14 AUSTRALIAN SKY & TELESCOPE April 2019
TWIRLING TELESCOPES
Althoughthetelescopecouldn’tlookasdeeplyasitcould
before, every 7 minutes it covered 80% of the sky. This took
theblindersoffthesatelliteandledtotheteam’sgreatest
discovery. “We had looked at the Crab Nebula many times
during the first two years for calibration purposes, because it
is,orwassupposedtobe,astable‘boring’source,”explains
Tavani. But forced to look again in 2010 due to being in a spin,
theteamnoticedtheCrabNebulawasfarfromastablesource:
It was emitting gamma-ray flares. “The result was almost
heretical, creating tremendous excitement in the community,”
he says. “We would have never discovered it if it were not for
this spinning mode.”To see and survey
More commonly, spinning is less
a happy accident and more an
integral part of how the space
telescope observes the sky. Just
when NASA’s Parker Solar Probe
wassettingoffonitsjourney
to ‘touch the Sun,’ scientists
finally lost contact with another
distinguished solar observatory
calledtheReuvenRamatyHighWTHE CRAB This composite of the Crab Nebula combines data from
across the electromagnetic spectrum: radio waves (red), infrared (yellow),
optical (green), ultraviolet (blue) and X-ray (purple). Fortuitous AGILE
observations helped reveal that, much to astronomers’ amazement, the
Crab produces gamma-ray flares. Why remains unclear.Precession rate: 1rph
22.5° half-angleSpin rate:
0.464 rpm
A-side
line of sightB-side
line of sightEarthSun1.5× 106 km1.5× 108 kmWMAP at L 2North Ecliptic PoleSouth Ecliptic Pole+90° +45° –45° –90°XWMAP’S STRATEGY Orbiting at L 2 , WMAP
gyrated in three ways (left). The spacecraft spun
around its axis about every half minute, and this axis
wobbled around in a circle every hour, tracing its two
lines of sight through a complex crisscrossed pattern
(centre). The craft continued this bobbing motion as
it revolved around the Sun, enabling it to scan the
whole sky (right).Energy Solar Spectroscopic Imager (RHESSI). However,
during its 16 years of operation RHESSI made a number
of discoveries about solar flares and other aspects of solar
physics and astrophysics. And none of it would have been
possible without spinning.
RHESSI’s aim was to make movies of solar flares in X-rays
and gamma rays in order to understand solar-flare physics.
But imaging X-rays — particularly high-energy or ‘hard’
X-rays — is difficult. This is because they don’t easily reflect
off mirrors due to their sub-atom-scale wavelengths, which
endow them with the ability to penetrate deeply into matter.
To get around this, a pair of grids known as a collimator
was placed in front of each of nine detectors. On their own,
these detectors couldn’t create an image; they solely picked up
the energy and arrival time of each detected photon. But by
allowing the spacecraft to spin, the area of the detector visible
through both grids from the Sun’s perspective changed with
time. When the solar source of the X-rays was slightly off-
centre, the X-ray photons were modulated, which the detectors
registered as a variation in photon intensity with time. This
difference encoded the location and size of the X-ray sources.
Computers on the ground then decrypted this information to
construct a series of snapshots of the Sun’s flares.
Many all-sky surveys also spin to map the known
universe. For example, the two most recent cosmic microwave
background (CMB) space observatories, NASA’s Wilkinson
Microwave Anisotropy Probe (WMAP, 2001–10) and the
European Space Agency’s (ESA’s) Planck (2009–13), each
deployed very different scanning strategies to interrogateWMAP: NASA / WMAP SCIENCE TEAM; CRAB NEBULA: X-RAY: NASA / CXC / SAO, OPTICAL: NASA / STSCI, INFRARED: NASA / JPL / CALTECH, RADIO: NSF / NRAO / VLA, ULTRAVIOLET: ESA / XMM-NEWTON