The Electro Magnetic Spectrum

Radio Waves
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.

Infra Red
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.

Optical Waves
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
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.


Snow T.P. & Brownsberger K.R. (1997) Universe: Origins and Evolution
Ridpath I. (1997) Oxford Dictionary of Astronomy