Sky and Telescope - November 2018 - 21

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bined with the telescope's extremely convex 4.2-m secondary and a slightly concave 3.8-m tertiary, it provides the ELT
with a perfect image quality over its 10 arcminute ﬁeld
TMT
(Plan B)
of view. Laser guide-star adaptive optics will be perTMT
formed using a 2.4-m ﬂexible mirror, consisting of six
(Plan A)
"petals." At just 2 mm thick and controlled by some
8,000 actuators, this will be by far the most complex
adaptive optics mirror ever produced. Finally, there's a
thin elliptical, 2.6-by-2.1-m ﬂat mirror. This ﬁfth mirror
stabilizes the image and steers the beam towards the science
instruments.
ELT
In the telescope's ﬁrst construction phase, with a price tag
GMT
n
of some $1.3 billion, the inner ﬁve rings of segments (210
in total) will remain empty, compromising the ELT's overall
sensitivity. Additional funds for the second and ﬁnal phase
would have already been available if Brazil had become ESO's
16th member state by now. However, in the spring of 2018 it
became clear that the accession agreement, which had already
scope. The double shutter of the enclosure measures 45 m in
been ratiﬁed by the Brazilian Congress in May 2015, will not
width, and at 74 m, the giant 5,000-ton dome is higher than
be signed by the president. "But," says de Zeeuw, "Poland -
the ancient Pyramid of the Sun near Mexico City.
home country of famous astronomer Nicolaus Copernicus -
Astronomy has come a long way since Galileo Galilei
did join ESO in 2015, and Ireland will probably follow in late
trained his small, home-built telescope at the Moon, the
2018. Moreover, Australia has entered a strategic partnerplanets, and the stars. Some four centuries ago, his observaship with ESO in July 2017, and I'm conﬁdent that they will
tions ushered in a whole new view of the universe we live
become a full member eventually."
in. The same will no doubt be true for the GMT, TMT, and
It's hard to imagine how impressive the Extremely Large
ELT, which are by far the largest eyes on the sky ever built
Telescope will be, when completed. At almost 1,000 square
by humans. I can't wait to take in the revolutionary cosmic
meters (over twice the size of a basketball court), its main
vistas they are going to yield.
mirror will be larger than all previous professional telescopes combined. The ELT has a total moving mass of more
than 3,000 metric tons. The Nasmyth instrument platforms
¢ S&T Contributing Editor GOVERT SCHILLING lives in The
on either side of the huge telescope structure will be large
Netherlands but has paid multiple visits to most major astroenough to accommodate a single 8.2-meter VLT Unit Telenomical observatories worldwide.
ea
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PROTESTORS: M OLLY SOLO M ON / H AWAI'I PUBLIC R A DIO; SIZE CO MPA RISON: SCOPES: ESO,
PY R A MID: DA NIEL CASE / CC BY-SA 3.0; M A P: LE A H TISCIONE / S&T, SOURCE ESO

Monster Telescopes at a Glance
Giant Magellan Telescope (GMT)

Thirty Meter Telescope (TMT)

Extremely Large Telescope (ELT)

International partnership
(U.S., Australia, Brazil,
South Korea)

International partnership
(U.S., Canada, China,
India, Japan)

European Southern Observatory
(15 member states,
see page 16 for list)

24.5 m

30 m

39.3 m

Paraboloid
(seven 8.4-m monoliths)

Hyperboloid
(492 1.44-m
hexagonal segments)

Ellipsoid
(798 1.45-m
hexagonal segments)

1,500 metric tons

1,430 metric tons

3,000 metric tons

Enclosure

Cylindrical
(56 m diameter, 63 m high)

Calotte dome
(66 m diameter, 56 m high)

Shuttered dome
(86 m diameter, 74 m high)

Location (altitude)

Cerro Las Campanas, Chile
(2,520 m)

Mauna Kea, Hawai'i (4,210 m) or Roque
de los Muchachos (La Palma, Canary
Islands), Spain (2,400 m)

Cerro Armazones, Chile
(3,050 m)

$1.05 billion

$1.4 billion

$1.5 billion

2024

2028

2024

Partnership

Aperture
Primary mirror design

Total moving mass

Estimated cost
First light

Altitudes rounded to the nearest 10 meters (3,000 meters is about 10,000 feet).

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