Sky and Telescope - January 2017 - 11

SCOPES
t FAST, seen here at its opening ceremony,
is now the world's largest filled-in, single-dish
radio telescope.

SCOPES SCENE: JEFF DAI; SCOPES WORK ERS: CHINESE ACA DEM Y OF SCIENCES

Chinese FAST Opens
for Business
ON SEPTEMBER 25TH the Chinese
Academy of Sciences announced the
completion of its new Five-hundredmeter Aperture Spherical Telescope
(FAST). With a dish 500 m across and
an area equal to 30 soccer ﬁelds, this
behemoth is now by size the largest
ﬁlled-in, single-dish radio telescope in
the world. A Nimitz-class aircraft carrier could easily ﬂoat in the dish from
bow to stern, with room to spare.
Only the 576-m diameter of Russia's
RATAN-600 system is larger than FAST's.
It's not all about size, though: RATAN600's collecting surface is a ring rather
than a dish, so it only has an effective
area of 12,000 m2. And FAST's effective
aperture is only 300 m, corresponding
to a collecting area of 70,000 m2. But
that's still more than twice the collecting area of Arecibo in Puerto Rico.
FAST is located in southern China in
the sparsely populated Guizhou Province. Like Arecibo, the telescope sits
inside a natural depression in limestonedominated terrain. It will use the surrounding, mountainous karst features
to block out radio interference, which
ofﬁcials have limited by relocating more
than 9,000 people from the surrounding
area to create a radio-free zone.
The telescope has already made its
ﬁrst observation and now enters an
extensive commissioning phase. This
is when bugs in the system manifest
themselves, and rumor has it they're
already cropping up. But assuming
scientists overcome any hurdles, they
will soon set to pursuing FAST's science
goals, which include hunting for faint
pulsars, mapping neutral hydrogen

in distant galaxies, and searching for
extraterrestrial signals.
FAST's expected resolution is 2.9
arcminutes at the key hydrogen emission
line of 1420 MHz (21 cm in wavelength),
compared with Arecibo's 3.4 arcminutes
for the same frequency. To achieve this
resolution, FAST uses actuators to push
and pull on a 300-m-wide subset of panels in order to morph its spherical surface
into a near-paraboloidal shape. Overall
the new instrument covers a slightly
smaller frequency band than its Puerto
Rican counterpart: 70 MHz to 3 GHz,
versus Arecibo's 47 MHz to 10 GHz.
Located at latitude 26°N, the radio
telescope looks straight up, allowing
Earth's rotation to "point" the dish as
the sky sweeps past. As with Arecibo,
FAST's receivers, suspended high above
the dish, are only partially steerable. But
the Chinese observatory's signiﬁcantly
deeper dish affords it a wider ﬁeld of
view: it can cover a swath of sky within
40° of its zenith, versus Arecibo's 20°.

FAST construction workers

FAST carries on the tradition of
large radio telescope construction that
started shortly after World War II and
culminated in the Puerto Rican observatory, which since 1963 held the title
of largest single, ﬁlled-in dish telescope.
It's possible that FAST's record-setting
dimensions will never be exceeded:
future radio observatories such as
ALMA and the upcoming Square Kilometer Array are "dish farms," using the
interference between the radio signal
collected by many dishes to assemble
high-resolution images.
■ DAVID DICKINSON

IN BRIEF
Pro-Am Team Finds
First Pulsing White Dwarf
A dedicated study by amateur and professional astronomers has revealed that the
long-watched variable star AR Scorpii is in
fact a binary system. Reported in the September 15th Nature, the observations show
that AR Sco comprises a cool red M star and
a white dwarf, the latter of which generates
a lighthouse-like beam of particles. The two
orbit each other every 3.6 hours in a precise
cosmic dance. The white dwarf lashes its
companion with its energetic beam as it
rotates, causing the whole system to brighten
and fade dramatically every 1.97 minutes.
■ MONICA YOUNG
See page 84 for amateur and study coauthor
Josch Hambsch's behind-the-scenes tale of
being part of the discovery.

Osiris-REX Bound
for Asteroid Bennu
On September 8th NASA's Origins, Spectral Interpretation, Resource Identification,
Security, and Regolith Explorer (Osiris-REX)
spacecraft launched from Cape Canaveral. Its
objective is to collect a sample of at least 60
grams (and as much as 2 kg) of the Earthcrossing Apollo asteroid 101955 Bennu and
bring the sample back to Earth in 2023 for
study. If successful, the souvenir will represent
the largest return of extraterrestrial matter
since Luna 24 traveled to and from the Moon
in 1976. Osiris-REX is scheduled to attempt
its land grab in July 2020. It's the third mission
in the New Frontiers program, after Juno and
New Horizons.
■ DAVID DICKINSON
Read more at https://is.gd/osirisrexlaunch.

Daily Alert for Asteroid Flybys
A new e-digest from the International Astronomical Union's Minor Planet Center gives the
public a head's up on asteroids passing near
Earth. The alert service, called the Daily Minor
Planet, sends an e-mail once a day to your
inbox with information about notable nearEarth objects. It includes the time of closest
approach, an orbit diagram, and other details.
The digest is a public outreach initiative and
isn't designed for observers; you'll need an
ephemeris generator to spot these objects.
■ CAMILLE M. CARLISLE
Learn how to sign up for the daily email at
https://is.gd/dailymp.

https://www.is.gd/osirisrexlaunch https://www.is.gd/dailymp

Sky and Telescope - January 2017

Table of Contents for the Digital Edition of Sky and Telescope - January 2017