A BIG BUCKET OF LIGHT:

Perched high above the clouds on Mauna Kea in Hawaii, the Keck I telescope joins a half dozen newly minted telescopes around the world that herald a new age of astronomy. These finely tuned instruments will provide the first images of planets in other solar systems as well as peel away the mysteries of how the first generation of galaxies came together during what Harvard astronomer John Huchra calls the extragalactic dark ages.

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.

Aluminum is the reflective material on most of the giant new telescope mirrors, but a microscopic silver coating will make Gemini North keenly sensitive to infrared light.

 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

Two VLT telescopes are now operating independently, but by 2006 will be yoked with two others currently under construction. Starlight collected by each of the four VLT mirrors will be piped through an underground labyrinth and focused onto a single detector, providing the resolution of one mirror some hundreds of feet across.

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.




Steward Observatory Mirror Laboratory
Large Binocular Telescope Project
The MMT Observatory
The Gemini Observatory
SUBARU Telescope
Hobby-Eberly Telescope