Sky and Telescope - May 2016 - 35

Star
Artiﬁcial
guide star

S&T: GREGG DINDERMAN

Deformable
secondary mirror

Primary mirror
Beam splitter

Beam splitter
Computer

Camera

Computer

The Next Generation: Megascopes
But the most expensive and ambitious spree of telescopes - and their accompanying adaptive optics - has
only just begun. Giant observatories currently under
development include the Thirty Meter Telescope on
Mauna Kea, the European Extremely Large Telescope in
Chile, and the Giant Magellan Telescope also in Chile.
When it comes to building billion-dollar behemoths,
adaptive optics is a must. Increasing the size of these
telescopes' primary mirrors would mean nothing if the
atmosphere were to limit their resolution to 1 arcsecond.
But cancel out the atmosphere, and a 30-meter telescope
could spot objects as small as 0.008 arcsecond across.
Megascopes will need improvements in laser technology if they're to implement AO. While the light from a
star inﬁnitely far away will fall through the atmosphere
in a cylinder - its rays of light perfectly parallel - a
sodium laser guide star's light falls through a tall cone
that peaks at the sodium layer. Since its rays aren't perfectly parallel, a single sodium guide star can't perfectly
mimic a star. The larger the telescope, the more that
diﬀerence begins to matter.
"The 30-meter telescopes are going to require not a
single laser but a grid of laser beacons, each of them with
their own cone, to try to reconstruct the atmospheric turbulence in three dimensions," Macintosh says. Demonstration systems have been deployed at the Gemini South
telescope and the Very Large Telescope. A similar system
is being readied for the Keck II telescope. Ghez can't wait
because she expects to see 10 times the number of stars
in the galactic center as she's seen before.
Even space-based telescopes will one day utilize a
form of adaptive optics. Though they ﬂy high above the
atmosphere, their AO will correct imperfections in the
optics themselves. Hubble, for example, slips between

Camera

TO INFINITY A laser
guide star shines down
from a layer in Earth's
atmosphere - a lot closer
than real stars, which might
as well be inﬁnitely far
away. As a result, artiﬁcial
starlight shines down in a
cone (left), rather than in
the cylindrical shape of real
starlight (right). So atmospheric turbulence won't
aﬀect light from an artiﬁcial
star in the exact same way
as light from a real star.
Future AO systems will use
multiple laser beams to correct for turbulence within a
larger area of the sky.

day and night roughly every 45 minutes. This change
warms and cools the spacecraft and changes its focus.
Although the beloved space telescope doesn't have AO,
its successor, the James Webb Space Telescope, will
launch with a built-in AO system.

Stumbling Blocks
It's safe to say that laser adaptive optics has revolutionized astronomy, but it does present several observing
challenges. Contrary to popular belief, artiﬁcial laser
guide stars don't allow astronomers to see any celestial
object on any clear night of the year. Astronomers still
need a second (real) guide star - albeit very faint - to
make a few basic corrections. With this restriction, laser
guide-star AO currently covers 70% of the sky, largely in
the galactic plane where there are more stars.
Also, sodium beacons don't work perfectly every day
of the year. The blanket of sodium in the upper atmosphere thickens every time Earth tumbles into a stream
of meteors. So the sodium layer will be densest September through December, after the brightest meteor
showers of the year (namely the Perseids, Geminids, and
Orionids) deliver their bits of sodium. Unfortunately,
this is when the weather is often poorer. During the
optimum observing conditions of summer, astronomers
may struggle to produce a bright-enough beacon of light.
Finally, there's one more minor - and sometimes
amusing - issue: local aviation. "Small private planes
are like moths," says Marshall Perrin (Space Telescope
Science Institute). "They're drawn to the light." When
Perrin was at Lick Observatory over a decade ago, his
team wouldn't operate the laser until 11 p.m., when most
of the general aviation was done. So-called "aircraft spotters" still work today at the largest telescopes to search
for planes.
Sk yandTelescope.com May 2016

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Sky and Telescope - May 2016

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