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Astronomers are planning to build a giant new telescope on a peak in the Andes Mountains in Chile. At the heart of the telescope are seven giant mirrors, each of them 27-feet across. The 21-ton block of glass that will become one of those mirrors is slowly cooling down after being cast at the University of Arizona in Tucson.

Earlier this month, NPR's Joe Palca visited Tucson, and has this report on what it takes to make an astoundingly large, astoundingly precise piece of optical equipment.

JOE PALCA, BYLINE: There's something delightfully incongruous about the location of the Steward Observatory Mirror Lab. It's under the stands at the University of Arizona football stadium.

Why there? Why not?

DR. ROGER ANGEL: When we wanted some space, and it was just used for parking some cars so this seemed like a good use.

(SOUNDBITE OF A FURNACE)

PALCA: Roger Angel is the master of making big mirrors for telescopes. For 30 years, he's been using a method called spin casting to make the largest solid telescope mirrors in the world.

As we talk, we're standing next to what looks like a dusky red enclosed merry-go-round. This is the furnace in which the mirror is being cast. In the old days, you made mirrors by ladling molten glass into a mold.

ANGEL: And in this case, we just put these chunks of solid glass. Lay them over the mold while they're cold.

PALCA: When they're ready to cast the mirror, the lid is put on the furnace with the mold and glass chunks inside. Then they slowly raise the temperature to 2,100 degrees Fahrenheit. At that temperature, the glass chunks melt turning into a clear, syrupy liquid that oozes into the mold. At this stage, the whole furnace is spinning at about six revolutions per minute.

This encourages the glass to flow into the shallow parabolic shape it will eventually become. The glass now in the oven won't come out for two more months. They cool it down slowly so there won't be any flaws.

Roger Angel has made several these 27-foot mirrors.

ANGEL: But the shape of this mirror is more challenging by about a factor of 10 than the previous ones that we've made.

PALCA: You see, casting the mirror isn't even the hard part. It's the grinding and polishing that's needed to obtain the final shape. And what makes things so hard is the final shape will be aspherical. Instead of being a shallow symmetrical bowl, one side of the mirror is higher than the other. It's a shape dictated by where the mirror will focus starlight once it's set in the telescope.

Not only is devilishly hard to grind and then polish a 27-foot aspherical mirror, without breaking it, it's hard to know when you've done it right. The differences in height across the surface are smaller than a millionth of an inch.

To make sure they've got the optics just right, they've built a special test rig with lasers and special test patterns.

(SOUNDBITE OF CONVERSATIONS)

ANGEL: Then there are more optics with aspheres and holograms, and a lot of stuff to compensate for the enormous asphericity of the thing that we're trying to test.

PALCA: I just want to say that I will personally try to ensure that asphericity enters the common usage of the English language.

(SOUNDBITE OF LAUGHTER)

PALCA: It's such a great word.

The giant mirrors will give astronomers two things they really want in a telescope: high sensitivity, so they can see really, really dim objects; and high resolution so they can see fine details. That's what Roger Angels mirrors will provide for the new telescope called the Giant Magellan Telescope, or GMT.

Wendy Freedman is an astronomer at the Carnegie Institution for Science and chair of the GMT board of directors. She says to get a sense of GMT's resolving power, imagine you're looking at the face of a dime.

WENDY FREEDMAN: And you were to take that dime and put it 200 miles away. Then with GMT, you could resolve the face of that dime. It's quite spectacular.

PALCA: Freedman says the resolution of the new telescope should let astronomers see planets around other stars. And its sensitivity should let them see some of the earliest objects to form in the universe. Freedman says astronomers can only imagine what they'll learn when GMT starts operating.

FREEDMAN: The opportunity for new discovery in astronomy generally follows when we make a big leap in resolution or sensitivity like this

PALCA: But those discoveries are a ways off. It will be a while before the giant mirrors are shipped to Chile and assembled into a telescope. Under the rosiest scenario, the telescope won't achieve first light, as it is known, until 2020. Still, Freedman and Roger Angel are convinced it'll be worth the wait.

Joe Palca, NPR News.

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