Invisible Astronomy
ATLAS OF THE UNIVERSE
T
he colour of light depends upon its wavelength – that
is to say, the distance between two successive wave-
crests. Red light has the longest wavelength and violet the
shortest; in between come all the colours of the rainbow –
orange, yellow, green and blue. By everyday standards
the wavelengths are very short, and we have to introduce
less familiar units. One is the Ångström (Å), named in
honour of the 19th-century Swedish physicist Anders
Ångström; the founder of modern spectroscopy, one Å is
equal to one ten-thousand millionth of a metre. The other
common unit is the nanometre (nm). This is equal to one
thousand millionth of a metre, so that 1 nanometre is
equivalent to 10 Ångströms.
Visible light extends from 400 nm or 4000 Å for
violet up to 700 nm or 7000 Å for red (these values are
only approximate; some people have greater sensitivity
than others). If the wavelength is outside these limits, the
radiations cannot be seen, though they can be detected
in other ways; for example, if you switch on an electric
fire you will feel the infra-red, in the form of heat, well
before the bars become hot enough to glow. To the long-
wave end of the total range of wavelengths, or electro-
magnetic spectrum, we have infra-red (700 nanometres
to 1 millimetre), microwaves (1 millimetre to 0.3 metre)
and then radio waves (longer than 0.3 metre). To the
short-wave end we have ultra-violet (400 nanometres to
10 nanometres), X-rays (10 nanometres to 0.01 nanometre)
and finally the very short gamma rays (below 0.
nanometre). Note that what are called cosmic rays are
not rays at all; they are high-speed sub-atomic particles
coming from outer space.
Initially, astronomers had to depend solely upon visi-
ble light, so that they were rather in the position of a
pianist trying to play a waltz on a piano which lacks all
its notes except for a few in the middle octave. Things
are very different now; we can study the whole range of
wavelengths, and what may be called ‘invisible astronomy’
has become of the utmost importance.
Radio telescopes came first. In 1931 Karl Jansky, an
American radio engineer of Czech descent, was using a
home-made aerial to study radio background ‘static’
when he found that he was picking up radiations from the
Milky Way. After the end of the war Britain took the lead,
and Sir Bernard Lovell master-minded the great radio▼ Antarctic Submillimetre
Telescope, at the
Amundsen-Scott South Pole
Station. The extremely cold
and dry conditions are ideal
for observations at
submillimetre wavelengths.UKIRT.The United
Kingdom Infra-Red
Telescope, on the summit
of Mauna Kea in Hawaii.
It has a 3.8-m (150-inch)
mirror. UKIRT proved to
be so good that it can also
be used for ordinary optical
work, which was sheer
bonus.▼ The Arecibo Telescope.
The largest dish radio
telescope in the world, it was
completed in 1963; the dish
is 304.8 m (approximately
1000 feet) in diameter.
However, it is not steerable;
though its equipment means
that it can survey wide areas
of the sky.The Lovell Telescope.
This 76-m (250-foot ) ‘dish’
at Jodrell Bank, in Cheshire,
UK, was the first really large
radio telescope; it has now
been named in honour of
Professor Sir Bernard Lovell,
who master-minded it. It
came into use in 1957 – just
in time to track Russia’s
Sputnik 1, though this was
not the sort of research for
which it was designed! It
has been ‘upgraded’ several
times. The latest upgrade
was in 2002; the telescope
was given a new galvanized
steel surface and a more
accurate pointing system.
Each of the 340 panes
making up the surface was
adjusted to make the whole
surface follow the optimum
parabolic shape to an
accuracy of less than 2 mm;
the frequency range of the
telescope was quadrupled.
The telescope is frequently
linked with telescopes
abroad to obtain very high
resolution observations.B Atl of Univ Phil'03stp 31/3/03 3:57 pm Page 20