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Every 10 years, about two dozen of the country's top astronomers and astrophysicists get together and make a wish list. On it are the new telescopes they'd most like to see built. At the most recent gathering, here's what topped the list: The Large Synoptic Survey Telescope. And for the many of us who don't know what that is, NPR's Joe Palca explains.

JOE PALCA, BYLINE: I'll assume you know large and telescope, so let's focus on the other two words in the name. For astronomers, a synoptic survey is a comprehensive map of every square inch of the night sky.

DR. SIDNEY WOLFF: We want to scan the entire sky over and over again for 10 years, and we will get over 800 images of every single patch of the sky.

PALCA: Sidney Wolff is in charge of building the new telescope. And why take 800 pictures of the sky over 10 years? Well, it's like taking a time-lapse picture of the sky. Anything that moves or changes will be easy to spot.

WOLFF: So one of the things we can do is that if there are any potentially hazardous asteroids out there that might impact the Earth and do significant damage, we will find them.

PALCA: Wolff says the new telescope will also easily pick up the flash of light from stars that explode overnight and become supernovas. It can also study things that are invisible. Seriously. Here's how that works. There's stuff between us and distant objects called dark matter. As the name suggests, dark matter doesn't give off light. Astronomers are pretty sure it's there; they just don't know what it's made of. To study dark matter, Wolff says the new telescope can look for changes in the shape of distant objects, changes caused by the invisible stuff.

WOLFF: It doesn't matter whether that matter emits light or not because we're measuring its gravitational effect, and dark matter produces gravitational effect.

PALCA: The telescope will ultimately be built on a mountain in Chile. At the heart of the telescope are three large mirrors that will capture starlight. Two of the mirrors are being built at a facility on the University of Arizona campus in Tucson.

WILLIAM GRESSLER: Even from this far away, you see the two surfaces. That's our tertiary mirror.

PALCA: William Gressler is a senior optical engineer for the Large Synoptic Survey Telescope or LSST as it's known. We're standing on a balcony overlooking a cavernous hall located under the university football stadium. Gressler is pointing to what looks like a giant round birthday cake at the other end of the hall. The cake is actually a single piece of glass 27 feet across. You can see what it looks like at Gressler says making mirrors for a survey telescope poses some special problems. Once the glass is mounted in the telescope frame, it will need to be swiveled fairly rapidly across the sky.

GRESSLER: Part of this survey is we can't take forever moving to the next part of the sky. So we actually move very quickly. Within five seconds, we can move from one piece of sky 3 1/2 degrees over and get ready for another exposure.

PALCA: Gressler says they've achieved that agility with some innovative mounting hardware that will take the stress off the mirror as it moves. Martha Haynes of Cornell University co-chaired the committee of astronomers that made the LSST top choice for the next big telescope. She says it beat the competition because nearly every astronomer needs a detailed map of the entire sky.

DR. MARTHA HAYNES: The surveys that are being proposed for the LSST will provide the data sets that a vast variety of astronomers will use to answer fundamentally different questions about the universe.

PALCA: Right now, all that's holding up the LSST is money. The mirrors were built with private donations. Federal science agencies have pledged support, but money isn't expected to start flowing until the 2014 fiscal year. Joe Palca, NPR News.

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