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WWW.ASTRONOMY.COM 27
expansion — are now thought
to account for 95 percent of the
known universe (25 percent dark
matter, 70 percent dark energy).
“We became astronomers
thinking we were studying the uni-
verse,” Rubin later said of the ram-
ifications of her game-changing
discovery, “and now we learn that
we are just studying the five or
10 percent that is luminous.”
The Rubin
Observatory
The Rubin Observatory, nearing
completion atop Cerro Pachón
in Chile, may help astronomers
probe the other 95 percent.
The telescope’s mammoth
digital camera — which, at the
size of a small SUV, is the world’s
largest — weighs several tons
and comprises 189 ultra-precise
CCD detectors of 16 megapixels
each, grouped in clusters of nine.
It employs three lenses, the larg-
est of which is 5 feet (1.5 m) in
diameter, principally to correct
chromatic aberration, and six
filters to capture the entire vis-
ible spectrum (plus a touch of
infrared and ultraviolet light).
The telescope’s mirror also
breaks new ground: It features
a tertiary embedded within the
primary, fabricated as a single
casting to maximize internal sta-
bility and minimize weight. This
integrated mirror system allows
the telescope to slew to new sky
positions in just five seconds,
meaning the observatory can
survey the entire Southern
Hemisphere sky down to 24th
magnitude every three days.
The resulting data f low will be
unprecedented for a single astro-
nomical instrument. Each night, it
will generate 20 terabytes of data
and issue some 10 million alerts of
any changes it detects in the sky,
mostly asteroids. Every alert will
be processed and broadcast world-
wide in less than 60 seconds.
During its 10-year Legacy
Survey of Space and Time (which
now uses the acronym LSST),
Rubin’s dedicated computers will
catalog some 20 billion galaxies
and a similar number of stars,
as well as several million new
supernovae and about 6 million
asteroids. By then, the observa-
tory will have surveyed every
part of the southern sky more
than 800 times, producing sev-
eral hundred thousand terabytes
of data. In doing so, the Rubin
Observatory may open the door
to understanding dark matter
using a phenomenon called grav-
itational lensing.
Based on Einstein’s theory of
general relativity, massive objects
act as lenses by bending and
amplifying light as it travels near
them. By observing how galaxy
clusters affect the light from
more distant objects behind
them, such as galaxies and bright
quasars, astronomers can calcu-
late the total mass of the cluster.
Comparing this to the mass of
the clusters’ visible matter reveals
how much invisible dark matter is
hiding within them.
Rubin made her
groundbreaking
discoveries of dark
matter using an
image tube
spectrograph
developed by
Kent Ford and
attached to the No. 1
36-inch telescope at
Kitt Peak National
Observatory. KPNO/
NOIRLAB/NSF/AURA
Rubin’s work showed that the outer regions of galaxies are rotating faster than
expected based on the mass of the galaxies’ visible matter. This figure, which
compares the observed rotation rate of stars and gas in M33 (white line) to the
expected rotation rate (gray line) means the galaxy has more mass than indicated by
its visible matter alone. That extra mass is dark matter. ASTRONOMY: ROEN KELLY, AFTER MARIO DE
LEO/WIKIMEDIA COMMONS