Cosmic time machine
The Giant Magellan Telescope will help astronomers demystify dark matter, probe black holes in distant galaxies and study planets around other stars
Feb. 21, 2011
It’s been more than eight decades since Paris, Texas banker William J. McDonald left most of his estate to the unsuspecting University of Texas at Austin. McDonald wanted his money used to establish an astronomical observatory. The surprised university had no astronomy department or faculty at the time.
After settling a legal struggle with McDonald’s disgruntled family heirs, the university forged a unique partnership with an established astronomical power to the north — the University of Chicago. The two universities built McDonald Observatory and never looked back. Decades later, the observatory became the sole property of the University of Texas. Since the late 1960s, through the observatory and its newly formed astronomy faculty in Austin, the university has developed one of the top 10 astronomy programs in the country. It is also one of the largest.
“Our goal is to be number one,” said McDonald Observatory Director David Lambert. “In order to maintain a frontier program in observational astronomy, we need to be a significant partner in what will be one of the world’s largest telescopes.”
To that end, the university is now engaged in another great astronomical partnership. This one is much larger: The university is part of a group of institutions working together to build the Giant Magellan Telescope (GMT).
GMT will be larger than any telescope in existence today. When it’s completed in about 2019 it will take advantage of seven large mirrors at a prime observing site in Chile to push the boundaries of astronomy.
The university joined the GMT consortium in 2004. Its participation to date has been funded by various offices within the university and a $1 million gift from George P. Mitchell of Houston.
The Carnegie Institution for Science was the project’s early leader. The group is now incorporated as the GMT Observatory. In addition to the university and Carnegie, American partners include Texas A&M University, Harvard University, the Smithsonian Astrophysical Observatory, the University of Arizona, and the University of Chicago. On the international front, the GMT has partners in the Australian National University, Astronomy Australia Ltd., and the Korea Astronomy and Space Science Institute.
The project’s total cost is estimated at $700 million. The university plans to have a 10 percent share, and is working to raise the $70 million needed.
GMT director Patrick McCarthy said the university brings valuable skills to the project.
“UT is one of the unique, or nearly unique [astronomy] departments left in the United States in which you have an outstandingly strong scientific group — they do theory, they do observation — and a group that still knows how to build scientific instruments, how to build and operate telescopes,” he said. “This is something that’s becoming increasingly rare at universities just because the cost is difficult, the long-term commitment from the funding source is hard to maintain.
“UT has the classic, old-school experimental astronomers that they build their own instruments,” he continued. “They go out and they do their own observations, and they and their postdocs and graduate students write great scientific papers. That’s the kind of tradition that we see as vital to a successful project like this.”
Discoveries in the last several decades of astronomy have pushed the envelope of astronomy to where larger telescopes are needed.
“The fundamental coin of the realm is the number of photons you collect, how much light you can manage to gather together, because more light lets you see fainter things,” said Astronomy Professor Dan Jaffe. “And eventually you reach a point where the science questions are driving you to need to look at fainter objects or to look at bright objects in much more detail. And those two things together mean you need a bigger telescope.”
Though GMT will be a ‘workhorse telescope,’ capable of studying all kinds of cosmic questions, in particular it should make contributions to some of the top astronomical mysteries today like the formation of planetary systems, enigmatic black holes and cosmology, the study of the universe on the largest scales — its birth, evolution and fate. How will GMT accomplish all of this?
GMT’s seven truly gigantic mirrors will work as one to probe the cosmos. Each will be 8.4 meters (27.5 feet) wide. Together, the seven will be as powerful as a single mirror 24.5 meters (80 feet) in diameter.
Why so big? Mirror size is crucial. GMT’s great resolving power will make images up to 10 times sharper than those from the Hubble Space Telescope. In fact, GMT will be able to take images of planets around other stars.
In addition to their resolving power, larger mirrors gather more light from distant stars and galaxies, seeing fainter objects at greater distances.
A large telescope mirror is a time machine. The more distant objects in space are, the farther back in time scientists are seeing them. That’s because it takes light time to travel from the star or galaxy to the telescope. Looking in the sky, you don’t see the sun as it is right now — you see it as it was eight minutes ago, when the photon of light departed the sun’s surface and began traveling to your eye at its fixed speed of 300,000 kilometers per second (186,000 miles/sec.). The distance to the sun is 8 light-minutes. The farthest galaxies seen with large telescopes are billions of light-years away.
Technology has always limited our exploration of the cosmos. Giant mirrors can only be made so large before they start to sag under their own gravity. That causes the precisely figured shape of the mirror’s surface to deform, making it unable to precisely direct the light from target objects into its instruments. Mirror sagging makes telescope images blurry.
In recent decades, engineers have worked to overcome the problem of making larger telescope mirrors. One way GMT’s design overcomes this problem is by using seven individual mirrors that work together. The mirrors are not made of solid glass, but have an ingenious honeycomb structure to their interior, making them weigh only 20 percent as much as a solid glass mirror.
The GMT mirrors are being made by the University of Arizona’s Steward Observatory Mirror Lab. Mirror construction is a long process. Work began on the first GMT mirror in 2006 and is just now being completed, GMT director McCarthy said. “It’s been a process that’s taken a little longer than we expected, but that’s why we started early. We’re sufficiently confident that we’re going to start manufacturing the second mirror this year.”
Though mirrors are extremely important, a telescope would be nothing without its instruments. A telescope gathers and focuses light so that it can be fed into an instrument that will do specific things to it, enabling specific kinds of studies. GMT will have a suite of two to four instruments; the final number has yet to be determined. Several groups are in competition to create these, and the field has been narrowed to six proposals.
“The instrument suite will have to allow a broad range of science to be done, make the telescope competitive, be able to function in the full range of weather conditions where observing is possible — for example, moonlight, no moonlight, cirrus clouds, no cirrus clouds, good seeing, bad seeing, and so on — and it will have to fit into the [instrument budget of] $80 million dollars,” Jaffe said.
Jaffe leads a team of instrument scientists whose work has been used on NASA’s SOFIA airborne observatory, the forthcoming James Webb Space Telescope (widely referred to as the successor to Hubble Space Telescope), and instruments at McDonald Observatory.
His proposal for a GMT instrument was one of the six selected for further study, and funded by the GMT consortium for half a million dollars. It is called GMTNIRS (GMT Near Infrared Spectrograph).
“It’s going to be very useful for anything that involves studying stars, because it will open up sensitive, high-resolution infrared observations of stars of any kind,” Jaffe said. “So, we can use it to study the variations of the abundance of the [chemical] elements across the galaxy, you can study the motions of stars very close to the black hole in the galactic center, we can study radial velocities of stars that might have planets around them, we can study protostars — very young stars. And then, we’ll be able to look at protoplanetary disks.”
The university is also involved in a second instrument study. Led by Darren DePoy of Texas A&M, the GMACS team includes the university’s Karl Gebhardt and Gary Hill.
The studies are due to be completed in June, Jaffe said. Then a panel will review them and make recommendations to the GMT board of directors to make the final choices for manufacture.
In addition to the telescope and its instrument suite, the observing site will be a contributing factor to GMT’s success. The giant eye on the sky will be built in the mountains of Chile, host to some of the best astronomical observing conditions in the world. European and American observatories have operated there for decades.
Turbulence in Earth’s atmosphere is an enemy of clear telescope images. But stable temperatures and smooth airflow from the Pacific Ocean make for low air turbulence at the GMT site. And because of low population, the site has almost none of the light pollution common in cities that makes faint cosmic targets hard to see.
The southern-hemisphere site also provides access to some amazing cosmic targets not visible from the northern hemisphere, including the center of the Milky Way galaxy and its satellite galaxies the Large and Small Magellanic Clouds.
But the telescope won’t be completely remote. It will be built at the Carnegie Institution’s Las Campanas Observatory. Founded in 1969, the observatory hosts all of the infrastructure GMT will need in this remote mountain location, including roads, water and electricity.
It will be many years before GMT is complete, but the university’s partnership will ensure the strength of its astronomy program far into the future. It “provides us with the opportunity to probe the frontiers of observational astronomy as they will exist in 2020,” McDonald Observatory Director Lambert said, “80 years after McDonald Observatory first opened its dome to study the stars.”
For more information, contact: Rebecca Johnson, McDonald Observatory, College of Natural Sciences, 512 475 6763;
Banner photo on the home page: Helix Nebula: After blowing off all of its gaseous atmosphere,
into space making a colorful shell, all that remains of the once-sunlike star at the center of
the Helix Nebula is a burnt-out cinder about the size of Earth. Credit: NASA, ESA,
C.R. O'Dell (Vanderbilt University), M. Meixner and P. McCullough (STScI)
Video animation credit: GMT Corporation