in habitable zones," says Jared Males, assistant astronomer at the University of Arizona's Steward Observatory. "What we want to do is look for biosignatures. We want to look for life on these planets."

Sophisticated spectrometers will measure the light collected by GMT to identify the compositions of celestial objects. When an exoplanet passes in front of the star it orbits from our perspective, the giant telescope's instruments will be sensitive enough to separate the light passing through the planet's atmosphere from the rest of the starlight. The light that passes through the atmosphere will have gaps in the electromagnetic spectrum—absorption lines that reveal the presence of specific molecules in that world's skies.

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"A really good biosignature is diatomic oxygen," says Patrick McCarthy. "All the oxygen in Earth's atmosphere is due to life."

Photosynthetic life is the only reason molecular oxygen exists in such abundance in Earth's atmosphere, but there are also natural geologic processes that can produce significant amounts of atmospheric oxygen (though if this were the case on another planet, other clues would tell us the oxygen wasn't from a biological source). More problematic, however, is the fact that life existed on Earth for more than a billion years before photosynthesis filled our skies with oxygen, and similar anaerobic microbes on other planets could happily thrive with no O 2 to speak of.

If that life is anything like the early life on Earth, however, it will produce methane, another intriguing potential biosignature for alien worlds. In fact, a recent study in Science Advances found that a combination of methane and carbon dioxide in an exoplanet's atmosphere would be a convincing biosignature of pre-photosynthetic life.

This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. NASA/JPL-Caltech​

The GMT will turn its gaze toward nearby exoplanets, such as Proxima b orbiting the closest star to us and the seven Earth-sized planets orbiting the star TRAPPIST-1 less than 40 light-years away. The scope will also look to thousands of other planets that astronomers are discovering all across the Milky Way to search for signs of life in alien atmospheres.

"You can ask whether the sky is blue on an exoplanet," says McCarthy. An atmosphere similar to Earth's would cause blue light to scatter more than other wavelengths.

The enormous telescope, perched in the high Atacama, will also be able to detect incredibly dim objects with its vast light-collecting surface. Early star formation will be studied with unprecedented precision, and galaxies will be discovered that are farther away, older, and less bright than any previously detected. In addition to searching the galaxy for life, the GMT will allow astronomers to study the most ancient eons of the universe, probing the firmament to test our fundamental laws of physics.



A Telescope for the Future

GMT diagram. Damien Jemison/GMTO

Since the 1990s, two revolutionary technologies have enabled telescopes to measure the cosmos with more precision than ever before. The first is interferometry, which uses multiple telescope observations to stitch together a picture with the same resolution as one enormous telescope, which is useful for doing things like imaging the surfaces of other stars. The test tower at the Mirror Lab uses interferometry to create contour maps of the glass surfaces.

The second major breakthrough in telescope technology, which the GMT will use, is adaptive optics.

"The telescope is basically always getting knocked out of focus from the atmosphere," says Laird Close, professor of astronomy at the University of Arizona. "So if you could use actuators to adjust the focus 1,000 times per second, you can have a telescope that is always in focus."

Keeping the telescope in focus is exactly what the adaptive secondary mirror is designed to do. This 3.25 meter diameter mirror will also have seven circular segments to match the primary mirror segments, except the secondary mirror segments will only be 1.6 mm thick.

"It's so thin that if you picked it up with your hands, it would break," says Close. "We spend a lot of time trying not to break the mirrors."

The adaptive secondary mirror will be constantly warped by hundreds of actuators to correct for the atmospheric refraction that bends light before it reaches the surface of our planet. Sodium lasers beamed from the telescope up into the sky will serve as guide stars to calibrate the adaptive optics system, which uses algorithms to predict atmospheric changes and warp the secondary mirror in real time.

GMTO

Even in the high Atacama Desert, atmospheric interference can significantly reduce the precision of astronomy observations. Adaptive optics systems allow telescopes like GMT to achieve unprecedented accuracy, similar to a telescope orbiting in space.

Other large ground-based telescopes—such as the Thirty Meter Telescope (TMT) planned for construction on the volcano Mauna Kea in Hawaii and the Extremely Large Telescope (ELT) which will live to the north of GMT in Chile—will have even larger primary mirrors than the Giant Magellan Telescope. These two telescopes, however, will use hundreds of hexagonal mirror segments that are each about a meter and a half across (492 segments for the TMT and 798 for the EMT), rather than seven gargantuan monolithic mirrors like GMT.

As a result, more gaps will exist in the light collecting areas of the TMT and ELT. The light will need to bounce around more before it reaches the instruments, and adaptive optics algorithms will be required to stitch everything together to a greater extent than with GMT, which can use a backup secondary mirror that is rigid to conduct observations while its flexible adaptive optics system is being cleaned and maintenced.

There is also the next flagship NASA space telescope, the James Webb Space Telescope (JWST), with 18 hexagonal mirror segments measuring 2.4 meters across for a total primary mirror aperture of 6.5 meters. James Webb will be dwarfed by the GMT's 24.5 meter aperture, but out beyond the atmosphere it will have an unparalleled view of the stars. Additionally, James Webb will take observations primarily in infrared, while GMT is optimized for light in the visible spectrum.



GMT construction site as of February 2017. GMTO

"I think it will be very complimentary to Webb," says McCarthy. "The visible part of the spectrum is very rich in information."

James Webb, for example, will be particularly suited to studying the very early universe, as light that has been traveling for billions of light-years is Doppler shifted significantly into the infrared part of the spectrum. Infrared light can also be detected piercing through clouds and nebulae in space, revealing the objects lurking behind. Optical observations, like those of Hubble and GMT, are well-suited to spectroscopy and detecting the compositions of bodies such as exoplanets. The sheer size of GMT will also make it capable of detecting incredibly dim and distant objects.

Before the Giant Magellan Telescope can power on, however, there is still much work to be done. The fifth mirror is currently cooling in the furnace, and three more need to be cast—one extra outer mirror to keep the telescope at full capability as mirrors are swapped out to be cleaned and recoated with aluminum. The 17-ton mirrors in their articulated, temperature-controlled, shock-absorbing cases also need to be shipped from Arizona to Chile—by way of port in either Houston or Los Angeles.

Everything is at stake. McCarthy stresses the need to avoid a situation like the first light of Hubble, when it was discovered that the mirrors were not perfectly shaped, requiring new instrumentation to be installed to account for the incorrectly focused beams of light. And, of course, there is the possibility of losing a mirror on the voyage across the ocean and up into the mountains.

"You have all your eggs," says McCarthy. "Do you put them all in one basket, or do you put them in multiple baskets? ... Maybe I'll just go with them because if the boat goes down, it won't be my problem."

GMTO

The Giant Magellan Telescope is truly a telescope of a new era. The first large-scale observatories were established by the scientific societies of the Enlightenment with the financial backing of monarchs, such as the Royal Observatory in Greenwich and the Paris Observatory. In subsequent centuries, wealthy benefactors sponsored the construction of giant telescopes, such as James Lick who is buried underneath the concrete of his eponymous observatory on the summit of Mt. Hamilton near San Jose, California. In the modern era, taxpayers foot the bill for major astronomy projects from agencies like NSF, NASA, and the European Southern Observatory (ESO).

The GMT project represents a paradigm shift. Private interests, international partners, universities, and science institutions across the world have come together to build this great monument to science in the high Atacama. Boeing is conducting fluid dynamics research for the telescope's housing structure, and NASA's Jet Propulsion Laboratory is providing mirror insight gleaned from the mistakes made on Hubble. Other than the University of Arizona's Steward Observatory, the GMT is sponsored by the Carnegie Institution for Science in Washington D.C., the São Paulo Research Foundation of Brazil, the Korea Astronomy and Space Science Institute, Harvard, the Smithsonian Institution, Arizona State University, the McDonald Observatory of the University of Texas at Austin, Texas A&M, the University of Chicago, Astronomy Australia Limited, and the Australian National University, among other private benefactors.

"We're going to be fighting tooth and nail for time," said Males, referring to the University of Arizona's need to share the telescope's observation time with so many other institutions. But the very fact that science agencies around the world are chomping at the bit to get involved with the Giant Magellan Telescope is a testament to its capability, versatility, and the profundity that astronomers expect to encounter through its 119-ton mirror, glinting on a mountaintop in the Atacama Desert.

"We do not make this to sell for a profit," says Dae Wook Kim. "We make it for all mankind."

