Sky and Telescope - May 2016 - 31

Deep (and Classiﬁed) Roots

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Many early scientists, even Isaac Newton, wrestled with
the problem of atmospheric distortion, but the real
advances didn't begin until the 1970s. That's when the
Pentagon was working on something seemingly unrelated to astronomy: it needed a way to focus
a laser beam on a distant target, which
meant protecting the beam from choppy
wind. At the same time, DARPA, an
agency within the Department of
Defense, wanted to identify satellites
launched by the Soviet Union.
Even at a good location, atmospheric turbulence smears out details
smaller than 1 arcsecond across. That's
good enough to see the cylinder-shaped
TI
IS
Hubble, which is similar in size to most
HR
YC
B
G
spy satellites, but not good enough to make
S SIN
PRO CE
out details. The military needed a way to do better.
If scientists could accurately measure how the
atmosphere is moving, they could send that information
along to a ﬂexible secondary (or tertiary) mirror. In principle, this deformable mirror would exactly cancel distortions introduced by the atmosphere into the primary
mirror's image, sculpting the rays of light (from a satellite or any other target) back to near-perfect alignment.
One of the ﬁrst AO demonstrations was installed in
1980 on DARPA's 1.6-meter telescope in Maui. It used
168 piezoelectric actuators, which expand or contract
in response to applied voltage, to very slightly bend a
deformable mirror. Today, AO systems might contain
many thousands of these mirror movers, thanks to
improvements in their manufacturing, positioning,

GEMINI OBSERVATORY

So on the fourth night, after the team of engineers
responsible for the tests headed to bed, Macintosh took
matters into his own hands. "That was the night where
we said, 'y'know, let's just point it at a damn planet
and see what it looks like.'" The team slewed the telescope toward Beta Pictoris, a star 63 light-years
from Earth with a hot, young giant planet
that orbits its star at almost twice the
distance that Jupiter orbits the Sun.
Then they waited - but not for long.
Within 60 seconds a lump had materialized on the screen before them.
The same detection prior to GPI
would have taken an hour to image
and days to process. Could the new
instrument capture an exoplanet in
only a minute? The astronomers ﬁlling
the room remained skeptical. They frantically grabbed their laptops and searched for any
papers that might show an image of the planet Beta
Pictoris b. They then scrutinized the screens, holding
their laptops sideways to better match the image's orientation on the observatory's computer screen. Sure
enough, the images aligned. Before their eyes was a
newborn gas giant, seen more clearly than ever before.
Spotting the exoplanet right oﬀ the bat was an
incredible feat, says GPI chief scientist James Graham
(University of California, Berkeley). "It doesn't require
any detailed analysis. It doesn't require crunching the
numbers. It's just completely evident in the raw data that
an exoplanet is there."
But the basis of GPI's success was decades in the
making. Even 2,700 meters (8,900 feet) above sea level,
Gemini South still sits beneath an ocean of air. So the
telescope uses adaptive optics (AO) to correct for the
turbulent atmosphere. In GPI, more than 4,000 actuators spaced just 400 microns apart deform the instrument's secondary mirror to exactly match and cancel out
atmospheric distortion. Without AO, light from planets,
stars, and galaxies would dance and distort, like pebbles
seen beneath a ﬂowing stream. The colossal observatory
wouldn't see any sharper than a backyard scope. But
with AO, images steady themselves, allowing astronomers to pick out ﬁne details.
The technology that makes this feat possible was
born in the 1970s in classiﬁed government meetings. A
few select Air Force scientists and astronomers worked
together to design early versions of laser guide-star
systems before the project was declassiﬁed in 1991. After
several decades of innovation, AO is still improving with
each new generation. Today, ground-based telescopes
such as Gemini South can exceed the clarity of the
Hubble Space Telescope. And the next generation of AOequipped mega-telescopes will push the boundaries of
sight even further.

BETA PIC Top: One of the ﬁrst images the Gemini Planet Imager took was of
the disk around Beta Pictoris, a star known to host a planet. With just a minute-long exposure, the newborn gas giant appeared on screens in the control
room. Above: The Gemini Planet Imager's ﬁrst-light images brought elation to
the team in the control room at the Gemini South telescope in Chile.

Sk yandTelescope.com May 2016

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Sky and Telescope - May 2016

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