Sky & Telescope - September 2016 - 48

OBSERVING

Celestial Calendar

Map Asteroid Shapes by Video
Join the worldwide project to time asteroid occultations precisely.
It's cheaper and easier than ever.
S TEPWISE OCCULTATION
Have you ever dreamed of doing
frontline solar-system research with a
small backyard telescope? You actually
can, even in an age when spacecraft study
planets, moons, asteroids, and comet
nuclei close up. One way is by making
a particular type of measurement that
citizen scientists can perform with very
high accuracy using simple equipment:
timing. In particular, timing the instants
when the edge of an asteroid covers and
uncovers a background star.
Thousands of asteroids await proﬁ ling, but the great space agencies are only
going to send $100-million spacecraft to a
carefully chosen few. That leaves thousands whose size and shape you can help
map with a 3-inch scope and a few accessories that cost about $700 all told.
It's done by video. When asteroidoccultation campaigns began in the
1970s, timings were done by eyeball.
You watched the star to be occulted and
shouted when it disappeared, and again

when it reappeared. A tape machine
recorded your shout along with radio
time signals playing in the background.
If everything went exactly right, you
could ﬁ x the time of the shout between
the time ticks to within a few tenths of a
second. Then you corrected for your estimated reaction time. It was barely good
enough to get a useful size and shape
for some asteroids, and it only worked if
enough observers were lucky enough to
be within the occultation track.
That was then. Far better is to record
the occultation with a modern, ultra-lowlight astronomical video camera, and
time-stamp each frame. This method
reliably times events to within 0.03 second, more than ten times better than the
eyeball method.
Predictions of occultation paths have
also much improved, allowing observers
to position themselves strategically. As
a result, amateur asteroid mapping has
blossomed.

From his home near Boston, S&T's
Dennis di Cicco recorded the asteroid
160 Una occulting a previously unresolved double star on January 24, 2011.
Watch his video at http://is.gd/unaoccn.

But we need more observers! People
used to think video occultation timings
were only for techies with soldering irons
and advanced computer-interface skills.
No more. Described here is everything
you need. And if you run into problems,
there's an online community eager to help.

The Parts
In addition to the telescope of your
choice, you'll need the items shown on
the facing page to successfully time
occultations. They're numbered from the
back of the telescope at left to the digital
video recorder at the right end:
1. Knight Owl 0.5 Focal Reducer, $29.95.
2. Knight Owl C-mount, to the left of the
Watec camera, $16.49.

INTERNATIONAL OCCULTATION TIMING ASSOCIATION (4X)

In these silhouettes obtained by IOTA observers, each yellow line represents one observer watching the target star. The gap in a line represents the
time when the star wasn't visible. If the lines are assembled according to the observers' locations and other known data, a silhouette of the asteroid
results. The more people, the clearer the proﬁle! Most asteroids turn out to be somewhat round, but a few are irregular, elongated, or a disconnected
binary pair. Kleopatra seems to be two elongated bodies touching almost end to end. See all results to date at asteroidoccultation.com/observations.

135 Hertha

506 Marion

Dec. 11, 2008

Mar. 7, 2015

50 km

September 2016 sky & telescope

50 km

93RMinerva
C O LO R F ILTE
S

Sept. 6, 2014

50 km

http://www.is.gd/unaoccn http://www.asteroidoccultation.com/observations

Sky & Telescope - September 2016

Table of Contents for the Digital Edition of Sky & Telescope - September 2016