How to build a 27-foot mirror under a football stadium

The Giant Magellan Telescope’s second mirror segment undergoes final polishing process. Photograph by Jules Zappone, Arizona Sonora News Service ©2017.

Beneath the east bleachers of  Arizona Stadium, the most delicate and perfected mirror-making process takes place.

The University of Arizona’s Richard F. Caris Mirror Laboratory is famous for building mirrors for some of the world’s most powerful telescopes. Engineers at the mirror lab are working on mirrors for the Giant Magellan Telescope; each mirror is about 27-feet in diameter.Upon completion, the GMT will be the largest telescope in the world and will revolutionize the way astronomers study the universe.

A 3D rendering of the GMT hangs on the wall of the mirror lab. Photograph by Jules Zappone, Arizona Sonora News Service ©2017.

“It will allow astronomers to study earth-like planets around other stars,” said Thomas Fleming, an astronomer and senior lecturer at the University of Arizona. “Also, the further away you can see in space, the further back in time you’re looking.”

Five of the seven GMT mirrors are at some point in this engineering process. But how are these massive mirrors made?

“The lifecycle begins with the glass,” says Stephen West, a senior research scientist who works in the mirror lab.

The glass is created and shipped from a company in Japan to the mirror lab. West said the glass has to be specifically made so how the glass expands and contracts due to temperature is uniform.

“Otherwise we’d have one area expanding this much [due to temperature] and then another area expanding more and that would mess up the shape of the mirror,” he said.

Before anything else, each piece of glass is tested for imperfections. West said that there is a certain criteria each chunk has to meet, so each chunk has to be looked at.

“Most of [the chunks] are fine,” he said. “But we have to get rid of the ones that aren’t.”

The glass for each mirror is shipped over from Japan in large chunks. Photograph by Jules Zappone, Arizona Sonora News Service ©2017.

Once individual glass chunks have been inspected, the team begins preparing the mold for the mirror. This process begins by bolting individual, hollow, hexagonal-shaped ceramic fiber cores to the foundation of the furnace. The cores are 80 percent hollow to help reduce the weight of the melted glass and reduce resistance to changing temperatures. As the glass melts, it flows down the spaces between the hexagonal cores, creating a honeycomb structure.

“[The honeycomb] structure helps us keep the mirror thermally uniform so when it’s put in the telescope, the outside temperature won’t warp the shape,” West said.

The bottom of each mirror has a number of holes, and each hole feeds into the honeycomb structure. Air taken from the environment is then blown into those structures as the mirror moves throughout the building process.

“We’re trying to keep the temperature across the mirror as close to that of the environment it’ll experience outside the lab,” West said.

Once the mold is completed the chunks of glass are hand-placed across the hexagonal cores and the furnace is sealed. According to West, about 36,000 pounds of glass are loaded onto the mold. Once the glass melts the furnace will begin rotating between 7-9 RPM to create the mirror shape.

“We find a rotation rate so that once the glass is melted, the centripetal force of the rotation pulls the glass into the shape we want,” West said.

This process of spin casting allows for the glass to evenly distribute across the mold and creates a surface that’s easier to grind and polish. The rotating furnace will spin for about three months.

After the newly-shaped mirror has cooled, the lid and walls of the furnace are lifted away and a spider-looking tool is glued to the mirror. The mirror is then lifted from the oven and attached to a steel handling ring that holds the mirror.

The spider-like lifting tool and the steel handling ring used to pick up and store the mirror after it is removed from the rotating furnace. Photograph by Jules Zappone, Arizona Sonora News Service ©2017.

“Anything we lift or support the mirror with, has to be done carefully to provide the right distribution of forces so the mirror doesn’t break,” West said. “And obviously we don’t want to break mirrors.”

The ring is also called the turning fixture and is used several times throughout the process; first is used to tilt the mirror on its side so the floor of the mold can be removed and then later used to flip the mirror from backside up to frontside up.

From there, the mirror goes to the Large Optic Generator, a machine that uses diamond wheels to aggressively shave off glass.

“So the mirror gets cast, then it comes-backside up-to the LOG. The machine diamond cuts the back and then fine grinds it to make it smoother,” West said. “The fine grinding is getting all the subsurface damage out of the glass and creating a flat backside of the mirror.”

The Large Optic Generator (LOG) grinds the backside of the GMT segment four to remove subsurface damage. Photograph by Jules Zappone, Arizona Sonora News Service ©2017.

Once the back is finished, the mirror is moved into a separate room where it is set on weight distributers until the front side is ready to begin its turn on the LOG.

The final shape is produced using stressed-lap polishing technology. This process is essential in getting the mirror to the shape necessary to bend light properly.

Once the mirror has been polished, the mirror is moved beneath the testing tower where the accuracy of its shape is tested using lasers.

“In the end, we’ll work at [polishing the mirror] for 3-4 days, then move it under the test tower and measure it. Then we send it back to the polishing machine to work out the errors,” West said.

The mirror will endure several rounds of polishing and testing before it reaches the perfect curved shape.

Jules Zappone is a reporter for Arizona Sonora News, a service from the School of Journalism with the University of Arizona. Contact her at

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