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October
9, 2008: A team of internationally renowned astronomers
and opticians may have found a way to make "unbelievably
large" telescopes on the Moon.
"It's
so simple," says Ermanno F. Borra, physics professor
at the Optics Laboratory of Laval University in Quebec, Canada.
"Isaac Newton knew that any liquid, if put into a shallow
container and set spinning, naturally assumes a parabolic
shape—the same shape needed by a telescope mirror to bring
starlight to a focus. This could be the key to making a giant
lunar observatory."
Borra,
who has been studying liquid-mirror telescopes since 1992,
and Simon P. "Pete" Worden, now director of NASA
Ames Research Center, are members of a team taking the idea
for a spin.
Right:
An artist's concept of a spinning liquid mirror telescope
on the Moon. Credit: Univ. of British Columbia.
On
Earth, a liquid mirror can be made quite smooth and perfect
if it its container is kept exactly horizontal and rests on
a low-vibration low-friction air bearing that is spun by a
synchronous motor having one stable speed. "It doesn't
need to spin very fast," says Borra. "The rim of
a 4-meter–diameter mirror—the largest I've made in my lab—travels
only 3 miles per hour, about the speed of a brisk walk. In
the low gravity of the Moon, it would spin even slower."
Most
liquid-mirror telescopes on Earth have used mercury. Mercury
remains molten at room temperature, and it reflects about
75 percent of incoming light, almost as good as silver. The
biggest liquid-mirror telescope on Earth, the Large Zenith
Telescope operated by the University of British Columbia in
Canada, is 6 meters across—a diameter 20 percent larger than
the famous 200-inch mirror of the Hale telescope at Palomar
Observatory in California. Yet when completed in 2005, the
Canadian Palomar-class liquid-mirror telescope cost less than
$1 million to build—only a few percent the cost of a solid-mirror
telescope of the same diameter--and, for that matter, only
a sixth of Palomar's original cost in 1948.
Those
economics are making astronomers sit up and begin noodling out
plans for a lunar observatory.
"Our
study [with Borra] started when I was still an astronomy professor
at the University of Arizona before I came to NASA in 2006,"
Worden recalls. "The real appeal of this approach is
that we could get an unbelievably large telescope on the Moon."
Mercury
is unworkable on the Moon: it's very dense and thus heavy
to launch, it's very expensive, and it would evaporate quickly
when exposed to the lunar vacuum. In recent years, however,
Borra and his colleagues have been experimenting with a class
of organic compounds known as ionic liquids. "Ionic liquids
are basically molten salts," Borra explains. "Their
evaporation rate is almost zero, so they would not vaporize
in the lunar vacuum. They can also remain liquid at very low
temperatures." He and his colleagues are now seeking
to synthesize ionic liquids that remain molten even at liquid-nitrogen
temperatures.
Below:
The University of British Columbia's 6-meter Large Zenith
Telescope uses a liquid mirror to scan the heavens. [more]
Much
less dense than mercury, ionic liquids are only slightly denser
than water. Although not highly reflective themselves, a spinning
mirror of an ionic liquid can be coated with an ultrathin
layer of silver just as if it were a solid mirror. Weirdest
of all, the silver layer is so thin—only 50 to 100 nanometers—that
it actually solidifies. In the vacuum of space, a liquid mirror
coated with a thin solid layer of silver would neither evaporate
nor tarnish.
A
liquid mirror can't be tilted away from the horizontal because
the fluid would pour out, destroying the mirror. But that
does not mean a liquid mirror telescope cannot be pointed.
Optical designers are now experimenting with ways of electromechanically
warping secondary mirrors suspended above a liquid mirror—or
even slightly warping the liquid mirror itself—to aim at angles
away from the vertical. Similar techniques are used to point
the great Arecibo radio telescope in Puerto Rico.
Furthermore,
says Borra, "if the telescope is located anywhere other
than exactly at the poles, with each rotation of Earth or
Moon it would scan a circular strip of sky. And the rotational
axis of the Moon wobbles with a period of 18.6 years; so over
a period of 18.6 years, the telescope would actually look
at a good-sized region of the sky."
Right:
The 1000-ft Arecibo radio telescope in Puerto Rico cannot
be moved, but it can still scan a wide swath of sky using
movable secondary mirrors. A lunar liquid mirror telescope
might employ similar techniques. [more]
Locating
a major liquid-mirror telescope near the lunar poles is appealing.
The telescope itself could reside near the bottom of a permanently
shadowed crater where it would stay at cryogenic temperatures,
desirable for the best infrared astronomy. Yet solar panels
could be erected on nearby permanently illuminated mountain
peaks to generate power to keep the mirror spinning.
The
fact that a liquid-mirror telescope always looks straight
up vastly simplifies its construction and reduces mass by
eliminating heavy mounts, gearing, and pointing-control systems
needed for a steerable telescope. "All you'd need is
the liquid-mirror container, which might be an umbrella-like
device that self-deploys, plus a nearly frictionless superconducting
bearing and its drive motor," Borra says. Worden estimates
that all the materials for an entire lunar telescope 20 meters
across would be "only a few tons, which could be boosted
to the Moon in a single Ares 5 mission in the 2020s."
Future telescopes might have mirrors as large as 100 meters
in diameter—larger than a football field.
"A
mirror that large could peer back in time to when the universe
was very young, only half a billion years old, when the first
generation of stars and galaxies were forming," Borra
exclaimed. "Potentially more exciting is pure serendipity:
new things we might discover that we just don't expect."
Says
Worden: "Putting a giant telescope on the Moon has always
been an idea of science fiction, but it soon could become
fact."
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Author: Trudy E. Bell
| Editor:
Dr. Tony Phillips | Credit: Science@NASA
more
information |
An
article describing a potential lunar telescope 20 to
100 meters across is "Metal Films Deposited on
Liquids and Implications for the Lunar Liquid Mirror
Telescope," by Ermanno F. Borra, Omar Seddiki,
Roger Angel, Daniel Eisenstein, Paul Hickson, Kenneth
R. Seddon and Simon P. Worden Nature vol. 447 (2007),
pp. 979-981.
This was not a new concept for Borra, who on May 20,
1991 published "The Case for a Liquid Mirror in
a Lunar-Based Telescope" in The Astrophysical
Journal (vol. 373, pp. 317–321). Borra, Worden,
Roger Angel, and eight other experts co-authored another
article called "A Lunar Infrared Telescope to Study
the Early Universe," published in The Astrophysical
Journal in 2008 (vol. 680, beginning p. 1582
An
account of the Palomar-class 6-meter telescope built
for only about half a million dollars appears in "The
Large Zenith Telescope: A 6 m Liquid-Mirror Telescope,"
by Paul Hickson et al., in the Publications of the Astronomical
Society of the Pacific in April 2007 (vol. 119,
pp. 444–455). For an account of earlier attempts, see
"Liquid Mirror Telescopes: History," by Brad
K. Gibson, published in the Journal of the Royal
Astronomical Society of Canada (vol. 85, no. 4,
pp. 158–171, 1991).
Ground-based
astronomers at the California Institute of Technology
and at other international centers are pursuing concepts
for ultralarge astronomical instruments on Earth using
liquid-mirror telescopes. Among those is the International
Liquid Mirror Telescope project.
Liquid
mirror telescopes are a reality at last, New Scientist,
June 2, 2008
NASA's
Future: US
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