space@mit-mc (02/13/85)
From: Ross Finlayson <rsf@Pescadero>
n104 2041 11 Feb 85
BC-TELESCOPES 2takes
(Science Times)
By WALTER SULLIVAN
c.1985 N.Y. Times News Service
NEW YORK - Radical new techniques for making giant telescopes are
being developed, holding the promise of opening up new views of the
universe. These large-scale optical devices, some of them planned to
be more than twice the size of the biggest telescopes operating
today, will be able to look far enough out across the universe, and
therefore back in time, to record its infancy. They should also be
able to look deep into the core of the Milky Way and into other
galaxies to seek out the nature of the mysterious ''engines''
generating vast amounts of energy there.
Conventional telescopes have been limited in size by the extreme
difficulty of casting very large blocks of glass, polishing them into
large mirrors of exactly the necessary shape, and then maintaining
that shape despite the effects of temperature and sag as they sweep
the skies. The new telescopes, all of them proven in tests but not
yet in full-scale operation, are being made possible by such
innovations as mass production of large mirrors in swirling baths of
molten glass, liquid mirrors formed of large, fast-spinning basins of
mercury, and the use of multiple conventional telescopes linked
together in the fashion of the large arrays of antennas used in radio
astronomy.
How soon the new methods will be put to use will depend on the rate
at which funds become available, and on success in overcoming
problems that will inevitably arise in bringing the new technologies
involved to full implementation.
The telescope most likely to achieve ''first light'' - the initial
passage of starlight through its optical system - is the 10-meter
(394-inch) telescope to be built in Hawaii by the California
Institute of Technology and the University of California. The project
seems almost assured by a conditional offer of $70 million to Cal
Tech by the foundation formed by the late W.M. Keck, founder of the
Superior Oil Co. His son, Howard B. Keck, is a trustee of the
California Institute of Technology.
The telescope, to be known as the Keck Observatory, would be far
larger than the Soviet Union's six-meter reflector in the Caucasus
mountains or the five-meter one on Mount Palomar in California,
currently the world's two largest reflecting telescopes. The mirror
for the Keck Observatory, formed of 36 hexagonal segments, would be
twice the width of the one at Palomar.
According to Dr. Marvin L. Goldberger, president of Caltech, it will
''provide answers to the most challenging and basic questions of the
universe.'' Among them, he said, are ''how the universe originated,
whether or not the universe is continually expanding or will
ultimately fall back on itself, why and how galaxies and stars formed
and evolved, and how the four basic forces of nature controlled the
early history of the universe.''
The mirror's segments will have to be formed in various asymmetrical
shapes to form a parabolic mirror when fitted together. They are to
be produced by a novel ''bend and polish'' method developed by Dr.
Jerry E. Nelson of the University of California, at Berkeley. The
glass is distorted according to an elaborate mathematical formula
developed by his colleague, Dr. Jacob Lubliner, and is then ground by
a machine producing curvature similar to that on the inside of a
sphere. When the tension is removed the glass springs into the
desired part of the mirror's shape.
During observations the segments will have to be constantly adjusted
to maintain the entire assemblage's precise shape despite sag and
temperature effects. Such a system is under development at Berkeley
as well as a way to cut the finished segments into hexagons without
altering them.
To test the grinding method, a small-scale segment has been ground
there; a full-scale one, two meters in diameter, is in the final
polishing stages in optical shops of the National Optical Astronomy
Observatories in Tucson, Ariz., which administers several national
observatories including the one on nearby Kitt Peak.
A site for the Keck Observatory has been allocated on a 13,600-foot
ridge atop Mauna Kea, a supposedly extinct volcano on the island of
Hawaii. It would join a growing population of observatories there,
including a 7.5-meter reflector planned by the Japanese. As pointed
out by Dr. Jesse L. Greenstein of Caltech in the February issue of
Physics Today, the Keck telescope, with its huge light-collecting
area, will be able to conduct observations far beyond the reach of
the Space Telescope to be launched in 1986.
A completely different approach has been proposed for the even
larger New Technology National Telescope that would probably be built
either on Mauna Kea or on 10,720-foot Mount Graham, 76 miles
northeast of Tucson. Its four mirrors, with the combined power of a
15-meter telescope, would ride in a single mount as in the Multiple
Mirror Telescope on Mount Hopkins south of Tucson.
It has been the success of that novel assembly, operated jointly by
the University of Arizona's Steward Observatory and the Smithsonian
Astrophysical Observatory, that led to the choice of such a design
over the one planned for the California telescope, although both
approaches were considered viable.
The six mirrors of the Multiple Mirror Telescope on Mount Hopkins,
each 1.8 meters (72 inches) in diameter, are installed like guns on a
large naval gunmount and their images combined by a system of optics.
Mirrors for the National New Technology Telescope, or NNTT, would be
more than four times as wide. Dr. Roger Angel and Dr. Neville Woolf
of the Steward Observatory in Tucson believe they can minimize their
cost by casting the mirrors from molten glass in a rapidly rotating
electric oven. A bowl of fast-spinning fluid assumes the parabolic
shape required for mirrors that focus light waves from a distant
source to a single point.
A two-meter (79-inch) mirror has already been cast by rotating the
oven 16 times a minute, although, according to Woolf, it took some
time to find a technician who could ride the control console of the
spinning oven without becoming dizzy.
This will not be a problem with the giant new oven, capable of
casting mirrors eight meters wide. It will spin 10 times a minute
under remote control. Housing for the oven is being built under the
university stadium.
While the glass surface that solidifies as the oven cools is
parabolic, it is far from perfect enough to serve as a mirror. For
further polishing a computer-controlled lapping device has been
designed to produce such a surface, rather than one that is spherical.
Budget cuts and constraints in Washington have diminished short-term
prospects for the NNTT, particularly if the California project moves
ahead, but development of the requisite technology is proceeding in
expectation that it will ultimately be built.
Meanwhile, Canadian astronomers have revived a long-discussed scheme
for producing large mirrors by spinning basins of mercury. In 1924,
as Mars began to make an unusually close approach to the Earth, David
Todd, head of the astronomy department at Amherst College, proposed
rotating a mercury-filled basin 50 feet wide at the bottom of an
abandoned mine shaft in Chile.
The shaft was so located that Mars would pass directly overhead, and
Todd said his instrument would make it appear only two miles away,
making it possible to determine whether the planet was inhabited.
Astronomers ridiculed the scheme, saying atmospheric effects would
produce a meaningless blur.
Nevertheless, astronomers at Laval University in Quebec have been
testing such mirrors. E.F. Borra of that group has proposed that
mirrors 30 meters (98 feet) wide could be achieved, and some with a
width of 1.65 meters (65 inches) have been tested.
The Canadians reported to an international conference on very large
telescopes in Garching, West Germany, last April that the results
''seem to indicate that large liquid mirrors are feasible.'' To
produce a sharp image, however, the rotation must be extremely stable.
Such a mirror would have to be aimed vertically, but by means of
movable mirrors it could scan the rotating canopy of the heavens. The
cost could be kept very low by placing the mirror at the bottom of a
standard farm silo.
The Steward Observatory group hopes its spinning oven can produce
mirrors not only for the National New Technology Telescope but for
its own projected observatory on Mount Graham and for a
multi-telescope European observatory in Chile.
The European Southern Observatory, based at Garching near Munich but
with its prime observing site at La Silla, Chile, is building a
3.5-meter (138-inch) New Technology Telescope as the final step
toward a Very Large Telescope, which would consist of four
eight-meter telescopes with the combined power of a 16-meter
instrument.
The tentative plan is to align the telescopes 30 meters (98 feet)
apart on a summit in northern Chile. They would be linked optically
to produce combined images. At infrared wavelengths they could also
be used in pairs for extremely detailed surveys of such critical
areas as the cores of the Milky Way and other galaxies, using the
technique known as interferometry.
The core of the Milky Way cannot be observed at visual wavelengths
because of intervening dust and gas. The California plan provides for
ultimate construction on Mauna Kea of a second large telescope linked
to the first for similar interferometric observations.
Interferometry has enabled radio astronomers, using multiple
antennas, to map distant sources of radio emission in extraordinary
detail. Because wavelengths of light are much shorter the technology
is far more demanding. The distances traveled by light waves from two
telescopes, when brought together to produce the interference
phenomenon, must be identical with high precision.
A French group led by Antoine Labeyrie has been testing this
approach at the observatory of CERGA (Centre d'Etudes et de Recherche
Geodynamiques et Astronomiques) in the Alps overlooking the French
Riviera. Two 10-inch telescopes are held in spherical mounts that can
be rotated in the manner that a circus animal, on its back, spins a
large ball with its feet.
''Reproducing the natural walking motion of animals requires very
sophisticated software,'' the meeting in Garching was told. The
mounts ride on a north-south rail line that permits various
separations. Trials at distances up to 52 feet have shown the method
applicable at infrared wavelengths (longer than those of visible
light) but susceptible to such subtle effects as Earth tremors that
vary path lengths through the array.
For Europe's Very Large Telescope the group has proposed an array of
four instruments in spherical mounts riding on a grid of north-south,
east-west rail lines. Several U.S. observatories hope to use
interferometry between smaller telescopes to track star movements
precisely enough to see if they are influenced by planetary
companions.
The University of Texas has been planning a 7.6-meter telescope in
West Texas but its prospects have been dimmed by the drop in oil
revenues that account for much of the university's income.
nyt-02-11-85 2353est
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