Light and Magic
On a clear night--with this telescope--you can see forever.
- By Eric Adams
- Air & Space magazine, July 2000
In the world of big, powerful telescopes, there is one accomplishment that none have been able to claim. It has no scientific value, really, but it would thrill both astronomers and the general public and ensure a lifetime of bragging rights to the first to do it.
Sitting in his office at the European Southern Observatory's Very Large Telescope in northern Chile, overlooking an expanse of brown hills and valleys leading to the Pacific Ocean five miles away, VLT director Roberto Gilmozzi smiles as he edges toward the revelation. "When this telescope is complete, it will have the angular resolution equivalent to that of a telescope with a mirror 132 meters [433 feet] in diameter," Gilmozzi begins. "That means that we will, if we wanted to, be able to resolve and photograph Apollo debris left on the moon."
Now, there are many, many other celestial targets that the ESO is much more interested in-distant galaxies, dim nebulas, extrasolar planets-but that would be a sensational stunt. Many thought it impossible, given how minuscule even the largest Apollo remnant-something about the size of a delivery van-would be from a quarter of a million miles away. But with the giant leap that the ESO is about to take in Chile, this and a lot of other accomplishments are indeed going to be possible. Gilmozzi is coy, though, about whether the moon shots will actually make it into the observing schedule. "It would certainly make for some good PR, wouldn't it?" he asks.
It would. It would also prove Gilmozzi's point: that the VLT is an exceptionally powerful instrument. Built atop a truncated mountain in the Atacama Desert, the VLT is actually four identical 8.2-meter-diameter reflecting telescopes-two of which are now operational-that can be used independently or be linked, through a process called interferometry, to create what is essentially a single ultra-large-aperture telescope. Technologies called active and adaptive optics, plus a prime geographic location-one of the driest places on earth-will enable the VLT to operate with virtually no atmospheric distortion, and its remote location will also give it the darkest skies possible. All of this will make the VLT capable of seeing farther and in greater detail than anything before it. It will capture images at least 50 times sharper than those obtained by the Hubble Space Telescope. It will answer many questions, and raise many new ones-and it will probably place Europe at astronomy's fore.
"The VLT is already the best-working telescope in the world," says British cosmologist Simon White, director of the Max-Planck Institute for Astrophysics in Munich. White visited the VLT last year to capture details of the rotation of spiral galaxies. "This is the first time in a century that the foremost optical astronomy instrument has been a non-U.S. facility. When complete, the VLT interferometer will open an entirely new range of phenomena for study-if it works to spec."
The VLT's potential isn't lost on the Americans. Says Robert Gehrz, president of the American Astronomical Society and a University of Minnesota physics and astronomy professor: "If they can get that thing to work as an interferometer, that's going to be a breakthrough that will make it the most powerful facility in the world. There's no question about that-and I'll probably be applying for time on it."
The VLT was conceived in 1977; in the years since, the ESO, a consortium made up of Germany, Italy, Denmark, France, Belgium, Sweden, Switzerland, and the Netherlands, has spent $500 million on it. All of its major advances have been tested at the New Technology Telescope in La Silla, Chile. But the VLT is one of the most complicated observatories ever built, and its success won't really be verified until 2006, when its four telescopes focus on the same speck of light and it begins coming up with answers to questions about galactic evolution, the insides of quasars and black holes, and, Gilmozzi promises, the precise nature of the planets orbiting stars other than our sun.
The VLT may as well be on another planet itself. Getting to Cerro Paranal, the mountain on which the VLT was built, is a multi-leg, often multi-day affair. Visitors fly first to Santiago and then 800 miles north to Antofagasta, an isolated, sprawling port city of 250,000 that exists primarily to support the dozens of mines in the region. Here, families flock to the rocky beaches-better for sunbathing than swimming-that span Antofagasta's western shore.
From the ocean, the trip east entails a bone-jarring 75-mile drive into the Atacama Desert, a dusty plateau on the edge of the Andes that is virtually devoid of vegetation and animal life. There is little but gently sloping mountains and vast fields of boulders that sit evenly distributed, as if placed by a machine. The wide dirt road, called the Old Panamerican Highway, is mostly used by the observatory and by a nitrate and iodine mine about 20 miles beyond the telescopes. There is nothing along the way, and trouble (breakdown, blown tire, accident) means either a long walk or a long wait. Visitors who don't take the ESO's shuttle and elect to drive themselves are instructed to call the observatory before leaving. If you don't show up in three hours, they send someone out to find you.
Eventually, a large white sign materializes, announcing the presence of the VLT. Behind it, a freshly paved road vanishes into the hills-a 280-square-mile region that Chile donated to the ESO in exchange for telescope time. A slow first-gear ascent leads to the guard shack and the observatory base camp, which sit 7,750 feet above sea level. From here, you can visually follow a two-mile road up the mountain's remaining 900 feet to its perfectly flat top, where four giant silvery cubes perch, with rocks dribbling over the sides-debris produced when the builders blew 90 feet off the top of Paranal in 1990.
The base camp below is a clean, orderly village that is mostly made up of bright white ship cargo containers that have been converted into surprisingly nice offices and dorm rooms. On one side of the camp sit a helicopter pad and a soccer field; on the other, a parking lot filled with white four-wheel-drive trucks bearing ESO logos on the doors. Scattered throughout are a two-story telescope service building, a platform with eight 20-foot-tall water tanks that get replenished twice daily by trucks from Antofagasta, a power station, and a dormitory being built for staff and visitors.
Beyond this, there is nothing. As workplaces go, Paranal has little appeal. Though serene and beautiful, it is also hot and dry, and far from any diversions. "Personally, I consider Paranal to be one of the better places on the Earth to read books," says VLT staff astronomer Gianni Marconi, a friendly 39-year-old Italian who spends his nights on the mountaintop operating the telescopes for visiting astronomers. "I'm used to walking far from the base camp to where human-produced noise disappears and I am disturbed only by the wind." Visitors aren't encouraged to take such walks, though: Two who wandered off last year quickly became disoriented in the featureless hills and ended up lost for two days.
But for astronomers, the lonely desert site has several advantages: the dry air, which makes for clearer skies and a low risk of condensation collecting on telescope mirrors; the distance from any sources of the urban light pollution that plagues much of the world; and the roughly equatorial positioning, which gives it access to objects in both northern and southern skies.
When the sun goes down and the night sky emerges, any doubts about why someone would travel so far for this are squelched. Scrolling up from the east, the Milky Way shines steadily against the deep-black sky, with dozens of fuzzy nebulas and star clusters visible to the naked eye. Two galaxies, consuming startlingly large swaths of sky, hover in the south: The Large Magellanic Cloud spans the width of about 14 full moons; the Small Magellanic Cloud, six. On this mountain, you feel as if you are staring out into space, rather than merely up at the stars.
On the telescope platform at dusk, the four 100-foot-square silver enclosures, each containing a five-story telescope, await instructions to open their doors and commence their night's work, either observation or, for the unfinished scopes, calibration and testing. All are cast in a shimmery reddish yellow from the sunset. Massimo Tarenghi, the VLT's project manager, stands amid a somewhat treacherous network of half-finished concrete channels and open pits that will soon contain the interferometry hardware. He wonders if the project will end up spoiling his colleagues: "Will we ever be able to work on a telescope that isn't at least as big as this?" he asks.
Though it isn't quite dark yet, the doors to Unit Telescope 1 slowly crank open and the dome rotates around to face south. This process helps ensure that the temperature of the air inside of the dome equals that of the air outside; inequalities would produce turbulence, which would distort the observed images.
Tarenghi notes that every major component of the telescope had a team of engineers dedicated to it. One group designed the 8.2-meter primary mirrors. Another focused the active optics system, which compensates for changes in the thermal conditions around the telescope and for tilt-induced weight changes that would alter the mirror's performance. Still another team mastered the $20 million secondary mirror system, which collects light focused by the larger primary mirror and directs it into the telescope's instruments. This system consists of a five-foot-long cylinder with a beryllium mirror on the bottom, and within it hundreds of pounds of electronics that constantly adjust the mirror's angle-another part of the telescope's active optics system. The secondary mirror hovers 40 feet above the primary and resembles a satellite in structure.
In each of the telescopes (UT1 through UT4), all of this hardware rests on a two-axis alt-azimuth mount that itself floats in a narrow track on a layer of oil only 0.05 millimeter thick. This makes for exceedingly smooth action-the whole 470-ton telescope can be moved with only a nudge of a hand. Of course, the motion of the telescopes is actually controlled by computers, from initial positioning to tracking of celestial objects during prolonged imaging and study. In the control building, a separate cluster of computers operates each telescope and provides continuously updated visibility conditions and tracking information. A fifth cluster of computers will call the shots when the interferometric mode becomes functional.
Though there is much for the VLT's engineers to brag about, several technological triumphs stand out. The first are the mirrors and their control mechanisms. The 8.2-meter Zerodur glass disks are among the largest single-piece telescope mirrors in the world. Manufactured in Europe, they were brought separately to South America on ships and then sent on a painstaking journey from Antofagasta to Paranal. Preceded by several grading trucks that smoothed out the dirt road, the trucks bearing the fragile mirrors drove a steady 3 mph. The trip took three days.
The 540-square-foot mirrors were aluminized in the ESO's mirror maintenance facility at Paranal, then inserted into basket-like cells and driven the final two miles up to the enclosures. These cells contain the 150 active optics actuators, small hydraulic pistons that flex the seven-inch-thick mirror in sub-millimeter increments.
Once light reflects off the actively controlled mirrors, it is further enhanced at an adaptive optics filter, which also employs a deformable mirror and works to counter errors, such as high-altitude thermal changes and atmospheric turbulence, that occur more rapidly than those the active optics can correct. These conditions are monitored by focusing on a single guide star; in response, the computer orders changes in the mirror's shape 100 times per second. Though several observatories around the world, most notably the Subaru Telescope atop Mauna Kea, Hawaii, are applying or developing active and adaptive optics, the VLT is by far its largest application, and the ESO was one of the first organizations to develop the technology. "When we were developing the active optics system in the 1980s, nobody was sure that this would work at this scale, and many people actually opposed the idea," Tarenghi recalls.
The other remarkable achievement taking shape on Paranal is the VLT interferometer. This setup, common in radio astronomy, capitalizes on the fact that if a telescope has a larger aperture, it can collect more of the light from objects in space. Interferometry enables the four telescopes to focus on the same object and gather its light as if the group were a single telescope as big as the combined distances between the individual telescopes-in the VLT's case, a distance of 426 feet.
Light beams collected by the four telescopes are deflected by mirrors into underground tunnels, where they are gathered at a single sensor. The sensor generates an image that is a cumulative product of the four beams. The trick is getting the light waves to meet at the sensor at the same time: As objects are tracked, the telescopes' relative positions change. Consequently, in the tunnel, the light is bounced off several retro-reflectors sitting on small, precisely positioned rail carts that move on 200-foot tracks to compensate for these changes. In addition, three small auxiliary telescopes, also moveable, on the surface will fill in the spaces between the four UTs to further punch up the resolution.
Final processing will prove that the VLT is far greater than the sum of its parts, with an angular resolution of 0.001 arcsecond. (The "celestial sphere" around Earth is 360 degrees; the full moon has an apparent size of 0.5 degree; a degree has 60 arcminutes; an arcminute has 60 arcseconds.) The Hubble, which sits above the atmosphere but has only a 2.4-meter mirror, can resolve to 0.1 arcsecond. The VLT's resolution is fine enough to capture detailed images of distant galaxies, clues about the chemical and biological composition of extrasolar planets-and snapshots of lunar rovers.
Putting all this to work means long nights on the mountain. In the evening, the base camp is pitch dark (no exterior lights are permitted, with the exception of a few dim safety lights) and increasingly silent as the lively chatter and music coming from the dorms of the ESO's 150-plus Chilean and European engineers and administrative and support staff gradually taper off. But atop Paranal, in the control building-already decorated with posters of some of the more spectacular images captured through UT1-a steady buzz of activity lasts until dawn. On average, the observing conditions (the "seeing") are considered excellent 350 days a year, a number envied by most observatories. On those evenings, the telescopes and their attached instruments work feverishly to flush every photon of light-borne information out of the sky.
The scheduling of science operations at the VLT is controlled at the ESO's headquarters in Garching, Germany. Astronomers compete for observing time, and if their proposal is accepted, they can travel to Chile to supervise the session themselves-as might be necessary for complicated or variable-dependent projects-or request that the VLT's staff astronomers, such as Marconi, conduct the program on their behalf. Marconi also operates the VLT for visiting astronomers, so that they don't lose time struggling with the technology.
The scientific programs Marconi helps execute are challenging, chosen to push the VLT's capabilities as far as possible. The results are sometimes breathtaking. "We made an observation three weeks ago which is particularly alive in my mind," Marconi recalls. "The target was a jet of material ejected from the famous active galaxy 3C273. While looking at the details obtained in the images, I forgot for a moment that I was on the ground."
The VLT will undertake a variety of scientific projects, including measuring the universe, studying galaxy structure and formation, and observing star birth and planetary system formation. Director Gilmozzi is confident that the VLT will push astronomy even further. "There's a final question that astronomers are asking, and that's whether we are alone in the universe," he says. "We will soon begin to ask how to detect biospheres and ecospheres on extrasolar planets, and the VLT will make those searches possible. I'm also sure that over the next several decades we will discover an enormous class of objects of which we have no hint today."
Along the way, ESO hopes to develop new strategies for studying astronomical phenomena. Instrumentation plays a key role in this. The VLT will initially be outfitted with 11 instruments-some as big as a room-capable of wide-range spectroscopy and other types of imaging. Some of these instruments have already paid big dividends for astronomers using UT1. Munich University's Rolf-Peter Kudritzki spent four nights at Paranal late last year using the VLT's FORS-focal reducer/low-dispersion spectrograph-to examine a newly discovered population of stars floating in the space between the galaxies of certain clusters. Though Kudritzki's team came well prepared, the members encountered some surprising results and consequently had to alter their program on the fly. "Many of our objects turned out to be completely different from what we expected," Kudritzki says."We detected a new class of extreme-emission-line galaxies at very high redshift." High redshifts indicate that the galaxies are moving away from our own very quickly and are at the edge of an expanding universe; because they are so far away, their light took billions of years to reach Earth, so we are seeing them as they were when the universe began. "I knew on the spot that I had something new and I was very excited about that," says Kudritzki.
Peter Barthel, an astronomer at the Kapteyn Institute in the Netherlands, spent several days at Paranal in late January. Collaborating with Willem de Vries of Lawrence Livermore National Laboratory in Berkeley, California, and Chris O'Dea of Baltimore's Space Telescope Science Institute, Barthel investigated distant galaxies that appear to be harboring small, young radio sources-potentially other galaxies. "The VLT data will tell if our ideas about these young radio galaxies make sense, and fortunately the data we gathered were excellent," Barthel says. "We got a number of new identifications with very faint galaxies. Though we still have substantial data processing and analysis left, we can, from the raw images, already see the faint host galaxies and study their spectra."
Barthel also devoted eight minutes of observing time to imaging the well-known, and significantly closer, Sombrero galaxy, M104. He is completing a study on black holes in the center of similar galaxies, and felt the VLT would help. It did-and it made for an impressive picture (one that the ESO made into a poster). "The raw Sombrero data were of sheer beauty," Barthel recalls. "We were making live pictures of a beautiful piece of nature."
Sunrise at Paranal brings a temporary halt to the exploration-and these days a continuation of construction, as there is still much work to be done on the telescopes, the interferometry tunnels, and the new dormitory building below. After coming off the mountain and grabbing a bite to eat in the cafeteria, astronomers usually head for their rooms, which are all clustered in a corner of the container camp and marked with signs reading "Silence: Astronomers Sleeping." For them, this lonely place, bathed in light from suns both near and far, is paradise.