The Design of Extremely Large Telescopes

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The Design of Extremely Large Telescopes

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In the late 1970s, astronomers had a problem: the scale of their telescopes no longer matched the size of their ambitions. To see deeper and deeper into our universe's past, they needed a bigger telescope. To build a bigger telescope, they needed a larger mirror, but mirrors larger than five meters had the pesky habit of deforming, producing bad images and frustrating attempts to surpass the resolution of the 5-meter Hale Telescope, first built in 1948.

"Many people thought [the Hale] was the biggest telescope that would ever be built," said Mike Bolte, director of the University of California Observatories.

When the Russians built a 6-meter telescope by the old methods in 1976, it produced awful, distorted images. Scientists across the world realized that a new design for the telescope had to be created.

The scientist who eventually found the key disruptive telescope technology was an unlikely UC-Berkeley physicist, Jerry Nelson (now at UC-Santa Cruz).

"When giving a talk, his manner was so matter-of-fact you would think he was discussing a new offering of municipal sewer bonds, not the world's largest telescope," the Los Angeles Times wrote of him. "Yet he was a persistent and capable scientist with a gift for devising elegant solutions to unexpected problems."

The elegant solution he designed β€” to build 36 smaller mirrors and fuse them together like a honeycomb β€” underpinned the construction of the Keck Observatories' twin 10-meter telescopes on Mauna Kea in Hawaii.

"Everybody thought that was an extremely risky thing. There was a big debate. Nobody trusted that it could be done," Bolte said. "The prototype was the first Keck 10-meter telescope. It's really a breakthrough that seems obvious, but it wasn't."

Telescope design has seen two distinct periods in the last hundredyears. First, astronomers switched away from the lens-usingrefracting telescopes to mirror-based reflecting telescopes. Thisopened the way for the Mount Wilson Observatory's 1.5-meter telescopein 1908, the 2.5-meter Hooker in 1917, and the Hale telescope atPalomer in 1948. The Keck inaugurated the next era of telescopebuilding via segmented mirror or mosaic construction.

In fact, Nelson's construction method and other segmented mirror designshave proven so flexible and scalable that three new telescopes that aremore than twice the size of the world's current record-holder arepreparing to leave the drawing board and enter the construction phase. Now, the Giant Magellan Telescope at 24.5 meters, the Thirty Meter Telescope, and the 42-meter European Extremely Large Telescope are expected to be completed within a decade. In the glacial world of large 'scope building, this is just around the corner.

These telescopes have two goals that could redraw our place within the universe, much as previous discoveries β€” like Edwin Hubble's discovery with a previous world-record holding telescope that the Milky Way was just one galaxy among many β€” redefined the centrality of our own galaxy.

First, the telescopes will bring the study of Earth-sized planetsaround local stars within human reach. We will be able to determine howrare Earths are, and by extension, how likely Earth-like life is toexist elsewhere in the galaxy. Second, by gathering more light thanever before, astronomers will be able to detect fainter objects thatare further back in our universe's history. They hope that the newtelescopes will see "first light," when the first stars formed out ofthe primordial universe's post-Big Bang mass.

Working in tandem with the segmented mirror design is a complementary technologyborrowed from the military called adaptive optics. Adaptive optics allows astronomers to account and correct fordisturbances in the Earth's atmosphere. It's a complex task thatrequires measuring the air within the telescope's view for temperaturevariations and adjusting the Keck's mirrors to compensate 2000 times asecond with tiny controllable magnets.

"Behind each magnet, there are a series of acoustic voice coils thatcan refresh the amount that that magnet is pulling at a rate of500-1000 Hz. At that frequency, we can correct for turbulence in the Earth's atmosphere that occurs," said Peter Wehinger, staff astronomerat the Stewart Observatory Mirror Lab, which is building mirrors forthe Giant Magellan Telescope.

Driven by faster and faster computers, the technology hasallowed ground-based telescopes to rival and in some cases surpass theHubble Space Telescope, and its successor the James WebbTelescope.

"All this really depends on things like Moore's Law, fast computers,technology to change shape of mirrors. We're like slaves totechnologies," said Taft Armandroff, director of the Keck Observatory."With the Keck we can get images with a better spatial resolution thanwe can get with Hubble Space Telescope."

With the technological challenges largely out of the way, what the newtelescope projects need now is money. And when it comes to funding theworld's largest telescopes, astronomers have often turned to philanthropists.

George Ellery Hale was a master of funneling astronomical amounts ofprivate money β€” particularly from Andrew Carnegie β€” into astronomyprojects. His fundraising skill was responsible for the Mount Wilsonand Hooker telescopes, both world record holders. While the Keck'sfunding history is convoluted, it eventually was built almost entirelywith a single family's money.

"All the way back to 400 years ago, there was some patron that paid forGalileo's telescope and that started an incredible trend that forefronttelescopes have almost always been privately funded," said UC's Bolte.

The Thirty Meter Telescope appears to have found that donor, in Intel'sGordon Moore. The Gordon and Betty Moore Foundation pledged $200 million for theconstruction of the telescope late last year.

The Giant Magellan Telescope is still waiting for its Keck or Carnegieor Moore, and they could struggle with the current economic climate.

Still, the odds are that at least one of these telescopes will be builtby 2018 and we could soon be exploring reaches of the universe β€” andtime β€” that no human has ever been seen before.

"The science questions are more compelling than ever. There's so muchto explore. It's really exciting to get these new tools," Armandroffsaid.

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(Via Wired)
 
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