Great Telescopes
ATLAS OF THE UNIVERSE
F
or many years the Mount Wilson 2.5-metre (100-inch)
reflector was not only the world’s largest telescope,
but was in a class of its own. It was set up through the
untiring energy of George Ellery Hale, an American
astronomer who not only planned huge telescopes but also
had the happy knack of persuading friendly millionaires
to pay for them! Hale had already been responsible for the
Yerkes refractor; later he planned the 5-metre (200-inch)
Palomar reflector, though he died before the telescope
was completed in 1948. The Palomar telescope is still in
full operation, and is indeed more effective than it used
to be, because it is now used with the latest electronic
equipment. What is termed a CCD, or Charge-Coupled
Device, is far more sensitive than any photographic plate.
In 1975 the Russians completed an even larger
telescope, with a 6-metre (236-inch) mirror, but it has
never been a success, and is important mainly because
of its mounting, which is of the altazimuth type. With an
altazimuth, the telescope can move freely in either direc-
tion – up or down (altitude) or east to west (azimuth).
This means using two driving mechanisms instead of only
one, as with an equatorial, but this is easy enough with the
latest computers, and in all other respects an altazimuth
mounting is far more convenient. All future large tele-
scopes will be mounted in this way.
The New Technology Telescope (NTT), at La Silla in
Chile, looks very different from the Palomar reflector. It
is short and squat, with a 3.5-metre (138-inch) mirror
which is only 24 centimetres (10 inches) thick and weighs
6 tonnes (13,440 pounds). Swinging a large mirror around
means distorting it, and with the NTT two systems are
used to compensate for this. The first is termed ‘activeoptics’, and involves altering the shape of the mirror so
that it always retains its perfect curve; this is done by
computer-controlled pads behind the mirror. With
‘adaptive optics’ an extra computer-controlled mirror is
inserted in the telescope, in front of a light-sensitive
detector. By monitoring the image of a relatively bright
star in the field of view, the mirror can be continuously
modified to compensate for distortions in the image due
to air turbulence.
The VLT or Very Large Telescope, at Cerro Paranal
in the northern Atacama Desert of Chile, is operated by
the European Southern Observatory. It has four 8.2-metre
(323-inch) mirrors working together. The first two were
operational by mid-1999, and the other two in 2001.
The Keck Telescope on Mauna Kea has a 9.8-metre
(387-inch) mirror which has been made from 36
hexagonal segments, fitted together to form the correct
optical curve; the final shape has to be accurate to a limit
of one thousandth the width of a human hair. A twin
Keck has been built beside it, and when the two tele-
scopes are operating together they could, in theory, be
capable of distinguishing a car’s headlights separately
from a distance of over 25,000 kilometres (over 15,
miles).
Some telescopes have been constructed to meet
special needs. With a Schmidt telescope, the main advan-
tage is a very wide field of view, so that large areas of
the sky can be photographed with a single exposure; the
United Kingdom Infra-Red Telescope (UKIRT) on
Mauna Kea was designed to collect long-wavelength
(infra-red) radiations, though in fact it has proved to be so
good that it can be used at normal wavelengths as well.▲ The New Technology
Telescope (NTT) at La Silla.
The NTT, at the site of
the European Southern
Observatory, has a mirror
3.5 m (138 inches) in
diameter. The telescope
is of very advanced design;
it moves only in altitude,
and the entire observatory
rotates. New techniques
such as active and adaptive
optics have been introduced,
and the NTT has proved to
be extremely successful.
It was completed in 1989.B Atl of Univ Phil'03stp 31/3/03 3:56 pm Page 18