The circle of glass casts a shimmery light like a fine piece of crystal. It weighs 16 tons and stretches 27 feet from edge to edge. Yet its understructure, a honeycombed lattice of one-inch-thick walls, is so precisely wrought that were it dinner-plate size, its filigree of glass would be wine-goblet thin. The surface, a sweeping parabola of Euclidean purity, seems perfectly matched to its function: to peer from a tiny speck in the universe called Earth into an unimaginably distant past when vast galaxies were still forming. In 2002, when it is polished and coated and then paired with a twin atop Mount Graham, Arizona, this mirror will open a new window on the cosmos.
The mirror is the brainchild of astronomer Roger Angel, whose boyish face masks a competitiveness that drove him to engage in one-upmanship with rival mirror makers. Angel’s mirror, shaped and polished, incongruously, in the bowels of the University of Arizona football stadium, is the largest piece of optical glass ever cast. But happily for astronomers,this sleek saucer is only one of an elite new generation of optics that promises to lead them beyond the solar system into unexplored regions of space.
Today, seven of these 8-meter-plus mirrors, four of them already set up 13,800 feet above sea level on Mauna Kea in Hawaii, are poised to begin the journey. Two Keck telescopes, whose jigsaw-style mirrors, assembled from hexagonal pieces, stretch a massive 10 meters across, have been operating on Mauna Kea since 1990 and 1996. Nearby, Japan’s wafer-thin 8.3-meter Subaru (the Japanese name for the Pleiades constellation) telescope collected its first images from space in January. And in the same neighborhood the Gemini North, an 8.1-meter telescope built by a seven-nation consortium that includes the United States, is scheduled to begin scanning the heavens any day.
Southern Hemisphere skies, with a panoply of stars that cannot be seen from the Northern Hemisphere, are about to be probed by the first two of four Europe-financed 8-meter instruments, called collectively the Very Large Telescope. One was scheduled to begin service atop Cerro Paranal in Chile in April. Chile’s Cerro Pachón will get the Gemini South observatory next year.
MIRROR IMAGE
Sailors refer to calm seas as glass, and astronomers in turn have their own nautical metaphor for the smoothness of each of the newest generation telescope mirrors: imagine the entire Atlantic Ocean without a single wave higher than a few inches.
The construction of such a spectacularly smooth mirror begins with blocks of rather humble-looking glass selected for a number of properties, from resistance to temperature-induced distortion to purity. The blocks are loaded into a large round mold and shoved into a vast oven set on slow hot bake. At 1300 degrees, the blocks begin to melt; at 1800, the glass forms a molten pond, flowing to fill every nook of the mold. After five days—when the temperature has reached 2100 degrees—the heat is turned down and the mirror spends many weeks in a controlled cooldown designed to eliminate thermal stresses.
Grinding and polishing removes tons of glass that lies between the blank mirror’s out-of-the-oven shape and its mathematically parabolic final form. The precision needed is so great that the first step in preparing the 8.4-meter mirror for the Large Binocular Telescope is polishing the back of the glass to inspect for flaws and ensure that it sits flat. Once the top is smoothed to within a few nanometers of perfection, a layer of aluminum vapor is deposited to provide a reflecting surface.
The process is fraught with hazards. The first European attempts to make revolutionary mirrors for the Very Large Telescope ended with tons of cracked—and useless—glass. And the mirror for the Large Binocular Telescope suffered a near-catastrophic leak during the casting phase. “We were able to bring it back to perfection,” astronomer Roger Angel says, “but the [mirror] was about a year in the furnace.”—Jeffrey Winters
This amazing new generation of telescopes will take mankind closer to the dawn of creation than ever before. Previously, the oldest light gathered by telescopes emanated from galaxies formed a few billion years after the Big Bang. Theorizing about what happened any earlier has been a bit like trying to describe what a toddler would look like by observing a 25-year-old adult. But astronomers expect the new instruments to show them a more youthful universe—“bits and pieces that came together to form the first galaxies,” says Angel, 59, who is head of the University of Arizona’s Steward Mirror Lab.
Closer to home, the big telescopes have an equally intriguing assignment: to see planets outside our solar system. In recent years astronomers have observed 18 stars whose motions indicate the gravitational pull of an orbiting planet. Scientists have even worked out planet densities and orbital paths, but so far none of these planets has actually been seen. The new generation of telescopes could not only put such planets on the map but, through a spectroscopic analysis of the light they reflect, determine their composition and—the ultimate question—whether they have the potential for harboring life.
“There are three or four planets that are far enough from their stars that they can be resolved by the Large Binocular Telescope,” says Angel. “We can actually record the photons from these planets.”
MULTIPLE EYES ON THE SKIES
Telescopes are fairly straightforward: incoming light bounces off a large curved mirror and is focused onto a smaller mirror that reflects it to a detector where the image of the sky is formed. The race to build ever larger telescopes is driven by the simple fact that the larger the aperture of the main mirror, the more light it collects and the sharper the image. But astronomers who want the precision of a giant reflector without its bulk and expense have devised schemes to squeeze more from less. Or, perhaps more accurately, more from many.
Although sharpness is related to the diameter of the mirror, it is in no way related to its area. That is, if one removed all but the edges of the reflector, the image would be much fainter but would not lose clarity. In 1978 the Multiple Mirror Telescope took this idea to an extreme, using six 72-inch mirrors to create the equivalent of a single 176-inch reflector. However, adjusting for flex and strain in the support structure proved problematic. A great deal of computerized fine-tuning was required to align the light from all six mirrors. To simplify the image-gathering process, the Multiple Mirror Telescope was converted recently to a single-reflector telescope, with a 6.5-meter mirror fresh from the Steward Observatory Mirror Lab.
While perfect alignment is the goal for some astronomers, there’s valuable information to glean from very slight misalignments of light from multiple mirrors. If the images are stacked on top of each other while traveling slightly different distances, the light waves from one image can cancel another out. By removing much of the starlight in this way, astronomers hope to detect very faint planets circling the star or achieve precision measurements of the stellar surface—feats that otherwise would require a single mirror hundreds of feet across. Astronomers plan to yoke the twin Keck telescopes in Hawaii, the four Very Large Telescopes in Chile, and the Large Binocular Telescope in Arizona into this sort of interferometer to tease apart the tangled web of the universe. —J. W.
The new telescopes could begin to unravel a host of other cosmic mysteries, too. Is the universe flat, round, or saddle-shaped? Why isn’t matter distributed uniformly throughout the universe? And most puzzling of all, where is the “missing matter” thought to make up 80 percent of the mass of the universe?
These are some of the questions astronomers have tried in vain to answer since the Hale telescope was installed on Mount Palomar, California, in 1948. With its 5-meter (200-inch) mirror, it was for several decades the most powerful light-gathering instrument ever devised. Yet despite an 80-fold gain in yield as electronic image detectors replaced photographic film, even the Hale’s superior muscle power fell short. By the 1980s scientists had to admit that they had wrung just about everything they could from it. “Detectors began approaching their theoretical limits on efficiency,” says John Huchra of the Harvard-Smithsonian Center for Astrophysics. “So we had to start building larger telescopes again if we were going to gather more light.” In telescope design there is no Bauhaus paradox: more is simply more.
Web ResourcesSteward Observatory Mirror LaboratoryLarge Binocular Telescope ProjectThe MMT ObservatoryThe Gemini ObservatorySUBARU TelescopeHobby-Eberly Telescope
Mirrors, however, had seemingly bumped up against the law of diminishing returns. To make them larger, using conventional design, you also had to make them thicker. Beyond a certain size, however, a mirror will spend the entire night dissipating the heat it absorbed the day before. Heat waves on the surface of a mirror distort the very images the mirror was designed to enhance. Furthermore, the weight of such a mirror, more than 100,000 pounds, would make it virtually unmanageable.
“One obvious alternative,” says Angel, his accent betraying his British origins despite more than a quarter century of living and working in the United States, “is to make mirrors wider but keep them relatively thin.” A thin piece of glass solves two fundamental engineering problems, weight and heat, but introduces a third: flexibility. Even minute changes in shape transform fine optical glass into a sort of celestial funhouse mirror.
JIGSAW PUZZLE MIRRORS
Three of the giant new telescopes, the two 10-meter Kecks and the Hobby-Eberly, skirted some of the weight problems associated with casting a mirror from a single piece of glass. Designed by Jerry Nelson, an astronomer at the University of California at Santa Cruz, the Keck mirrors are made up of 1.8-meter-wide hexagons, 36 for each, which when assembled form a perfect parabola. This design made it easier to transport the glass, which was cast in Germany and polished in California and Massachusetts, but also introduced a formidable shaping challenge. Since the precise curvature of a hexagon depends on where it lies in the parabola, polishing had to be subtly customized to each piece. Shaping the Kecks’ segmented mirrors was like painting sections of a landscape on individual pieces of plaster and then assembling them in a seamless mural.
The Hobby-Eberly’s mirror, also assembled from pieces, is a full meter larger than the Kecks’. It has a spherical, not a parabolic, curve, making it fundamentally simpler and not as expensive to build. Each of its 91 segments is congruent with every other segment, which makes them notably less costly to polish. Such mirrors sacrifice sharpness, but HET is used primarily for spectroscopy—the chemical analysis of matter according to the light it emits or absorbs. “The design allows us to get a large number of spectra in a short amount of time,” says project scientist Larry Ramsey of Penn State, which, together with the University of Texas, Stanford, and two German universities, owns the telescope. HET’s simplified mounting mechanism—it can be rotated but is fixed at an angle of 55 degrees above the horizon—produced further savings. In all, HET cost $15 million, compared with $93 million for Keck I and $77 million for Keck II. —M. L.
One group of mirror builders has battled flexibility by constructing its reflectors from a ceramic material that is more rigid than glass and supporting them in a cradle that continually adjusts for any deflection. This is the method being used on the Very Large Telescope and the Subaru. Their mirrors, each no more than about 8 inches thick and weighing about 50,000 pounds, are mounted on a thicket of actuators—hydraulic pistons that make continuous adjustments to restore the mirror’s shape when it is bent by the wind or distorted as it is moved from one position to another. A laser monitors the shape, and the necessary adjustments are calculated and controlled by a computer.
The Mirror Lab’s Roger Angel, who had come to Columbia in 1967 to work in physics but then followed his adviser into astronomy, wanted a more elegant solution: a mirror that was thin and rigid. He started his search in the early 1980s with a distinct disadvantage: “I knew nothing about glass,” he admits. But that didn’t deter him. He began his education, where else, but in the garage of his house in Tucson, Arizona, where he took to melting Pyrex custard cups in a homemade oven to see how hot glass behaved. As the project grew over the years, he moved it into a corner of the astronomy department, then to the University of Arizona’s optical shop, then into an abandoned synagogue, and finally, in 1985, into the new mirror lab he had convinced the department to build.
By then Angel, his collaborator Neville Woolf (another transplanted Brit), and graduate student John Hill had developed a design for mirrors thick enough to be stiff, but also lightweight and quick to dissipate their internal heat: a disk, smooth on one side and honeycombed on the other. “It wasn’t an original idea,” says Angel, who credits George Ritchey, designer of the 60-inch and 100-inch telescopes on Mount Wilson at the turn of the century, with having first dreamed up such a mirror. Ritchey, however, never figured out how to make it.
Rather than trying to carve out excess glass—an unimaginably tedious procedure—Angel and Woolf decided to cast their mirrors mostly hollow. They do this by melting glass over an assemblage of hexagonal columns made from a heat-resistant ceramic foam very similar to the material used in space shuttle tiles. After the glass cools, the foam is pulverized with high-pressure jets of water and removed. What’s left is a thin layer of glass supported by an inch-thick honeycomb structure that is three feet deep at the perimeter and 18 inches in the center. Its weight of 16 tons is about one-fifth what it would have weighed had it been built conventionally. When the mirror goes into use, air will be blown across the honeycomb structure so that the glass will maintain ambient temperature. No nighttime cooldown period will be required.
WHAT'S NEXT?
While many astronomers are queuing up for time on the giant telescopes just coming on line, others are drawing plans for even larger telescopes. If these behemoths are built, they will make the twin Keck telescopes, today’s largest, look like opera glasses.
One future option is to extend the linked-telescope approach of the Keck, the Large Binocular Telescope, and the Very Large Telescope. If linking two or four telescopes gives great results, the thinking goes, why not 16? The telescopes would be placed in a circle like pearls on a two-mile-long necklace, each feeding light to a central facility that would combine the light into one incredibly sharp image. Such a telescope would have the light-gathering ability of 50 Hubble telescopes and the resolving power of a single mirror some half-mile across. It could pick out the flag planted on the moon by the Apollo 11 astronauts or resolve an asteroid near Alpha Centauri.
Another radical plan being pushed by European astronomers is for a single mega-telescope. Dubbed the Overwhelmingly Large (OWL) telescope, it would have a primary mirror made of some 2,000 smaller ones—each the size of the Hubble. Placed together, they would create a single reflecting surface more than 100 yards across with a light-gathering capacity ten times greater than all the telescopes ever built. Even its backers admit it would take a decade to grind all the mirrors, but once built, it could pick out brown dwarfs in neighboring galaxies or supernova explosions from 10 billion light-years away.
Astronomer Matt Mountain, director of the new Gemini Telescope in Hawaii, says this future generation of gargantuan earthbound telescopes would make it possible to study individual stars in some of the earliest galaxies or determine the atmospheric gases of distant planets. But before astronomers get carried away, there’s the question of price. Would these telescopes be worth the billion dollars it would take to build them? “At some point, it becomes more economical to put them in space,” Mountain says. “We’re not at that point yet, but it’s fast approaching.” —J. W.
