| Radio Waves: | Frequency Range: <3x1011 Hz | Wavelength Range: >1mm |
Of all of the types of elcetro magnetic radiation, radio waves have
the lowest frequency and the longest wavelength. The atmosphere of the
Earth is transparent to radio waves with wavelengths of a few millimetres
to about twenty metres. As such radio telescopes can be ground based. Radio
telescopes consist of very large dishes constructed of metal plates which
focus the radio waves to a point above the centre of the dish where the
receiver is located. Radio telescopes have to be very large because the
long wavelengths of the EM radiation result in poor resolution. Today the
majority of radio astronomy is done using interferometry which acts to
mitigate the poor resolution obtained. The largest radio telescope in the
world in located in Arecibo, Puerto Rico. It is 300 metres in diameter
and is constructed in a natural bowl shaped depression.
Radio astronomy began in the1930s but did not really take off until
after the Second World War. Numerous sources of radio waves have been detected
such as radio galaxies and quasars as well as emssions from the centre
of the Milky Way Galaxy.
| Microwaves: | Frequency Range: 3x1011 - 1013 Hz | Wavelength Range: 1mm - 25um |
Microwaves have a long wavelength, though not as long as radio waves.
The Earth's atmosphere is transparent to some wavelengths of microwave
radiation, but not to others. The longer wavelengths (waves more similar
to radio waves) pass through the Earth's atmosphere more easily than the
shorter wavelength microwaves. Microwave telescopes need to be large, but
not as large as radio telescopes.
Microwave Background Radiation (or Cosmic Background Radiation) is
the primordial radiation field that fills the universe, having been created
in the form of gamma rays at the time of the big bang. It has now cooled
so that its temperature today is 2.73K and its peak wavelength is near
1.1mm (in the microwave portion of the EM spectrum).The Microwave Anistrophy
Probe (due to be launched in 2000) will study small fluctuations in the
Microwave Background Radiation.
| Infrared: | Frequency Range: 1x1013 - 4x1014 Hz | Wavelength Range: 25um - 750nm |
Some wavelengths of infrared radiation pass through the Earth's atmosphere,
while others are blocked - this gives rise to 'infrared windows' which
can be measured from the ground. The main atmospheric constituents that
prevents infrared radiation from reaching the Earth's surface is water
vapour, and, to a lesser extent, Carbon Dioxide. As such infrared telescopes
are located in high, dry places such as the extinct volcano Mauna Kea.
Far infrared radiation (>4000nm) is emitted by cool objects such as planets
and newly forming stars but it does not penetrate as far into the atmosphere
as near infrared and as such we must place the detectors higher up. The
Kuiper Airborne Observatory is an aeroplane modified to carry a 1-meter
infrared telescope up to 12 km above sea level. This eliminates 99% of
the atmospheric water vapour. The IRAS satellite was launched in 1983 and
collected information on the very long wavelengths that hardly penetrate
the atmosphere at all.
A second source of interference with measuring infrared radiation is
the heat of the telescope itself. Therefore an infrared telescope must
be cooled to a low temperature, especially if measuring the far infrared.
Liquid helium was used on the IRAS satellite, and the limited supply of
it was what ended IRAS's working life.
| Visible: | Frequency Range: 4x1014 - 7.5x1014 Hz | Wavelength Range: 750nm - 400 nm |
Visible light makes up only a tiny part of the spectrum, but it is the
part that is most important to us. It ranges from red light (longest wavelength)
through yellow, green and blue to violet (shortest wavelength). Visible
light is not blocked by the Earth's atmosphere, although clouds and dust
can scatter some of the light back. However, the clarity of any image can
be affected by atmospheric factors such as turbulence, city lights and
pollution. As such Earth-based telescopes are situtated in high, dry places
to minimise the effects of the Earth's atmosphere. The largest telescope
in operation today is he 10m Keck telescope at Mauna Kea in Hawaii. However,
telescopes placed in space eliminate atmospheric interference completely,
as well as any problems caused by bad weather. The most famous orbiting
telescope, the Hubble Space Telescope has been used to observe storms on
the outer planets, volcanoes on Io, new planetary systems forming and galaxy
formation during the early universe. The Hubble Space Telescope also operates
in the near infra-red and the near ultra-violet.
| Ultraviolet: | Frequency Range: 1015 - 1017 Hz | Wavelength Range: 400nm - 1nm |
Ultraviolet radition can be split into the shorter wavelength far ultraviolet
and the longer wavelength near ultraviolet (the boundary between the two
being at approximately 200nm). The extreme ultraviolet range overlaps with
the far ultraviolet at wavelengths of between 1 and 100nm). Ultraviolet
radiation is absorbed by Ozone at an altitude of between 20 and 40 km.
As such ultraviolet telescopes must be placed into space. Ultraviolet astronomy
began after the second world war using rockets - previous attempts using
balloons could only study the very near ultraviolet. The Hubble Space Telescope,
as well as being an optical telescope, can 'see' in Ultraviolet light and
has continued the work of. other ultraviolet telescopes including the International
Ultraviolet Explorer, the IUE, launched in 1978. This telescope discovered
hot haloes of gas surrounding many galaxies, including our own, as well
as studing novae and binary stars. Ultraviolet astronomy has also been
carried out by Skylab and the two Voyager space probes.
| X-rays: | Frequency Range: 1017 - 1020 Hz | Wavelength Range: 1nm - 1pm |
X-ray radiation is absorbed by the Ozone in the Earth's upper atmosphere
in common with other high energy wavelengths of EM radiation. X-rays are
classified as being either 'hard' (shorter wavelengths) or 'soft' (longer
wavelengths). The first celestial X-ray source, other than the Sun,
Scorpius X-1 was detected in 1962 by a sounding rocket. The first X-ray
satellite, Uhuru, was launched in 1970 and it carried out the first X-ray
survey of the sky. More recent X-ray satellites include BeppoSax, the Einstein
Observatory and Rosat. Detectable X-ray emissions come from high energy
processes such as stellar wind, a shockwave from a supernova and hot gases
in stellar coronae. Other X-ray sources discovered later include active
galactic nulcei and hot white dwarfs.
| Gamma Rays: | Frequency Range: 1020 - 1024 Hz | Wavelength Range: <10-12 m |
Gamma rays have the shortest wavelength and highest frequency of all
EM radiation. Only the very highest energies can reach the surface, the
rest are absorbed by Ozone in the Earth's upper atmosphere. Gamma rays
are produced in areas of extremely high temperature, density and magnetic
fields. Gamma ray observations were first taken in the 1960s on the Apollo
and Ranger missions. The first sky surveys were done in the 1970s by the
SAS-2 and COS-B followed up by the HEAO satellites in the late 70s and
Granat in the eraly 90s. Gamma rays of above 100GeV require instruments
larger than can be carried on satellites and for these energires the Earth's
atmosphere itself is uesd as a detector, with optical telescopes used to
record the Cerenkov radiation produced by the photons.
References:
Snow T.P. & Brownsberger K.R. (1997) Universe: Origins and Evolution
Ridpath I. (1997) Oxford Dictionary of Astronomy