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Inside The Design Of The Largest Telescope On The Planet

The Giant Magellan Telescope will be enormous, but its design was inspired by something small: the lotus flower.

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The arid climate of Chile's Atacama Desert is already one of the best places in the world to stargaze. But when the Giant Magellan Telescope opens there in the Las Campanas Observatory in 2025, it will allow humans to see the universe more clearly than ever.

The $1 billion telescope is the first of several next generation telescopes expected to come online; the GMT will be about 10 times more powerful than Hubble, allowing humans to see into space with unprecedented detail. The secret to the GMT's amazing fidelity? Seven mirrors, each around 28 feet in diameter, arranged in an 80-foot lotus pattern.

How do you design a telescope meant to peer back to the earliest moments of the universe? "When you design a telescope, you need to make engineering choices based on the questions you want to answer," says Patrick McCarthy, president of the Giant Magellan Telescope Organization. In the GMT's case, McCarthy said they wanted to make "the biggest light bucket we could build" so that they could explore extra-solar planets, and the earliest days of the universe. And that meant taking tips from Mother Nature.

Designing A Time Machine

First, let's talk about the history of telescope design. The earliest telescopes were refracting telescopes. Created by European lens makers in the early 17th century, refracting telescopes worked by lining up two lenses so that an object viewed through them appeared magnified. By the late 19th century, though, telescope makers had pretty much reached the limits of what could be accomplished with refracting telescopes: Past about 40 inches in diameter, glass lenses tended to fracture and crack under their own weight.

At this point, reflecting telescopes—which use mirrors instead of lenses—became popular. Reflecting telescopes were originally invented around the same time as refracting telescopes, when the only way to reflect light was to polish metal until it was shiny. But metal quickly tarnishes, which made reflecting telescopes impractical for centuries. "Astronomers would literally spend all day polishing their mirrors, and all night observing," says McCarthy. It was exhausting.

At the end of the 19th century, astronomers finally perfected the process of making mirrors out of glass. These mirrors were as shiny as metal, but didn't tarnish. Soon, reflecting telescopes broke through the 40-inch barrier, and grew to 60-, 100-, and even the 200-inch Hale Telescope. But by 1950, reflecting telescopes had seemingly topped out, too. You just couldn't reliably make mirrors larger than that without cracking. After three grueling attempts, the Soviet Union managed to build a 240-inch mirror for its BTA-6 telescope in 1975, but for all practical purposes, 200 inches was the limit for decades.

In 1993, astronomers figured out a way to get past the 200-inch limit. Built in Hawaii, the W.M. Keck Observatory featured a 33-foot primary mirror, which they created by using 36 smaller, hexagonal mirrors with identical optical qualities to work together as a single larger unit.

Since then, at least four other multi-mirror telescopes have been built around the world, and all of the big next generation telescopes, such as the European Extremely Large Telescope (or E-ELT), use segmented primary mirrors to provide images of the far universe with unprecedented clarity.

A Blueprint Found In Nature

What sets the Giant Magellan Telescope apart from other next gen telescopes primarily has to do with the way its mirror segments have been designed. While a telescope like the E-ELT will eventually be made up of 798 hexagonal segments, the Giant Magellan Telescope is made up of seven round mirrors, arranged in a lotus pattern and polished into one optical surface. They're just a lot bigger.

When so many other new telescopes are using so many more segments to make up their primary mirrors, why is the GMTO opting to use fewer? According to McCarthy, it all has to do with minimizing points of failure. Between polishing them, stringing them together, and controlling them, the more mirror segments your telescope has, the more things there are that can go wrong. Only having seven larger segments keeps things comparatively simple. In fact, thanks to material science and manufacturing advances, the Giant Magellan Telescope's mirrors will each be more than 330 inches in diameter.

But the size of the mirrors is not the only design consideration for a telescope like the GMT. There was also the question of the mirrors' shapes, and how they should be arranged. These factors affect in the way the final image in a telescope appears, says McCarthy. For example, the cross-shaped light pattern we often associate with stars has nothing to do with the way they actually appear, outside of a reflecting telescope. Instead, the cross-pattern is created by the struts that hold a telescope's secondary mirror behind its primary mirror. Likewise, square mirrors in a telescope will result in more cubist final images, says McCarthy, while triangular mirrors will make stars look more triangular.

But the universe is mostly spherical. That's why the GMT uses round mirrors, and it's also why the segments have been aligned in a lotus pattern. "In optics, the lotus pattern is just the most natural and efficient way to combine circular apertures," he says. According to McCarthy, the goal of a telescope is to create the most efficient light bucket, and that's something natural selection did with the lotus flower long ago. "There's a very good reason they have the shape that they have."

When the GMT finishes completion in 2025, McCarthy and his colleagues hope that the telescope will be used to tell us more about how the universe formed after the Big Bang, and possibly detect life on other planets. But even he's not really sure what they're going to see when they turn it on a decade from now. "We build new telescopes to open up a discovery space, and to learn about things we don't even know of yet," he says.

"It's like what George Ellory Hale, the founder of the Mount Wilson Observatory, said to financiers when they asked him what he expected to discover with his telescope. 'If I knew that, I wouldn't have to build it.'"

All Photos: via Giant Magellan Telescope

Slideshow Credits: 08 / Ray Bertram/University of Arizona; 09 / Ray Bertram/University of Arizona; 10 / Ray Bertram/University of Arizona; 11 / Ray Bertram/University of Arizona; 12 / Ray Bertram/University of Arizona;

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