Sky and Telescope - November 2018 - 16

for in a technique known as adaptive optics (S&T: May 2016,
In subsequent decades, however, most of the limiting factors in constructing much larger instruments have been over- p. 30). Evidently, the venerable 200-inch Hale Telescope was
not an end point, but just another milepost along the road
come. One of the developments that made this possible was
to astronomy's ever-larger eyes on the skies. The same is now
the return, in the late 1970s, to small and relatively cheap
true for Keck and the VLT.
alt-azimuth telescope mounts, as opposed to the bulky, asymmetric equatorial mounts of the past. The big advantage of an
equatorial mount is that the diurnal rotation of the night sky
Seven-Eyed Magellan
can be tracked by rotating the telescope at a constant speed
If you want to see an example of what's next, you need to
around just one axis, parallel to the axis of the Earth. But
drive to Arizona Stadium in Tucson, home ﬁeld of the Arizona
today's computer-controlled stepping motors no longer have
Wildcats. Unknown to most of the visiting football fans (and
an issue with moving a huge telescope around two axes at
probably to quite a number of players), the University of Arithe same time, with continuously varying speeds, as required
zona's Richard F. Caris Mirror Laboratory is located underfor a much more compact alt-azimuth mount. A smaller and
neath the stadium's east wing. Here's where ﬁve of the seven
more symmetrical mount leads to huge cost savings for both
thin 8.4-m mirrors for the future Giant Magellan Telescope
the telescope construction and
(GMT) have already been
the enclosure, which can also
spun-cast in a giant rotating
oven (S&T: Mar. 2014, p. 24).
be much smaller.
The ﬁrst segment has been
Even more important was
polished to its ﬁnal surface
the advent of thin-mirror techaccuracy, while the next four
nology. Older generations of
mirrors are currently being
mirrors had to be thick enough
worked on.
to withstand gravity, wind
load, and temperature changes
"With a focal length
precision requirement of 300
without losing their shape. But
microns, it's a big challenge,"
thanks to active support by
computer-controlled actuators,
says GMT Organization vice
president Patrick McCarthy
which compensate for possible
(Carnegie Observatories), "but
deformations by these enviwe steadily keep on moving
ronmental changes, today's
forward."
telescope mirrors can be as
p FUTURE ELT SITE Taken in early 2018, this image shows the
At Cerro Las Campanas in
thin as 10 to 20 centimeters
early foundations for the dome and telescope structure of ESO's
northern
Chile, construction
without losing their required
Extremely Large Telescope (ELT), which will perch at an altitude of
work
for
the
new telescope
curvature stability. Moreover,
some 2,500 meters (8,300 feet) on Cerro Armazones in the Chilean
Atacama Desert.
started earlier this year. The
telescope builders successfully
mountain's summit was
started to experiment with
already leveled in 2012, and when Miguel Roth (who was the
large, jigsaw-like mirrors, consisting of relatively small interobservatory director at the time) drove me up to the plateau
locking hexagonal segments, which are easier and cheaper to
in the spring of 2013, the outline of the GMT enclosure was
produce (and transport!) than a single monolithic mirror.
marked with white boulders. Since then, the road has been
As a result of these new technologies, quite a number of
graded, and residence buildings for construction workers
8- to 10-m class optical telescopes are operational right now,
have been erected. "The site is big enough to accommodate
including the twin 10-m Keck Telescopes at Mauna Kea and
a second GMT," Roth proudly told me. Who knows what the
the four 8.2-m Unit Telescopes of ESO's VLT in Chile. By
using laser-produced artiﬁcial guide stars in the upper atmofuture will bring? The W. M. Keck Observatory on Mauna
Kea also consists of two identical telescopes, and the Las
sphere, wavefront sensors to precisely measure the incoming
Campanas Observatory is already home to the twin 6.5-m
light, and thin, rapidly deformable mirrors in the telescope's
light path, even atmospheric turbulence can be compensated
Magellan Telescopes.

Megascope Partnerships

EXTREMELY LARGE TELESCOPE

GIANT MAGELLAN TELESCOPE

European Southern Observatory (ESO) member states:
Austria Belgium Czech Republic Denmark Finland
France Germany Italy The Netherlands Poland
Portugal Spain Sweden Switzerland United Kingdom

Arizona State University Astronomy Australia Limited Australian National
University Carnegie Institution for Science FAPESP: the São Paulo Research
Foundation (Brazil) Harvard University Korean Astronomy and Space Science
Institute Smithsonian Institution Texas A&M University University of Arizona
University of Chicago University of Texas at Austin

N OV E M B E R 2 018 * SK Y & TELESCOPE

ESO / G. HÜDEPOHL

Extremely Large Telescopes, Part I

Sky and Telescope - November 2018

Table of Contents for the Digital Edition of Sky and Telescope - November 2018