The Design of Extremely Large Telescopes
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 — to build 36 smaller mirrors and fuse them together like a honeycomb — 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 hundred years. First, astronomers switched away from the lens-using refracting telescopes to mirror-based reflecting telescopes. This opened the way for the Mount Wilson Observatory's 1.5-meter telescope in 1908, the 2.5-meter Hooker in 1917, and the Hale telescope at Palomer in 1948. The Keck inaugurated the next era of telescope building via segmented mirror or mosaic construction.
In fact, Nelson's construction method and other segmented mirror designs have proven so flexible and scalable that three new telescopes that are more than twice the size of the world's current record-holder are preparing 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 — like Edwin Hubble's discovery with a previous world-record holding telescope that the Milky Way was just one galaxy among many — redefined the centrality of our own galaxy.
First, the telescopes will bring the study of Earth-sized planets around local stars within human reach. We will be able to determine how rare Earths are, and by extension, how likely Earth-like life is to exist elsewhere in the galaxy. Second, by gathering more light than ever before, astronomers will be able to detect fainter objects that are further back in our universe's history. They hope that the new telescopes will see "first light," when the first stars formed out of the primordial universe's post-Big Bang mass.
