Other Wavelength Regions

All wavelengths of the electromagnetic spectrum enter the Earth from the sky. However, as mentioned in Sect. 3.1, not all radiation reaches the ground. The wavelength regions absorbed by the atmosphere have been studied more extensively since the 1970's, using Earth-orbiting satellites. Besides the optical and radio regions, there are only some narrow wavelength ranges in the infrared that can be observed from high mountain tops.

The first observations in each new wavelength region were usually carried out from balloons, but not until rockets came into use could observations be made from outside the atmosphere. The first actual observations of an X-ray source, for instance, were made on a rocket flight in June 1962, when the detector rose above the atmosphere for about 6 minutes. Satellites have made it possible to map the whole sky in the wavelength regions invisible from the ground.

Gamma Radiation. Gamma ray astronomy studies radiation quanta with energies of 105-1014 eV. The boundary between gamma and X-ray astronomy, 105 eV, corresponds to a wavelength of 10-11 m. The boundary is not fixed; the regions of hard (= high-energy) X-rays and soft gamma rays partly overlap.

While ultraviolet, visible and infrared radiation are all produced by changes in the energy states of the electron envelopes of atoms, gamma and hard X-rays are produced by transitions in atomic nuclei or in mutual interactions of elementary particles. Thus observations of the shortest wavelengths give information on processes different from those giving rise to longer wavelengths.

The first observations of gamma sources were obtained at the end of the 1960's, when a device in the OSO 3 satellite (Orbiting Solar Observatory) detected gamma rays from the Milky Way. Later on, some satellites were especially designed for gamma astronomy, notably SAS 2, COS B, HEAO 1 and 3, and the Compton Gamma Ray Observatory. The most effective satellite at present is the European Integral, launched in 2002.

The quanta of gamma radiation have energies a million times greater than those of visible light, but they cannot be observed with the same detectors. These observations are made with various scintillation detectors, usually composed of several layers of detector plates, where gamma radiation is transformed by the photoelectric effect into visible light, detectable by photomultipliers.

The energy of a gamma quantum can be determined from the depth to which it penetrates the detector. Analyzing the trails left by the quanta gives information on their approximate direction. The field of view is limited by the grating. The directional accuracy is low, and in gamma astronomy the resolution is far below that in other wavelength regions.

X-rays. The observational domain of X-ray astronomy includes the energies between 102 and 105 eV, or the wavelengths 10-0.01 nm. The regions 10-0.1 nm and 0.1-0.01 nm are called soft and hard X-rays, respectively. X-rays were discovered in the late 19th century. Systematic studies of the sky at X-ray wavelengths only became possible in the 1970's with the advent of satellite technology.

The first all-sky mapping was made in the early 1970's by SAS 1 (Small Astronomical Satellite), also called Uhuru. At the end of the 1970's, two High-Energy Astronomy Observatories, HEAO 1 and 2 (the latter called Einstein), mapped the sky with much higher sensitivity than Uhuru.

The Einstein Observatory was able to detect sources about a thousand times fainter than earlier X-ray telescopes. In optical astronomy, this would correspond to a jump from a 15 cm reflector to a 5 m telescope. Thus X-ray astronomy has developed in 20 years as much as optical astronomy in 300 years.

The latest X-ray satellites have been the American Chandra and the European XMM-Newton, both launched in 1999.

Besides satellites mapping the whole sky, there have been several satellites observing the X-ray radiation of the Sun. The first effective telescopes were installed in

Fig. 3.31. (a) X-rays are not reflected by an ordinary mirror, and the principle of grazing reflection must be used for collecting them. Radiation meets the paraboloid mirror at a very small angle, is reflected onto a hyperboloid mirror and further to a focal point. In practice, several mirrors are placed one inside another, collecting radiation in a common focus (b) The European Integral gamma ray observatory was launched in 2002. (Picture ESA)

Fig. 3.31. (a) X-rays are not reflected by an ordinary mirror, and the principle of grazing reflection must be used for collecting them. Radiation meets the paraboloid mirror at a very small angle, is reflected onto a hyperboloid mirror and further to a focal point. In practice, several mirrors are placed one inside another, collecting radiation in a common focus (b) The European Integral gamma ray observatory was launched in 2002. (Picture ESA)

the Skylab space station, and they were used to study the Sun in 1973-74. In the 1990's, the European Soho started making regular X-ray observations of the Sun.

The first X-ray telescopes used detectors similar to those in gamma astronomy. Their directional accuracy was never better than a few arc minutes. The more precise X-ray telescopes utilize the principle of grazing reflection (Fig. 3.31). An X-ray hitting a surface perpendicularly is not reflected, but absorbed. If, however, X-rays meet the mirror nearly parallel to its surface, just grazing it, a high quality surface can reflect the ray.

The mirror of an X-ray reflector is on the inner surface of a slowly narrowing cone. The outer part of the surface is a paraboloid and the inner part a hyperboloid. The rays are reflected by both surfaces and meet at a focal plane. In practice, several tubes are installed one within another. For instance, the four cones of the Einstein Observatory had as much polished optical surface as a normal telescope with a diameter of 2.5 m. The resolution in X-ray telescopes is of the order of a few arc seconds and the field of view about 1 deg.

The detectors in X-ray astronomy are usually Geiger-Müller counters, proportional counters or scintillation detectors. Geiger-Müller and proportional counters are boxes filled with gas. The walls form a cathode, and an anode wire runs through the middle of the box; in more accurate counters, there are several anode wires. An X-ray quantum entering the box ionizes the gas, and the potential difference between the anode and cathode gives rise to a current of electrons and positive ions.

Ultraviolet Radiation. Between X-rays and the optical region lies the domain of ultraviolet radiation, with wavelengths between 10 and 400 nm. Most ultraviolet observations have been carried out in the soft UV region, at wavelengths near those of optical light, since most of the UV radiation is absorbed by the atmosphere. The wavelengths below 300 nm are completely blocked out. The short wavelength region from 10 to 91.2 nm is called the extreme ultraviolet (EUV, XUV).

Extreme ultraviolet was one of the last regions of the electromagnetic radiation to be observed systematically. The reason for this is that the absorption of interstellar hydrogen makes the sky practically opaque at these wavelengths. The visibility in most directions is limited to some hundred light years in the vicinity of the Sun. In some directions, however, the density of the interstellar gas is so low that even extragalactic objects can be seen. The first dedicated EUV satellite was the Extreme Ultraviolet Explorer (EUVE), operating in 1992-2000. It observed about a thousand EUV sources. In EUV grazing reflection telescopes similar to those used in X-ray astronomy are employed.

In nearly all branches of astronomy important information is obtained by observations of ultraviolet radiation. Many emission lines from stellar chromospheres or coronas, the Lyman lines of atomic hydrogen, and most of the radiation from hot stars are found in the UV domain. In the near-ultraviolet, telescopes can be made similar to optical telescopes and, equipped with a photometer or spectrometer, installed in a satellite orbiting the Earth.

Fig. 3.32. (a) The European X-ray satellite XMM-Newton was launched in 1999. (Drawing D. Ducros, XMM Team, ESA) (b) FUSE satellite has photographed far ultraviolet objects from Earth orbit since 1999. (Graphics NASA/JHU Applied Physics Laboratory)

The most effective satellites in the UV have been the European TD-1, the American Orbiting Astronomical Observatories OAO 2 and 3 (Copernicus), the International Ultraviolet Explorer IUE and the Soviet Astron. The instruments of the TD-1 satellite included both a photometer and a spectrometer. The satellite measured the magnitudes of over 30,000 stars in four different spectral regions between 135 and 274 nm, and registered UV spectra from over 1000 stars. The OAO satellites were also used to measure magnitudes and spectra, and OAO 3 worked for over eight years.

The IUE satellite, launched in 1978, was one of the most successful astronomical satellites. IUE had a 45 cm Ritchey-Chretien telescope with an aperture ratio of ff 15 and a field of view of 16 arc minutes. The satellite had two spectrographs to measure spectra of higher or lower resolution in wavelength intervals of 115-200 nm or 190-320 nm. For registration of the spectra, a Vidicon camera was used. IUE worked on the orbit for 20 years.

Infrared Radiation. Radiation with longer wavelengths than visible light is called infrared radiation. This region extends from about 1 micrometre to 1 millimetre, where the radio region begins. Sometimes the near-infrared, at wavelengths below 5 m,

Fig. 3.33. Refractors are not suitable for infrared telescopes, because infrared radiation cannot penetrate glass. The Cassegrain reflectors intended especially for infrared observations have secondary mirrors nodding rapidly back and forth between the object and the background near the object. By subtracting the brightness of the background from the brightness of the object, the background can be eliminated

and the submillimetre domain, at wavelengths between 0.1 and 1mm, are considered separate wavelength regions.

In infrared observations radiation is collected by a telescope, as in the optical region. The incoming radiation consists of radiation from the object, from the background and from the telescope itself. Both the source and the background must be continually measured, the difference giving the radiation from the object. The background measurements are usually made with a Cassegrain secondary mirror oscillating between the source and the background at a rate of, say, 100 oscillations per second, and thus the changing background can be eliminated. To register the measurements, semiconductor detectors are used. The detector must always be cooled to minimize its own thermal radiation. Sometimes the whole telescope is cooled.

Infrared observatories have been built on high mountain tops, where most of the atmospheric water vapour remains below. Some favourable sites are, e.g. Mauna Kea on Hawaii, Mount Lemon in Arizona and Pico del Teide on Tenerife. For observations in the far-infrared these mountains are not high enough; these observations are carried out, e.g. on aeroplanes. One of the best-equipped planes is the Kuiper Airborne Observatory, named after the well-known planetary scientist Gerard Kuiper.

Fig. 3.34. The most effective infrared satellite at present is the American Spitzer, launched in 2003. (Drawing NASA)

Balloons and satellites are also used for infrared observations. The most successful infrared observatories so far have been the InfraRed Astronomy Satellite IRAS, the European Infrared Space Observatory ISO, and the present-day Spitzer (originally SIRTF, Space InfraRed Telescope Facility). A very succesful satellite was the 1989 launched COBE (Cosmic Background Explorer), which mapped the background radiation in submillimetre and infrared wavelengths. The Microwave Anisotropy Probe (MAP) has continued the work of COBE, starting in 2001.

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