Sky and Telescope - May 2016 - 34

Adaptive Optics

BEFORE AND AFTER Without adaptive optics, a near-infrared image
of Uranus appears blurry (left). When the same image is taken with AO
technology turned on (right), the faint and fuzzy ring resolves into several distinct rings, and small storms within the atmosphere are revealed.

TUNING IN TO
SGR A* Astronomers used complex techniques
to reveal action in
our galaxy's center
before adaptive
optics (top).
With the advent
of AO (bottom),
astronomers
have pinpointed
and tracked the
minute motions
of individual stars
as they careen
around Sgr A*, our
galaxy's supermassive black hole.
The larger square
ﬁelds are 6″ on a
side, insets span
1″, and all images
were taken at
wavelengths near
2.2 microns.

Adaptive
Optics OFF

Adaptive
Optics ON

T. NAKAJIMA / S. DURRANCE; S.
KULKARNI / D. GOLIMOWSKI /
NASA; M. CHUN / NICI TEAM

UCLA GALACTIC CENTER GROUP / W.M. KECK OBSERVATORY LASER TEAM (2)

HEIDI B. HAMMEL AND IMKE DE PATER / WMKO

Find a full gallery of before-and-after images at
http://is.gd/adaptiveoptics.

ADAPTIVE OPTICS REVOLUTION The AO-enabled Palomar Observatory discovered the ﬁrst brown dwarf, Gliese 229B, tucked within the glare of
its companion star (left). Hubble followed up a year later (center) to help pin
down its orbit. After a decade, an image captured by the Near-Infrared Coronagraph and AO system on the Gemini North telescope (right, in a slightly
larger ﬁeld of view) shows how far the technology has come.

May 2016 sky & telescope

at 1 micron, but only because it ﬂies about 350 miles
above Earth's surface. With the advent of adaptive optics,
one of Keck's 10-meter telescopes (among the largest
observatories in the world) can resolve details as ﬁne as
0.04 arcsecond - producing images more than twice as
crisp as Hubble's.
In 1999, the Keck Observatory placed its ﬁrst natural
guide-star system on Keck II and a year later on Keck
I. With better deformable mirrors and even the ability
to separate light into its constituent wavelengths, this
system was radically improved in comparison to earlier
AO counterparts.
Such razor-sharp vision enabled astronomers to peer
into the crowded environment at the center of the Milky
Way Galaxy. Before getting access to Keck's adaptive
optics, Andrea Ghez (University of California, Los Angeles) and her team had used a camera that took exposures
every few milliseconds to create stacked images of this
region. But from 1995 to the present day, Ghez's team has
been able to watch stars orbiting the Milky Way's center at
closer distances than ever before. These previously hidden
stars whip as close as 45 astronomical units to the center,
at speeds up to 12,000 kilometers per second (roughly
4% the speed of light). Yet the center appears empty. The
source of gravity that's ﬂinging the stars in their speedy
orbits can't be anything but a supermassive black hole.
Although Ghez herself is modest about this achievement, other astronomers think her research is without a
doubt the best example of AO's successes. "I don't have
words to express how stunning that is," says GPI's Macintosh. "It's both just visually stunning to watch as a human,
and scientiﬁcally it's really, really important."
Then in 2003, Keck II upgraded its AO system to
use a sodium laser sent from Max's team at Livermore
(Keck I followed suit a few years later). Peter Wizinowich, who leads AO development at Keck, has spent
the past two decades improving the technology. He has
commissioned new lasers, better deformable mirrors,
and faster code to work seamlessly between the two. As
such, Keck's AO system is the most productive one in
astrophysics: to date, it's responsible for roughly 70% of
refereed-science papers that use adaptive optics.
Orbits seem to be the theme of Keck's ﬁrst AO
results. A couple years after Ghez's work, a team including Macintosh used the Keck II and Gemini North
telescopes to directly image three pinpricks of infrared
light around HR 8799, a 6th-magnitude star in the
constellation Pegasus. In theory such direct images of
exoplanets could reveal their composition, climate, and
even possibilities for life. Though this ﬁrst image wasn't
yet up to that task, the discovery laid the groundwork for
direct-imaging systems (S&T: Oct. 2015, p. 16).

http://www.is.gd/adaptiveoptics

Sky and Telescope - May 2016

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