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.
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.
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.”
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.
Steward Observatory Mirror Laboratory
Large Binocular Telescope Project
The MMT Observatory
The Gemini Observatory