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04.10
Engineering a New Window on the
Universe
By
Charles Blue
It’s easy to appreciate how
science and fundamental research advance
engineering. What’s less obvious, however, is
that the opposite is also true; engineering and
innovation advance scientific research.
Nowhere is that more clear than
in the development of what will be one of the
world’s most powerful and sophisticated
scientific instruments, the aptly named Thirty
Meter Telescope, or TMT.
When completed in 2018, the TMT
will be the largest telescope ever constructed,
giving astronomers unprecedented clarity to
study alien planets around distant stars, the
turbulent region near the black hole at the
center of our Milky Way Galaxy, and even the
very first stars at the edge of the observable
Universe.
Engineering the Eye of the
Telescope
When we say 30 meters (nearly
100 feet) we’re referring to the main mirror, or
eye, of the telescope. Compared to the
telescopes of today, thirty meters is a
remarkable increase in size and power. The
current generation of large optical telescopes
has mirrors that range from eight to ten meters
in diameter. By going all the way to 30 meters,
TMT will have nine times the light collecting
power and three or more times the resolution of
these world-class science instruments.
But this increase in size and
power comes with equally impressive engineering
challenges. Not the least of which is the fact
that a single mirror 30 meters in diameter would
be simply too massive to build and too unwieldy
to transport.
The solution to this dilemma was
developed in the mid 1990s by Jerry Nelson for
the twin Keck Telescopes on Mauna Kea in Hawaii.
Rather than a single 10-meter piece of glass,
the Keck mirrors are made up of 36 individual
segments that are kept in place so precisely
that they function as
one. This revolutionary design removed many of
the limitations on building giant optical
telescopes.
Today, engineers are scaling-up that technology
for TMT, which will have an astounding 492
segments. To keep all these segments in perfect
alignment, TMT will use 1,476 edge sensors to
monitor the shape of the mirror and 1,476
high-precision actuators to continually adjust
the orientation of the segments, ensuring the
telescope remains in perfect focus.
Though certainly a remarkable engineering
achievement, TMT’s primary mirror is only a part
of the story. For TMT to achieve its full
potential, engineers also must contend with
Earth’s chaotic atmosphere.
Engineering Clearer Views
Ground-based telescopes have a
number of advantages over space-based
telescopes. They are less restricted by weight
and size, and they also make it possible to
continually access the telescope for repairs and
upgrades.
But
by avoiding the cost and challenges of putting a
telescope into orbit, astronomers instead have
to contend with less-than-ideal viewing
conditions. Because our atmosphere is in
constant motion, with different thermal layers
and pockets of turbulence, it distorts the light
from objects in space. This gives stars their
quintessential twinkling appearance, but it also
blurs the sharpness of images and reduces the
amount of science that can be performed.
To
overcome this limitation, many telescopes now
use an innovation known as Adaptive Optics (AO).
These AO systems essentially take out the
twinkling of starlight, allowing telescopes to
study objects as clearly as if the telescope
were in space.
Many
adaptive optics systems now harness powerful
lasers to create what amount to artificial stars
high in Earth’s atmosphere. This is done by
making use of a thin, natural layer of sodium
atoms in the upper atmosphere. A laser with
precisely the right yellow-orange wavelength is
able to excite these sodium atoms, causing them
to glow, much like a sodium vapor streetlight.
By
carefully monitoring the backscatter of the
laser light, a device called a Wavefront Sensor
is able to continually measure atmospheric
turbulence. The sensor then sends commands to
hundreds-to-thousands of actuators that push and
pull on a small flexible, or deformable, mirror,
the heart of an AO system. The actuators
precisely change the shape of this mirror many
of hundreds of times each second, creating the
exact but opposite warping that the atmosphere
is causing. This returns the light wave to its
original flat shape, creating a sharper image
and enabling better science.
When
combined with the unprecedented size of the
primary mirror, this AO system and other
cutting-edge instruments will ensure TMT becomes
the most capable and advanced telescope ever
constructed.
The
current schedule has TMT beginning on-site
construction near the summit of Mauna Kea,
Hawaii, in 2011. The TMT project is an
international partnership among the California
Institute of Technology, the University of
California, and the Association of Canadian
Universities for Research in Astronomy. The
National Astronomical Observatory of Japan
joined TMT as a Collaborating Institution in
2008. The National Astronomical Observatories of
the Chinese Academy of Sciences joined TMT as an
Observer in 2009.
Charles Blue is the media relations specialist for
the Thirty Meter Telescope Project, and has more
than 20 years of strategic communications
experience in science, engineering, and
technology. He has worked as public information
officer for the National Academy of Engineering
and the National Radio Astronomy Observatory. He
also served as the writer/editor for the
National Science Foundation’s Directorate for
Engineering.
Comments may be submitted to
todaysengineer@ieee.org.
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