Breaking the Mold

As the glass cools on his latest giant mirror, Roger Angel keeps pushing telescope design. His next one might even find Earth-like planets around other stars

Not long after he founded the Mirror Laboratory at the University of Arizona's Steward Observatory in 1984, astronomer Roger Angel threw some Pyrex custard cups into a backyard kiln to get a feel for how borosilicate glass melts. He has been playing with fire ever since. On a sweltering July day in Tucson, his seventh giant mirror takes shape in a 200-ton rotating oven underneath the campus football stadium. After a week of heating, the borosilicate blocks in the oven have reached 1,170 degrees Celsius and begun to liquefy, sending molten glass seeping over hexagonal pillars to form a 21-ton honeycomb that is 8.4 meters (28 feet) in diameter, roughly a meter high, but nowhere thicker than three centimeters.

Usually Angel would be in the control room, nervously watching the hot, red carousel spin five times a minute, just fast enough to pull the solid top of the glass honeycomb into the desired gentle curve. To Angel, after all, this process represents the birth both of a new era of astronomy and of the Giant Magellan Telescope (GMT), which, by banding together seven huge mirrors with sophisticated control mechanisms, would outdo the Hubble Space Telescope. And the first time his team made a mirror of this size, the mold leaked two tons of glass, causing months of delay. This first GMT mirror is trickier still, Angel explains, because "it is wickedly curved into an extreme shape that no one has ever made before at this scale."

But Angel had to leave the control center this morning to zip over to a hotel, where he is trying to persuade some of the high rollers of science funding that the GMT is worth its $400-million price. Wendy Freedman, director of the Carnegie Observatories, which is a GMT partner, says she has full confidence in Angel and his mirror-casting abilities. As a cosmologist, Freedman is keen to use the GMT to solve mysteries about dark energy and the "dark ages" before stars and galaxies formed. But those are not the questions that most excite Angel.

"I got into this because we have been searching for another Earth for the past two millennia, and for the first time it seems possible to find planets like ours around other stars," he says in a British accent tempered by almost four decades in the U.S. "You can now sit down and plan seriously what kind of telescopes you would need to image an Earth-like planet and to take its spectra," to look for the chemical signatures of liquid water and alien life.

Angel did those calculations before he came up with the basic design of the GMT, and he figures that with a bit of luck and some technical wizardry, it may be able to pick evidence of an Earth-like world out of the glare of its star. His characteristically unconventional solution is to arrange the primary collectors like the petals of a daisy, with one symmetric mirror in the center surrounded by six off-axis partners, each distinctly lopsided in shape. The assembly will gather 4.6 times as much starlight as does one of the California Institute of Technology's two 10-meter Keck Telescopes, currently the world's largest.

To snap legible photographs of such planets, GMT will use advanced adaptive optics to make images that are 10 times as sharp as those from Hubble. Each primary mirror will focus its photons onto a separate, 1.1-meter secondary reflector, a thin membrane attached to 672 actuators. Computers will then cancel out much of the atmospheric blurring by making 1,000 subtle adjustments to the shape of each secondary mirror every second. This past summer Angel and his co-workers installed a similar adaptive optics system in the Multiple Mirror Telescope and demonstrated that it boosted the resolving power of the instrument to its theoretical maximum.

"When we started describing our plans for the GMT, lots of people told us it would never work," says Peter A. Stritt┬┐matter, director of Steward Observatory. Angel faced similar skepticism 20 years ago when he and his colleague Nick Woolf first proposed making large mirrors with honeycomb backs, a strategy that cuts the weight by four fifths and enables the mirror to settle to the temperature of the night air within half an hour.

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