Time - Man of the Year/Time_Man_of_the_Year_Compact_Publishing_3YX-Disc-1_Compact_Publishing_1993.iso/moy/042792/0427300.000

Text File | 1992-09-10 | 9KB | 183 lines

                                                                                SCIENCE, Page 56

Shoot for the Stars


A fresh generation of telescopes will open a new era of
astronomical discovery

By J. MADELEINE NASH/TUCSON


    Twleve summers ago, University of Arizona astronomer
Roger Angel swung by a Tucson pottery shop to pick up some
firebricks for a backyard kiln. Then he purchased some glass
ovenware at a nearby hardware store. A few days later, he
materialized in a graduate student's doorway, brandishing a
couple of Pyrex custard dishes melted to a misshapen blob. "We
can make telescope mirrors out of this!" Angel exclaimed. Thus
began a monumental and quixotic effort to reinvent the central
light-gathering surface of the telescope, from its initial
design to its final polishing.

    This month, many years and millions of dollars later, that
effort culminated in a spectacular success: the casting of one
of the world's largest telescope mirrors, a single 6.5-m
(21-ft.) circle of glass that sometime in 1994 will be hauled
by flatbed truck to the top of Arizona's Mount Hopkins, where
it will tilt skyward like a giant Cyclopean eye.

    These are heady days in the rarefied world of telescope
making. Not since the 1934 casting of Mount Palomar's 5-m mirror
-- a record size at the time -- has there been more innovation
or competition to push the edge of possibility. In the clear air
above Hawaii's Mauna Kea, the Keck I Telescope's mammoth 10-m
mirror, built of 36 separate segments, is nearing final assembly
-- a 10-month process was completed last week. Four years from
now it will be joined by the Keck II, an equally monstrous twin.
By then, the European Southern Observatory hopes to have
positioned the first of four 8.2-m telescopes atop a high peak
in the Chilean Andes. Japanese astronomers and other groups
around the world will be constructing telescopes of similar size
and daring before the end of the century.

    Collectively, this new generation of ground-based
instruments will open an extraordinary new window on the cosmos.
"What we can look forward to," says Caltech astronomer Maarten
Schmidt, "is the biggest gain in telescope power in the past 50,
maybe even 100 years." It should bring into focus the most
distant quasars yet and even planets orbiting other stars.

    The intellectual seeds for this technological renaissance
were sown more than a decade ago, when Angel and a handful of
other pioneers began contemplating the challenge of building
more powerful telescopes. Very quickly, they were forced to
consider radical new approaches to mirror design. Simply scaling
up old models would have been hopelessly expensive and unwieldy.
"A large mirror can't look like a small mirror," explains
Angel, "for pretty much the same reason that an elephant can't
look like a fly. If it did, its legs would collapse under its
own weight."

    The central conundrum confronting designers was this: how
to make a telescope mirror that could hold its shape against
gravitational sag and gusting winds yet retain the capacity to
make rapid adjustments to fluctuating temperatures. As mirror
size increases, these two requirements begin to dictate
different, and quickly contradictory, solutions. Very thick
mirrors resist physical deformation extremely well, but because
they retain so much heat, they tend to generate shimmering
currents in the cold night air that play havoc with astronomers'
observations. Very thin mirrors, on the other hand, have ideal
thermal properties but a daunting physical handicap: as the
telescope pans across the sky, a thin mirror will bend and
wobble as if made of rubber.

    Between this Scylla and Charybdis, mirror designers are
charting a variety of bold, new courses. By designing the Keck
Telescope mirror as a mosaic of small segments, each the size
of a dining-room table, astronomer Jerry Nelson of the
University of California, Berkeley was able to make his mirrors
both rigid and thin. But to provide images of pinprick
sharpness, each segment must be kept perfectly aligned with its
neighbors, a task handled by an elaborate electronic network.

    By contrast, the mirrors designed for the European
Southern Observatory consist of a single, vast expanse of glass,
thin (17.7 cm) and very flexible. To control wobbling and
stabilize the orientation, these mirrors, like giant catcher's
mitts, will be constantly readjusted by 180 computer-activated
steel "fingers." A prototype mirror has already proved its
worth. A flaw identical to the one that crippled the Hubble
Space Telescope was easily corrected by adjusting the mirror's
shape.

    Angel's approach relies less on intricate control systems
and more on vitreous wizardry. The 10-ton mirror he and his
colleagues plan to install in Arizona -- merely a warm-up for
some 8-m versions -- boasts a light-collecting surface that is
nearly as wide as a house is tall, yet it averages only 2.8 cm
thick. What prevents this marvel from fracturing under its own
weight is a supporting truss composed of thousands of glass ribs
that are cast as part of the mirror's underlying structure.
Arrayed in a striking hexagonal pattern, the ribs form an airy
honeycomb that confers on the mirror the structural strength of
solid glass at one-fifth the weight. Because the hexagonal cells
are hollow, air can be circulated through them to keep the
mirror in constant thermal balance.

    Although the conceptual design appears straightforward,
the casting of a honeycomb mirror requires considerable
technical know-how -- and time. Angel's team tackles the job in
their hangar-like mirror lab located, improbably enough, under
the stands of the University of Arizona football stadium. In
the center of the lab is a huge round furnace. To make a
mirror, a complex ceramic mold is assembled inside the furnace
and filled with glittering chunks of Pyrex-type glass. Once the
furnace lid is sealed, the temperature will slowly ratchet up
over a period of several days, at times rising no more than 2
degrees C in an hour. At 750 degrees C (1382 degrees F), when
the glass is a smooth, shiny lake, the furnace starts to whirl
like a merry-go-round -- an innovation that automatically spins
the glass into the parabolic shape traditionally achieved by
grinding. At about 1150 degrees C, the liquid glass oozes into
the mold, filling the cells of the honeycomb.

    Cooling the mirror is an equally painstaking process that
takes many weeks. Reason: if one section of the glass cools
faster than another, it will contract more quickly, creating
stresses that lead to cracking. When finally unmolded, the
mirror will still require months of tedious polishing to remove
any imperfections.

    Why devote so much time and energy to increasing the size
of telescope mirrors? The quest is driven by science. To
understand how the universe evolved from the Big Bang to its
present form, astronomers strive to capture ever more fleeting
flecks of light that emanated from ancient galaxies billions of
years ago. A 10-m mirror increases their chances by providing
a light-gathering surface that is four times the area of a 5-m
mirror. Even bigger gains will be possible if astronomers
proceed with plans to link huge telescopes like the Keck I and
Keck II together, combining their light-catching power. The laws
of physics serendipitously ensure that such telescopic arrays
will also provide sharper images -- if spatial distortions in
the new thinner mirrors can only be held to a minimum.

    Of course, that is a big if. All the new mirror designs
are pushing the technological frontier, and already some
surprisingly nettlesome problems have arisen. "Naturally, the
challenges have come in places we least expected them," says
physicist Terry Mast, one of the scientists who is helping build
the Keck Telescope. For instance, the laborious procedure
developed for polishing the Keck's 36 mirror segments turned out
to warp them. A system of special harnesses has now been
developed to bend the segments to the correct curvature. So far,
Angel's mirrors appear to be free of serious problems, though
concerns persist that the honeycomb structure could interfere
with "seeing" by leaving a subtle quilted pattern on the
surface. Far outweighing any potential negatives, Angel
believes, is the fact that his mirrors, unlike the Keck and
European mirrors, do not require fancy computerized controls to
keep them optimally configured. "When we succeed in casting a
mirror," says Angel, "we've produced a piece of glass that makes
everything else easy."

    Right now, which design will prove best is anyone's guess.
"We'll know in 50 years," says Mast. But whatever the ultimate
outcome of this ethereal competition, it is clear that Angel's
creative hand will shape telescopes built for many years to
come. He and a team of graduate students are among many
astronomers racing to devise an "adaptive optics" system that
corrects for the turbulence of the earth's atmosphere. The
system affords ground-based instruments the heady illusion of
operating in the clairvoyant emptiness of space. Angel, in other
words, is on the verge of endowing his telescope mirrors with
wings.