This month, a dome will be slipped over the first of 10 1.4-meter telescopes in an optical interferometer.


Telescope array will spy on spy satellites, star surfaces, and black holes

At a time when astronomers are building billion-dollar telescopes with mirrors 30 meters across, the 1.4-meter instrument being installed this month atop South Baldy Mountain in New Mexico may seem like a bit player. But over the next few years, nine more identical telescopes will join it on the grassy, 3200-meter summit, forming a Y-shaped array that will surpass any other optical telescope in its eye for detail. When it's complete around 2025, the $200 million Magdalena Ridge Observatory Interferometer (MROI) will have the equivalent resolution of a gigantic telescope 347 meters across.

MROI's small telescopes can't match the light-gathering power of its giant cousins, so it will be limited to bright targets. But by combining light from the spread-out telescopes, it is expected to make out small structures on stellar surfaces, image dust around newborn stars, and peer at supermassive black holes at the center of some galaxies. It will even be able to make out details as small as a centimeter across on satellites in geosynchronous orbit, 36,000 kilometers above Earth, enabling it to spy on spy satellites.

That's one reason why the U.S. Air Force, which wants to monitor its own orbital assets and presumably those of others, is funding MROI. "They want to know: Did the boom break or did some part of the photovoltaic panels collapse?" says Michelle Creech-Eakman, an astronomer at the New Mexico Institute of Mining and Technology in Socorro and project scientist on MROI. But if the facility succeeds, its biggest impact could be on the field of astronomy, by drawing new attention to the promise of optical interferometry, a powerful but challenging strategy for extracting exquisitely sharp images from relatively small, cheap telescopes.

MROI, like optical interferometry itself, has made slow progress. The U.S. Navy began funding the facility in 2000, but lost interest and pulled out in 2011. Air Force support is not assured beyond the array's first three telescopes. "If for some horrible reason they failed, it would be a disaster for us all," says Theo ten Brummelaar, associate director of the Center for High Angular Resolution Astronomy (CHARA), an array of six 1-meter optical telescopes on Mount Wilson in Los Angeles, California.

Radio astronomers have had it easier. The long radio wavelengths mean data from separated dishes can be recorded, digitized, time-stamped by an atomic clock, and combined later for analysis. But optical interferometry is far trickier: The short wavelengths of visible light, running at terahertz frequencies, cannot yet be digitized by any electrical system. So the light must be merged in real time, with nanometer precision.

Boiling cells of plasma were seen on a distant red giant star using an optical interferometer in Chile.


In the 1990s, advances in fiber optics, lasers, and computers developed to the point where the Keck Observatory's twin 10-meter telescopes, separated by 85 meters atop Mauna Kea in Hawaii, could be made to work as an optical interferometer. But the system needed at least four additional NASA-funded "outrigger" telescopes to reach its full potential—and the outriggers were canceled in 2006 after protests from native Hawaiians, who consider the summit of Mauna Kea sacred. "Interferometry is still something of a dirty word around NASA," says Gerard van Belle, chief scientist at the Navy Precision Optical Interferometer (NPOI) near Flagstaff, Arizona.

Many astronomers have overlooked optical interferometry's recent achievements, says Tabetha Boyajian, an astronomer at Louisiana State University in Baton Rouge, who used CHARA to perform a survey of stellar sizes. She says astronomers are sometimes surprised to learn about the technique's capabilities. "You hear, ‘Oh wow, how can I use that for my science?’” she says.

CHARA and other optical arrays have imaged the squashed shapes of rapidly rotating stars, captured roving sunspots on stellar surfaces, filmed binary companion stars swapping material, and witnessed objects whipping around the Milky Way's central black hole in real time. After linking up the four 8.2-meter telescopes of the Very Large Telescope in the Atacama Desert of Chile, researchers with the European Southern Observatory last year were able to discern boiling convective cells on the face of a star 530 lightyears away. A new infrared instrument should enable that interferometer to image the warm dusty disks where planets are forming around other stars.

And despite its previous setbacks, NASA is supporting interferometry on the twin 8.4-meter instruments at the Large Binocular Telescope Observatory atop Mount Graham in Arizona. This year, scientists there announced they had used an interferometric technique to show that many young solar systems contain less dust than expected—good news for astronomers hoping to image exoplanets directly.

Once complete, MROI's telescopes will be more widely separated than any other interferometer, enabling its superior resolution. It will also test new technologies for combining starlight from multiple telescopes that could simplify the process. If it lives up to expectations, the observatory could give optical interferometry a boost in future decadal reviews, community exercises that set budget priorities for NASA and the National Science Foundation. By the time of the 2030 decadal survey, John Monnier, a physicist at the University of Michigan in Ann Arbor and a CHARA member, wants solid support for the Planet Formation Imager, a 12-telescope, 1-kilometer-baseline optical interferometer that would be able to resolve dusty disks not just around young stars but newborn planets—the grist for moons and rings.

By then, NASA could be ready to put an optical interferometer in space, too. In 2007, the agency abandoned plans for the Terrestrial Planet Finder, an orbiting array of four telescopes designed to image planets around other stars. But it may now be more receptive to space interferometers, as giant space telescopes like the 6.5-meter James Webb Space Telescope, due to launch in 2021, strain the limits of rocket cargo loads.

"At some point [interferometry is] the only way we can address questions that are burning on the frontiers of astronomy," Van Belle says.