‘Magic Mirrors’ Will Power The Giant Magellan Telescope

Everyone says it’s important to have a clear, concise mission statement.

What Project Manager James Fanson, who leads the team building the Giant Magellan Telescope said about their work, may constitute the best (unofficial) one ever: “We’re on a journey to build the world’s most powerful telescope.”

That’s about as clear and concise as it gets.

Clarity like that can only help when your end goal is an instrument ten times as powerful as the 32-year-old Hubble Space Telescope and four times as powerful as the recently commissioned James Webb Space Telescope. Pure science as the driving force helps too. “It’s a fact that the things we’re most interested in observing are very far away and very faint,” Fanson explained. “The bigger an instrument you can build, the clearer the image will be. You want to build the biggest telescope your technology and budget will support.”

The sheer size of this endeavor requires lots of backing. The Giant Magellan Telescope’s founders include six major American universities along with the Carnegie Institution for Science and the Smithsonian Institution, plus five international partners in Australia, Brazil, Israel and South Korea. Today, the international consortium announced a new investment of $205 million to fund the construction of the telescope’s 12-story structure at Ingersoll Machine Tools in Illinois, to continue work on the telescope’s mirrors at the University of Arizona’s Richard F. Carris Mirror Lab, and to construct scientific spectrograph instruments in Texas.

That’s critical funding for certain, but the scientific depth of the founding consortium is also critical for the telescope’s technological hurdles. “Advances in telescopes are really driven by technology,” Fanson said. “Making eight- to ten-meter scopes took the advance of computerization.” But the giant in the name of this project refers to the much-larger effective 25.4-meter diameter of the main mirrors, so Fanson’s team is breaking new ground in technology. (The Giant Magellan Telescope is a reflecting telescope–a Gregorian optical-infrared telescope, to be exact, consisting of a parabolic main light-collecting mirror and a secondary mirror that reflects the image into the lens system.) “Our design for the telescope is unique. The primary mirror has seven segments, each 8.4 meters in diameter. Each segment is the world’s largest mirror! Manufacturing them was the first technological challenge. Six of the segments are the same, but off-axis–they take on a potato-chip shape. Those mirrors have to be polished to within one-millionth of an inch. If their surfaces were the surface of the earth, their tallest mountain would be two inches.”

On the one hand, telescope construction is simpler for this one than for Hubble and Webb, because the Giant Magellan Telescope will be ground-based. “We’re building at the Carnegie Institution for Science’s Las Campanas Observatory in the very southern part of the Atacama Desert in Chile,” said Fanson. “You want an area that’s dry, since moisture in the air absorbs light. You need consistently good weather. And you want an area where the air is smooth, since you get distortion from air currents. Our location on the edge of the ocean means we have smooth ocean currents.” On the other hand, smooth air is not distortion-free like the vacuum of space, and that’s where technology plays an even greater part–specifically, in the telescope’s secondary mirrors, where computerization will help address the air distortion. “The wavefront of the light becomes wrinkled by the atmosphere,” he continued. “We intend to defeat the effect of the atmosphere with adaptive optics. Moment to moment, the image changes. We correct for that by using a deforming mirror to achieve a diffraction-limited image. We have seven segments for our 1.1-meter secondary mirror. Each is a deformable mirror. We’re building the 4Th generation of this type of mirror, one that’s faster than any previous one. We’re in the process of building the first segment.”

The glass of the secondary mirrors is a mere two millimeters thick, and the back surface of each segment has 675 magnets bonded to the glass. Each magnet has a corresponding computer-driven electromagnetic actuator that deforms the mirror’s surface 2,000 times per second to clean up the distorted light reflections they receive from the primary mirrors prior to sending them to the telescope’s scientific instruments. Each of the secondary mirror’s seven segments receives, corrects, and transmits the light reflected from one of the seven segments of the primary mirror.

“The telescope’s spectrum range is from 320 to 25,000 nanometers,” said Fanson. That represents a wavelength range from optical to infrared. “We’ve also optimized the optical design, with the Gregorian offering several advantages, including a compact plate scale arc, a wide field of view of 20 arc-minutes, and the ability to carry multiple instruments at one time.”

Currently, two segments of the telescope’s primary mirrors are complete. The third is receiving the final touches on its polishing and is in final testing. Three others are in various stages of fabrication. “Each one takes about four years to complete,” Fanson said. “The telescope’s expected life is 50 years, and we can make improvements to it over its lifespan.” Work is underway in France and Italy on the first secondary mirror segment. Construction of a 40,000 square foot facility at Ingersoll Machine Tools in Rockford, Illinois to build the telescope structure is complete, as is the earthwork in Chile for the pouring of the telescope’s foundation.

Fanson eagerly anticipates the new capabilities his project will deliver. “With this next generation of scopes, we’re going to be able to do some things that have never been done,” he explained. “We ask ourselves, ‘What is our universe? What is our place in it? Are we alone?’ We’ve discovered other planets that we’d like to be able to explore, to find out what they’re like and whether they have any sign of life. The nearest ones with the potential for water are just beyond our ability to detect their reflected light. They’re just on the edge of our ability to explore, and the offer profound opportunities for discovery. These new telescopes are going to allow us to push back the frontier of our knowledge in every dimension–not just planets, but black holes, dark energy and dark matter, and galaxy formation. In my career working on telescopes, every time there’s a new one, we discover things we couldn’t even have foreseen.”


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