Tucson, Ariz. – A recipe for how to make an 8.4-meter telescope mirror:

About 44,000 pounds of glass chunks.


  1. Load glass chunks into 27-foot diameter mold.
  2. Place mold in giant spinning oven and set for five revolutions-per-minute.
  3. Cook for four days at 2,000 degrees Fahrenheit.
  4. Gradually decrease the heat to room temperature and rotation rate to zero over three months.
  5. Uncover

Listen to the story:

A highly polished telescope mirror. This segment, part of the Giant Magellan Telescope, is 8.4 meters, or 27 feet, in diameter.

A highly polished telescope mirror. This segment, part of the Giant Magellan Telescope, is 8.4-meters, or 27-feet, in diameter.
(Photo by Mark Duggan)


The Steward Observatory Mirror Lab at the University of Arizona has been making some of the world’s largest telescope mirrors for more than 20 years.

Scientists have perfected the math needed to produce near-perfect mirrors consistently.

Mirror making is so specialized that the lab has fabricated most of its equipment.

Among the many mirror projects at the lab, the Giant Magellan Telescope takes years to create.

The first of seven 8.4-meter (27-foot) mirrors for the GMT was cast in 2005. GMT1, as it’s known, has been waiting years for its siblings to catch up.


Raw glass from Japan, which will eventually be loaded into a mold, placed into a centrifuge and formed into a telescope mirror.

Raw glass from Japan is loaded into a mold, placed into a centrifuge, and formed into a telescope mirror.
(Photo by Mark Duggan)


When complete, the GMT’s seven mirrors will be shipped to a remote Chilean mountaintop and installed in a honeycomb layout slightly off-axis from each other, creating a single optical surface 24.5 meters (80-feet) in diameter.

The GMT’s camera will take high-resolution images of the night sky, allowing astronomers to see previously unknown amounts of celestial detail.

According to Roger Angel, founder and Scientific Director of the Mirror Lab, today’s telescopes can see only the brightest planets around distant stars.

“With the Giant Magellan Telescope, we should be able to see lots of them,” he said. “That’s completely made possible by the bigger aperture. It’s simply not possible with what we have now.”

In recent years, the lab has built several other large mirrors, including the 8.4-meter Large Synoptic Survey Telescope. With two different slopes of concavity, the unique design has been the lab’s most ambitious single mirror project. The dual curvature of the surface creates both a primary and tertiary mirror on a single piece of glass.

The LSST will be a what’s known as a ‘wide-field survey’ telescope, taking snapshots of large areas of the night sky. Like the GMT, it will be located on a mountaintop in Chile.

A computer-controlled polishing machine works the surface of the Large Synoptic Survey Telescope mirror. A close inspection will show the two levels of concavity in the mirror.

A computer-controlled polishing machine works the surface of the Large Synoptic Survey Telescope mirror, creating two levels of concavity.
(Photo by Mark Duggan)

Jim Burge explained that the final polish is the most exacting process at the Mirror Lab. A computer-controlled polishing tool slowly wears the surface down.

“The key to making an accurate shape is to measure the shape you have and compare that to the ideal shape,” said Burge. “Then run the polishing tool to wear it down until it gets closer and closer to perfect.”

Burge compared it to sanding a table, but with tolerances of only one-millionth of an inch. Anything more would alter the mirror’s shape enough to affect observations.

“For the telescope’s performance, it’s all about the surface,” Burge said. “The light reflects off a thin metal film that’s on top of a polished glass surface. The shape of that film has to be almost mathematically perfect.”

The Mirror Lab has furnished optics for some of the world’s largest telescopes, including the Large Binocular Telescope, Magellan I and II, the MMT Observatory, and the Vatican’s facility in eastern Arizona.

Their giant eyes have brought the distant reaches of the universe a bit closer.

Pin It on Pinterest

Share This