Giant Mirrors Capture the Universe

What does it take to create the perfect 27-foot parabolic mirrors for the world’s largest telescopes?

“Building a telescope is a little like building a cathedral,” says Roger Angel, PhD, University of Arizona professor of astronomy and optical sciences and member of the National Academy of Sciences since 2000. It takes time and patience.

The Steward Observatory Mirror Lab (SOML) lies beneath Arizona Stadium, home of the UA Wildcat football team. While play on the field moves at a fast and furious pace, spin-casting the world’s largest telescope mirrors deep within the lab below is slow and methodical, requiring absolute perfection.

SOML is unique in the world; no other facility on the planet has the combination of engineering expertise – from optical to mechanical to thermal – and the equipment to manufacture mirrors as large and sophisticated as the ones made here.

From the day the raw chunks of glass arrive to the day that glass is shipped out the door as a new, perfect telescope mirror, it takes about four years. Every year, SOML receives a new shipment of glass, and every year a new mirror goes out the door. Thus, at any given time, the lab has four mirrors in production.

Twenty-Seven-Foot Eyes

With its small diameter, the human eye lets in enough light to allow us to see well enough to function in our world, day or night. But with the little light our feeble eyes can gather, the stars in our night sky are mere twinkles.

The mirror of the Hubble Telescope, on the other hand, measures 94.5 inches (just over 7.5 feet) from one edge to the other; that mirror has allowed astronomers to capture some of the most miraculous images of the universe humanity has ever seen.

“If you want higher resolution and more light, you need to put more than one mirror together and make them work together,” explains Angel.

Today, the largest telescope in the world is the Large Binocular Telescope on Mount Graham in Arizona, with two 27-foot mirrors made in the SOML.

“That (the Large Binocular Telescope) has worked so well that now we want to make one with seven 27-foot mirrors working all together,” explains Angel. He is talking about the Giant Magellan Telescope (GMT), which will be more than 80 feet in diameter. The GMT will be completed in about 10 years and be located on a mountain top in Chile.

“When you put seven of them together,” says Angel, “you make images which are 10 times sharper than the Hubble telescope.”

Big Questions, Big Answers

“How did we come to exist? How did life originate?” ponders Peter Strittmatter, PhD, Director of the Steward Observatory. “How did the galaxy in which we live come about? The telescopes we’re building are all designed to address those fundamental questions of the origins of life."

Answering such questions requires open-minded, innovative thinking – the kind that Angel brings to solving problems. “Roger Angel has revolutionized the making of large mirrors for telescopes,” says Strittmatter.

In the case of building giant mirrors, he had to develop processes that involve thermal engineering as well as mechanical engineering; it was more than just an optical engineering problem.

“What makes these mirrors get the best images, you have to control the shape and everything, but you have to control their temperature, they have to be exactly the right temperature to make a good image,” Angel says. “If it gets out of hand, you can stress it or crack it.”

To achieve his vision, Angel had to engineer the entire production system, from start to finish.

“What’s been the most fun,” he says, “is not any one of these things, (e.g., optical or mechanical engineering)…it’s actually thinking about the system – the system of making things, the system of using things. So it’s putting all the pieces together that’s fun.”