As twilight envelops Mount Wilson, a 5,700-foot peak near Los Angeles, Harold McAlister begins his night of stargazing by retracing the footsteps of the late astronomer Edwin Hubble. Night after night during the 1920s, Hubble headed up this same tree-lined path to scan the heavens through the 100-inch Hooker telescope—the most powerful in the world. What he saw was a bizarre universe extending far beyond the Milky Way, composed of multiple galaxies flying away from one another at breakneck speed. That discovery eventually led to the extraordinary theory about the origin of everything, called the Big Bang. Now, some 80 years later, McAlister pauses along the footpath to gaze with reverence at the huge white dome protecting the famous old telescope. "That 100-inch instrument is more important than the space telescope they named after Hubble," he says. "It's the most important telescope of the 20th century." Then the Georgia State University professor puts his head down and moves on. The stars are crisp above the mountain tonight—a good opportunity for him to stare at them with an entirely new kind of machine for scanning the universe. Passing behind the old observatory, he enters a long corrugated-steel building marked Beam Combining Lab and arrives at the nerve center of an optical interferometer, a revolutionary device scattered across the mountaintop and composed of six conventional telescopes, 3,100 feet of light pipes, and 20 computers. It promises to transform Mount Wilson's reputation from that of keeper of a famous old telescope to the new center of cutting-edge astronomy. This is the largest of a half-dozen interferometers under construction around the world. It is called the CHARA (Center for High Angular Resolution Astronomy) Array, and its ability to see into space with incredible detail—50 times finer than any single-mirror telescope ever built—promises to bring the night sky into incredibly sharp focus. For example, CHARA could zoom in on an illuminated object on the moon as small as a man. "If that man were driving a car," McAlister says, "we could distinguish one headlight from another." More important, CHARA can distinguish one star from another. That may seem odd, but most stars viewed through even the largest and newest conventional telescopes look much as they do to the naked eye—tiny dots of light, dimensionless and deceptive. Spectrographic analysis reveals that most of those pinpoints are likely to be two stars—binaries—or even more stars: Castor, in the Gemini constellation, for example, looks like a single star but is actually six balls of fire dancing around one another. Solo performers like our sun are the exception, not the rule. Soon interferometers will help astronomers figure out why stars tend to flock together and how they behave as they age. Eventually, those lessons will come back home, telling us what our sun was like in the past and exposing threats we can expect from it—giant flares, perhaps, or periods of dimming that could trigger an ice age. Interferometers will open up the heavens anew: "We'll make thousands of stellar measurements that have never been done before," McAlister says. Interferometry is also likely to be a boon to planet hunters. If CHARA can detect individual planets around binary stars, as expected, the census of extra-solar planets will grow immensely. The more planets found, the more likely the prospects of finding planets that could support life. Searching for extraterrestrial planets could be the ultimate fulfillment of Hubble's visionary work that began here more than eight decades ago. "The 100-inch telescope allowed us to think that the universe is broad enough and old enough for many other civilizations to have existed out there," says Robert Jastrow, director of the Mount Wilson Institute. "CHARA will restore the glory of Mount Wilson by examining stars closely for signs of ourselves."
The CHARA (Center for High Angular Resolution Astronomy) Array collects starlight from six separate telescopes via an elaborate conduit system. Two vacuum pipes, eight inches in diameter, protrude from W2, one of a western pair of telescopes. The center pipe carries light from W2; the left carries light from W2's more distant twin, W1. Light beams from all the telescopes ultimately end up in the central Beam Combining Lab.
McAlister enters a clean room in the Beam Combining Lab and slips on booties over his shoes. Inside, CHARA associate director Theo ten Brummelaar fusses over a table of delicate optical mirrors where light waves from CHARA's six separate telescopes are combined. Tired-eyed and unshaven, ten Brummelaar has spent months struggling with complicated calibration problems trying to get all six beams of light to meet at the same spot at the same time—the key to making interferometry work. By contrast, the key to making better conventional telescopes is to build wider and wider mirrors. But both conventional and interferometry telescopes operate on a principle that's not exactly intuitive. When it comes to seeing detail, their ability increases as their baseline measurement increases. The baseline is the diameter across the telescope from one edge to the other; as it increases, the telescope's angular resolution increases. The surface area of the mirror is not important to sharpness and detail. Two small mirrors, one at each end of the baseline, would work just as well as a huge mirror that spans the opening. So scientists began to think about placing individual mirrors much farther apart, collecting their light, and combining the separate light waves from each telescope. The idea was popularized in the late 1800s by Nobel Prize-winner and astronomer Albert Michelson. Michelson took a swath of black cloth and cut two small slits in it, so that when he placed it over the 12-inch lens of his telescope, only two slits of glass showed. He pointed his masked telescope at Jupiter's moons. The moons were dimmer with the mask on because less light was collected. But Michelson discovered that only two small samples of light gave the same angular resolution as an entire 12-inch lens. And using his crude instrument, he was able to measure the diameter of the moons. "All that matters for angular resolution is the length of the baseline," says McAlister, glancing at an architectural drawing of CHARA that hangs on the control room wall. The bird's-eye view shows six small telescopes laid out in a Y formation over the mountaintop, each feeding its collected starlight into the Beam Combining Lab via vacuum tubes. What held true for Michelson's two-holed interferometer with a 12-inch baseline, McAlister says, also holds true for CHARA—a giant six-holed interferometer with a 1,080-foot baseline. But as Theo ten Brummelaar is quick to point out, there is a catch—figuring out how to synchronize light waves from six different telescopes. It requires cutting-edge optics, superfast computers, and new engineering invented from scratch.
