The age of behemoths.
George Ellery Hale had a unique talent for raising private funds to pay for scientific research. He used his skills to build the world's largest telescope --four times. For nearly three decades, the 5-meter atop Palomar Mountain outside San Diego, California, designed by and named for Hale, remained the most powerful research-quality telescope in the world.
Over the past decade, however, a new breed of bigger and smarter telescopes has humbled Palomar's giant, and today Hale would no doubt be proud--if not envious. These new machines, with the aid of state-of-the-art computers and sophisticated optics, are providing unprecedented views of cosmic denizens near and far. A rush of astonishing new discoveries has begun, and much more excitement is in store as these "grand masters" reach their full potential.
Walking Among Giants
Probably best known among the new behemoths are the twin Keck telescopes. They reign atop Mauna Kea--a dormant 13,796-foot (4,205-meter) Hawaiian volcano that boasts some of the clearest and darkest skies on Earth (S&T: August 2001, page 40). Built in the early 1990s, each of the Kecks--jointly run by Caltech and the University of California--has a hexagonal light-gathering surface 9.8 meters (32 feet) across. Also sharing Mauna Kea's otherworldly landscape are the 8.2-meter Subaru, owned by Japan, and the 8.1-meter Gemini North, operated through a seven-country coalition.
Another kingdom of big telescopes rules from the mountains of northern Chile, offering astronomers access to southern skies and exceptionally clear nights. Here in the arid plateaus of the Atacama Desert, the European Southern Observatory has built four 8.2-meter behemoths, collectively called the Very Large Telescope (VLT). Only 625 kilometers (388 miles) away, Gemini's southern twin sits atop Cerro Pachon, high in the Andes, not far from the two 6.5-meter Magellan telescopes on Las Campanas.
More stargazing machines in the "8-meter class" are cropping up in Texas, Arizona, South Africa, and the Canary Islands off the western coast of Spain. This big-glass boom is far from serendipitous. It came from the combination of ingenuity, monumental advances in computer and electronic technologies, and loads of cold, hard cash--the Keck twins cost more than $150 million. The radical new telescope designs alone have transformed the field far beyond the wildest dreams of Hale.
Palomar Mountain's 5-meter mirror--Hale's crowning achievement--is a bulky beast, more than 2 feet thick and weighing 20 tons. To keep the mirror from losing its shape and sagging under its own weight, the primary requires a massive support structure. Simply scaling up the 5-meter's unwieldy design to an instrument twice the size would be an engineer's nightmare.
Jerry Nelson (University of California, Santa Cruz), the driving force behind Keck, got around the problem by using thirty-six 1-meter mirrors that act as a single 10-meter light bucket. Meanwhile, University of Arizona astronomer Roger Angel pioneered another clever way to make the huge mirrors seen on the Magellans. Angel's mirrors are surprisingly light-weight and mostly hollow; they employ a honeycomb-like structure to ensure stiffness. He makes them through a process known as spin casting, in which a molten-glass mirror cools down within a spinning mold, and a parabolic shape forms naturally. For the VLT, European engineers took a third approach. They made exceptionally thin mirrors--just 20 centimeters (8 inches) thick for an 8-meter mirror. This fragile design (also seen on Gemini and Subaru) relies on computer-controlled pistons to preserve the mirror's shape as the telescope moves around.
Making Great Instruments Even Better
But it isn't only about size. Modern technological wizardry is helping the giant new telescopes see fainter, farther, and more clearly than any of their predecessors. Chief among the advances is adaptive optics (S&T: October 2001, page 30), a sophisticated technique that negates our atmosphere's blurring effect on starlight. Even though a 10-meter telescope can detect light from vanishingly faint objects in the sky, its uncorrected images are not much sharper than those of a 10-centimeter scope.
Adaptive optics measures the changing distortions of light waves as they travel through the atmosphere and compensates for them hundreds of times per second by flexing a thin, deformable mirror in the light path. The mirror is pushed and pulled from behind by hundreds of tiny motors so that its shape exactly cancels out the effects of the roiling air above. A second, comparatively inefficient technique is known as speckle interferometry. With speckle, astronomers combine thousands of ultrashort exposures to characterize and remove atmospheric disturbances.
Until speckle interferometry and adaptive optics, the only way astronomers could escape atmospheric smearing was to put their telescopes in space. That's why the Hubble Space Telescope (HST), even with its relatively puny 2.4-meter mirror, delivers sharper images than any ground-based observatory. But adaptive-optics systems now allow giants like Keck and Gemini to rival Hubble's picture quality--and often surpass it at infrared wavelengths, where the seeing is better.
Discoveries Near and Far
Thanks to their enormous size and smart design, the new generation of telescopes is revolutionizing astronomy. As Harvard astronomer Robert Kirshner says, these behemoths "make it possible to do things that were impossible before." The result is a flood of new discoveries stretching from the farthest reaches of the universe to our celestial backyard.
Kirshner and his colleagues around the world often use large telescopes such as Keck, the VLT, and the 6.5-meter MMT Observatory in Arizona. His team studies distant supernovae in remote galaxies to determine the ultimate fate of the universe. Will it expand forever, or eventually slow to a halt and collapse into a "big crunch"? The answer lies in measuring changes in the cosmic expansion rate, which involves finding distance indicators that represent the earliest epochs in cosmic history. Type Ia supernovae--white-dwarf stars that undergo thermonuclear explosions--are ideal because they are identical in the way they erupt and are bright enough to be seen across vast stretches of space.
But "it's not enough just to discover supernovae," explains Kirshner. "We need to follow them as they get fainter." That's because astronomers use the rate at which a supernova fades to accurately determine its peak brightness. Knowing the peak is pivotal to determining an accurate distance. Moreover, they need a spectrum to ensure that the supernova is indeed a Type Ia. "The new 8-meter telescopes are making a big difference," says Kirshner. His team uses 4-meter-class telescopes and Hubble to search for supernovae but relies on their larger brethren to provide spectra and follow-up observations. By comparing the brightness of these beacons--and thus their distances--Kirshner's team has helped determine how much the cosmic expansion rate has changed over time.
The surprising answer has been hailed as one of the greatest astronomical discoveries in recent years. The universe is accelerating. Paradoxically, though, the gravitational pull of all the matter in the universe should slow the cosmic expansion. The best explanation to date, first suggested by Albert Einstein purely on theoretical grounds, is that space itself has a repulsive force known as the "cosmological constant." This force counteracts gravity.
While Kirshner's targets are at the farthest reaches of the visible universe, literally billions of light-years away, those of Geoffrey Marcy (University of California, Berkeley) lie within a mere 300 light-years. Marcy leads the world's most prolific extrasolar-planet search team. He and his colleagues break down starlight into its individual wavelengths and then look for lines in the spectrum that sway as a star wobbles due to an orbiting planet's gravitational tug (S&T: June 2001, page 34). "Ultimately," Marcy declares, "it's all about photons. The more photons we collect--the more light we gather from our target stars--the easier it is for us to look for planets in their midst," he explains. "It's truly awesome what Keck is doing for us," says Marcy. "It's improved our precision tremendously."
Almost all of the 70-odd "exoplanets" found to date are suspected to be gaseous monsters--many are more massive than Jupiter, and most hug their stars in tight orbits. But with Keck, Marcy's team can pick out the signature of lower-mass planets in wider orbits that barely perturb the stars they orbit. Within a few years Marcy hopes to find planets only a few times more massive than Earth. "That would be cool," he beams, "because those planets are likely to be rocky, where water could puddle into streams, lakes, and oceans, and water could really be in liquid form and act as the solvent for biochemistry." Finding wet, habitable worlds around other stars is an important step toward answering the grand question of extraterrestrial life.
The new giant telescopes are also providing sharper-than-ever views of celestial bodies much closer to home. Planetary scientist Imke de Pater (University of California, Berkeley) uses Keck's adaptive-optics system to study planets and moons in the outer solar system with startling clarity. Observing at near-infrared wavelengths, de Pater's team has seen patchy clouds on Uranus as well as the bright Epsilon ring and three other narrow ringlets surrounding the planet. "Since Uranus rotates every 17 hours, in half a night you can watch these clouds move across," says de Pater, pointing to a dramatic image on her computer screen. "We're seeing an incredible amount of detail never seen before on Uranus," she adds.
Saturn's moon Titan, the only satellite with a substantial atmosphere, is another of de Pater's favorite targets. "The big question here is whether Titan has oceans on its surface," explains de Pater. "We don't know the answer yet." Jupiter's moon Io, one of the four discovered by Galileo, is also on the observation list. Io harbors more volcanic activity than any other body in the solar system. "Last February  at Keck, we saw a whopping outburst on Io, much brighter than the satellite itself in the infrared," says de Pater. While spacecraft such as Voyager and Galileo have provided snapshots of Io's outbursts at a given time, ground-based observations like de Pater's are necessary to create a long time line of volcanic activity. She also plans to use Gemini and the VLT to monitor Io's volcanoes and to search for low clouds on Titan.
Caltech's Michael E. Brown uses Keck to study Kuiper Belt objects (KBOs)--icy bodies within the outer edges of the solar system. KBOs range in diameter from just a few kilometers to 1,000 km or more. "They are the true primordial remnants of the early days of the solar nebula," says Brown, "They have been in a deep freeze for 4 1/2 billion years." Brown uses Keck to obtain spectra that provide clues to their makeup. "These things are so faint that you just can't do it without a 10-meter telescope," he explains. Already Brown and others have found evidence of organic molecules in these primitive bodies.
King of the Mountain?
With a six-year head start, the Keck twins have delivered much more science than their siblings. But the competition has toughened. In 2001 a team of astronomers led by Danielle Alloin (European Southern Observatory) used the VLT to find evidence for a gigantic black hole in the nucleus of the nearby galaxy M77 in Cetus. Meanwhile, Gemini is emerging as a dominant player in infrared astronomy and adaptive optics. The northern telescope's image of the center of our galaxy is one of the sharpest ever. In the past year Subaru has also produced formidable scientific results.
Have these new ground-based titans made space telescopes like HST obsolete? Far from it. Hubble still holds the lead in visible-light imaging because current adaptive-optics systems work well only in the infrared, and they can't detect ultraviolet light blocked by the ozone layer. As Kirshner points out, "You need to use the right tool for each job." Hubble's unique capabilities nicely complement those of ground-based facilities, and it continues to make remarkable discoveries. In fact, NASA hopes to launch a 6-meter successor to Hubble, dubbed the Next Generation Space Telescope (NGST), within the coming decade. On Earth, some infrared wavelengths can't penetrate the atmosphere, and others are overwhelmed by the glare of a terrestrial telescope's own heat. Suspended in the cold vacuum of space, however, NGST will peer deep into stellar embryos shrouded in dust and search for baby galaxies in the distant universe.
As for ground-based telescopes, the next step is to use two or more behemoths in unison to make even sharper observations. The technique of combining light from multiple telescopes, called interferometry, is used routinely in radio astronomy. But applying it to infrared and visible light is far more difficult because those wavelengths are typically 10,000 times shorter than radio waves. Combining two beams therefore demands astonishing precision--cumulative spacing errors as small as a fraction of a micron render observations useless.
The early signs are promising. Just a few months ago, scientists combined the light from the two Keck telescopes into one optical pattern (S&T: June 2001, page 28). The VLT interferometer, which will eventually combine light from all four telescopes, has also passed its initial tests. Within a few years these combined giant telescopes will sharpen our view of the heavens yet again, perhaps allowing us to take a picture of a planet around another star for the first time.
But all that is just a prelude to the next generation of super-giant telescopes already on the drawing boards. Keck's designers, including Nelson, hope to follow up with a 30-meter mega-Keck, called the California Extremely Large Telescope (S&T: January 2001, page 41). Other astronomers in the U.S. and Europe are considering a variety of designs for 30- to 50-meter monsters (S&T: August 2000, page 52). The most ambitious plan yet, presented by engineers of the European Southern Observatory, is for an Overwhelmingly Large Telescope, with 2,000 mirror segments making up a light-gathering area larger than a football field. If these grandiose plans come to fruition, we ain't seen nothing yet.
Using a telescope isn't what it used to be. The giant new star machines--with a high premium on their observing time, an ever more complex suite of instruments, and locations on inhospitable mountaintops--are changing the way astronomy is done.
The Keck Observatory strongly discourages its users from going to its site on Mauna Kea. At an altitude of nearly 4,200 meters (14,000 feet), the summit air has just half the oxygen available at sea level, and the wind chills often dip well below freezing--not exactly a pleasant working environment. Instead, the observers run the show via computers from a remote observing facility 70 kilometers away in Waimea, while a telescope operator sits at the summit. A closed-circuit TV system and a host of computers provide scientists with all the same information and controls they would have if they were at the summit. "I love this setup," says Geoffrey Marcy (University of California, Berkeley). "It's a lot easier to sleep, work, and think at sea level." Harvard's Robert Kirshner agrees: "You really don't lose anything by being in Waimea, and you gain a lot in terms of wear and tear."
At other premier observatories, like the VLT and Gemini, some fraction of the observing is done in "service mode." Staff scientists carry out the observations and mail the data back to the investigators. The arrangement is new to ground-based astronomy, but space telescopes like Hubble operate entirely in this mode. The main advantage is efficiency. Observatories can keep all the approved programs in a "queue" and run them when conditions are just right. Astronomers are spared having to travel thousands of miles and to master a complicated new instrument. Moreover, projects requiring just a few images or spectra need not apply for a full night of time. While the system works well most of the time, some astronomers lament missing the chance to make last-minute changes to their program or to make on-the-spot judgments about their results.
Long gone are the days when Edwin Hubble observed at the eyepiece of Mount Wilson's 100-inch telescope with his "night lunch" close by to cure late-night hunger pangs. Soon astronomers will be able to observe with a telescope on a different continent while sitting in their own office and pop out to the local McDonald's for a "midnight" snack.
Today's top observatories are much more international than their predecessors. The reasons for these cross-border collaborations are as much financial and political as they are scientific. The new behemoths are often too expensive for a single nation to bear all the costs. Besides, it's a lot easier to persuade a national government to pay for "big science" if neighboring--or rival--countries are also doing it.
"Among our politicians, Gemini and the Southern Observatory for Astrophysical Research (SOAR) are regarded as prestige projects, of which they are proud," explains Albert Bruch of the National Laboratory of Astrophysics in Brazil. His country owns a 2.5 percent share of the Gemini twins and a 30 percent share of SOAR, a new 4-meter survey telescope in the Andes. Bruch expects the payoff to include a growing awareness of astronomical research in Brazil.
Perhaps the nation that has benefited the most from the current telescope boom is Chile. In return for providing prime real estate for American and European observatories, Chilean astronomers get a share--typically 10 percent--of observing time on all telescopes located in their country. "That's giving us a chance to do first-rate science in a competitive atmosphere," says Diego Mardones (University of Chile).
How do observatories ensure that all the partners get their fair share? Each of the seven member countries in the Gemini consortium has its own time-allocation committee (TAC) to review and rank observing proposals from astronomers in that country. In the end, an international "super-TAC," which includes one member from each nation, makes the final schedule using a complex accounting formula. "So far, it has worked very smoothly," says Canadian astronomer Jean-Rene Roy, an associate director of Gemini. "We follow the recommendations of the national TACs and just try to resolve duplicates and overlaps." The process works "to my fullest satisfaction," concurs Bruch. However, the European Southern Observatory, whose telescopes provide access to nine member states in addition to Chile, handles time allocations very differently. Proposals from all countries go to one central TAC, which has several panels to cover different research areas.
RAY JAYAWARDHANA, a Miller Research Fellow in astronomy at the University of California, Berkeley, frequently uses Keck, Gemini, and the VLT to study the formation of planetary systems.
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|Title Annotation:||changing views of the heavens; includes related articles|
|Publication:||Sky & Telescope|
|Date:||Feb 1, 2002|
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