108 PART 1^ |^ EXPLORING THE SKY
New-Generation Telescopes
For most of the 20th century, astronomers faced a serious limita-
tion on the size of astronomical telescopes. Traditional telescope
mirrors were made thick to avoid sagging that would distort the
refl ecting surface, but those thick mirrors were heavy. Th e 5-m
(200-in.) mirror on Mount Palomar weighs 14.5 tons. Th ese tra-
ditional telescopes were big, heavy, and expensive.
Today’s astronomers have solved these problems in a number of
ways. Read Modern Astronomical Telescopes on pages
110–111 and notice three important points about telescope design
and ten new terms that describe astronomical telescopes and their
operation:
Traditional telescopes use large, solid, heavy mirrors to focus
starlight to a prime focus, or, by using a secondary mirror, to
a Cassegrain focus. Some small telescopes have a Newtonian
focus or a Schmidt-Cassegrain focus.
Telescopes must have a sidereal drive to follow the stars, and
an equatorial mounting with easy motion around a polar axis
is the traditional way to provide that motion. Today, astron-
omers can build simpler, lighter-weight telescopes on alt-
azimuth mountings that depend on computers to move the
telescope so that it follows the westward motion of the stars
as Earth rotates.1
2
Large Binocular TelescopeThe mirrors in the VLT
telescopes are each
8.2 m in diameter.Only 6 of the mirror
segments have been
installed in this photo.Active optics, computer control of the shape of telescope mir-
rors, allows the use of thin, lightweight mirrors—either
“fl oppy” mirrors or segmented mirrors. Reducing the weight
of the mirror reduces the weight of the rest of the telescope
and makes it stronger and less expensive. Also, thin mirrors
cool faster at nightfall and produce better images.High-speed computers have allowed astronomers to build
new, giant telescopes with unique designs. A few are shown in
■ Figure 6-12. Th e European Southern Observatory has built the
Very Large Telescope (VLT) high in the remote Andes Mountains
of northern Chile. Th e VLT actually consists of four telescopes,
each with a computer-controlled mirror 8.2 m in diameter and
only 17.5 cm (6.9 in.) thick. Th e four telescopes can work singly
or can combine their light to work as one large telescope. Italian
and American astronomers have built the Large Binocular
Telescope, which carries a pair of 8.4-m mirrors on a single
mounting. Th e Gran Telescopio Canarias, located atop a volcanic
peak in the Canary Islands, carries a segmented mirror 10.4
meters in diameter and holds, for the moment, the record as the
largest single telescope in the world
Other giant telescopes are being planned with segmented
mirrors or with multiple mirrors (■ Figure 6-13). Th e Giant
Magellan Telescope will carry seven thin mirrors, each 8.4 meters
in diameter, on a single mounting. It will be located in the
Chilean Andes and is planned to
have the light-gathering power of
a 22-m telescope. Th e Th irty
Meter Telescope, now under
development by American astron-
omers, will have a mirror 30 meters
in diameter, composed of 492
hexagonal segments. Th e European
Extremely Large Telescope is being
planned by an international team.
It will carry 906 segments, mak-
ing up a mirror 42 meters in
diameter. Other very large tele-
scopes are being proposed with
completion dates of 2016 or later.3
■ Figure 6-12
The four telescopes of the VLT are
housed in separate domes at Paranal
Observatory in Chile (Figure 6-10). The
Large Binocular Telescope (LBT) carries
two 8.4-m mirrors that combine their
light. The entire building rotates as the
telescope moves. The Gran Telescopio
Canarias contains 36 hexagonal mirror
segments in its 10.4-m primary mirror.
(VLT: ESO; LBT: Large Binocular Telescope
Project and European Industrial Engineer; GMT:
ESO; GTC: Gara Mora, IAC)