Philips Atlas of the Universe

EXPLORING THE UNIVERSE

Type of

radiation

Gamma rays

0.00010.001

X-Rays Ultra-

violet

Infra-red

Visible

Radio waves

Wavelength

Radiated by

objects at a

temperature

of...

0.01 0.1 1 10 100 1000 nanometres

1 10 100

10 millimetres

1 10 100 centimetres

1 10 100 1000 metres

100,000,000

10,000,000

1,000,000

100,000

10,000 1000 100 10 1 degrees above absolute zero

1000 microns

1

down to 0.001 of a second of arc, which is the apparent

diameter of a cricket ball seen from a range of 16,000

kilometres (10,000 miles).

The sub-millimetre range of the electromagnetic

spectrum extends from 1 millimetre down to 0.3 of a

millimetre. The largest telescope designed for this region

is the James Clerk Maxwell Telescope (JCMT) on Mauna

Kea, which has a 15-metre (50-foot) segmented metal

reflector; sub-millimetre and microwave regions extend

down to the infra-red, where we merge with more ‘con-

ventional’ telescopes; as we have noted, the UKIRT in

Hawaii can be used either for infra-red or for visual work.

The infra-red detectors have to be kept at a very low

temperature, as otherwise the radiations from the sky

would be swamped by those from the equipment. High

altitude – the summit of Mauna Kea is over 4000 metres

(14,000 feet) – is essential, because infra-red radiations

are strongly absorbed by water vapour in the air.

Some ultra-violet studies can be carried out from

ground level, but virtually all X-rays and most of the

gamma rays are blocked by layers in the upper atmos-

phere, so that we have to depend upon artificial satellites

and space probes. This has been possible only during

the last few decades, so all these branches of ‘invisible

astronomy’ are very young. But they have added immea-

surably to our knowledge of the universe.

telescope at Jodrell Bank in Cheshire; it is a ‘dish’, 76

metres (250 feet) across, and is now known as the Lovell

Telescope.

Just as an optical collects light, so a radio telescope

collects and focuses radio waves; the name is somewhat

misleading, because a radio telescope is really more in

the nature of an aerial. It does not produce an optical-

type picture, and one certainly cannot look through it;

the usual end product is a trace on a graph. Many people

have heard broadcasts of ‘radio noise’ from the Sun and

other celestial bodies, but the actual noise is produced

in the equipment itself, and is only one way of studying

the radiations.

Other large dishes have been built in recent times; the

largest of all, at Arecibo in Puerto Rico, is set in a natural

hollow in the ground, so that it cannot be steered in the

same way as the Lovell telescope or the 64-metre (210-

foot) instrument at Parkes in New South Wales. Not all

radio telescopes are the dish type, and some of them

look like collections of poles, but all have the same basic

function. Radio telescopes can be used in conjunction

with each other, and there are elaborate networks, such

as MERLIN (Multi-Element Radio Link Interferometer

Network) in Britain. Resolution can now be obtained

The Very Large Array, in

New Mexico, is one of the

world’s premier radio

observatories. Its 27

antennae can be arranged

into four different Y-shaped

configurations. Each

antenna is 25 m (82 feet) in

diameter, but when the

signals are combined

electronically it functions as

one giant dish, with the

resolution of an antenna

36 km (22 miles) across.

The electromagnetic

spectrum extends far

beyond what we can see

with the human eye. These

days, gamma-ray, X-ray and

ultra-violet radiation from

hotter bodies and infra-red

radiation and radio waves

from cooler are also studied.

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