97 98 The Development of Astronomical Observation Methods Historical Introduction to Our Knowledge of the Electromagnetic Spectrum

The great advances in research are often associated with (Charge Coupled Device), with very high quantum the invention or introduction of new types of instru• yields, thereby offering a further enhancement of the ments. The telescope, the clock, the photographic plate, possibility of observing extremely faint light sources, photometer, spectrograph, and finally the whole arsenal especially those of extragalactic origin. of modem electronics and space travel each are associ• The wavelength region which is accessible to as• ated with an epoch of astronomical research. However, tronomical observations from the Earth's surface is equally important- and we should not forget this- is limited by the transmission of the Earth's atmosphere the creation of new concepts and approaches for the (Fig. 11.1). analysis of the observations. Scientific attainments of The "optical window" includes the near ultraviolet genius are indeed always based upon a combination of and the near infrared, in addition to the visible region. the formulation of new concepts and of instrumental On the short-wavelength end, it is limited by the absorp• developments, which only together can achieve an ad• tion due to atmospheric ozone, 03, near A. = 300 nm; vance into previously unknown realms of Nature. We on the long-wavelength end, by the absorption of wa• are tempted to agree with Simon Stevin (1548-1620), ter vapor, HzO, at about A.= 1 J.Lm. Out to about 20 J.Lm, "Wonder en is gheen wonder". some observations are still possible in several narrow The invention of the telescope (G. Galilei, 1609; windows. J. Kepler, 1611) opened a new era for astronomy Only in the radiofrequency range does the atmo• with previously unsuspected observational possibili• sphere again become transparent. The "radio window" ties, due to the enormous increase in magnification and is bounded on the shortwave end at A. ::::: 1 to 5 mm light-gathering power. or v ::::: 300 to 60 GHz by the absorption due to at• Beginning with Galileo's refracting telescope with mospheric water vapor and oxygen; on the longwave a diameter of only 2 em, larger and larger optical tele• end, at A. ::::: 50 m or v ::::: 6 MHz by reflection from the scopes were constructed, culminating in those of the ionosphere. 20th century with mirrors (in one piece) of 3.5 up to 6 m Although the propagation of long-wavelength elec• in diameter. The 1990's brought the construction and to tromagnetic waves in free space was discovered in 1888 some extent the use of a new generation of large tele• by H. Hertz, available radio receivers were for a long scopes in which the primary focusing elements are made time not sufficiently sensitive to detect radio emissions of several parts and are adjusted by active and adaptive from cosmic sources. Following the fortuitous discovery optics, so that their mirror surfaces are equivalent to by K. G. Jansky in 1932 of the radiofrequency emis• diameters of 10 to 20 m. sions of the Milky Way in the meter wavelength region The accessible spectral region of visual observa• (A.= 12 to 14m), G. Reber, beginning in 1939, car• tions is limited by the sensitivity of the human eye; ried out a survey of the heavens at A.= 1.8 m using visible light occupies only a small portion of the elec• a 9.5 m parabolic mirror antenna (in the garden of his tromagnetic spectrum, from about 400 to 750 nm in house!). During the 2nd World War, in 1942, J. S. Hey wavelength, from violet through blue, green, and yel• and J. Southworth detected the radio emissions of the low to red. Only in the 19th century, after the invention active and of the quiet Sun, using receivers which had in of the photographic plate, did a light detector for astro• the meantime been improved for use in radar apparatus. nomical observations become available which, on the In 1951, various researchers in Holland, the USA, one hand, stores and "integrates over" the incident light, and Australia almost simultaneously discovered the and on the other, possesses sensitivity beyond the visible 21 em line of interstellar hydrogen, which had been pre• spectral region. Finally, in the 1970's, highly sensitive dicted by H. C. van der Hulst; its Doppler effect opened solid-state detectors were developed, such as the CCD up enormous possibilities for investigating the motions The Development of Astronomical Observation Methods

99 Fig. 11.1. The transmission lkeV 1MeV lGHz v lOOOGHz leV of the Earth's atmosphere m I dm I em I mm ~-~ ~ Infrared uv for electromagnetic radi• Waves OpticalI I ation as a function of the - 8 region Oz wavelength). (lower scale) or the frequency v and -7 OJ the corresponding photon energy Ey = hv (upper - 6 scale). The altitude h [km] (and the corresponding - 5 Optical pressure P in units of the window pressure at ground level, - 4 RadiowindoW ---.j H Po::::: 1bar) at which the intensity of the incident - 3 radiation is reduced to one-half its initial value is - 2 shown, and the maximum 20 altitudes for observations -1 10 from aircraft and balloons

0 ~-L--~--L-_L~u_ __L_ ~~ULL-L-~--~--~~--~--~_Jo are also indicated 100m 1m lmm 111 m lpm ).

of interstellar matter in our Milky Way galaxy and in due to atmospheric oxygen, 0 2; then the X-ray and other cosmic formations. Since the 1950's, an almost ex• finally the gamma ray regions. Although we can investi• plosive development of radio astronomy has occurred, gate the solar spectrum continuously from wavelengths owing to the construction of individual telescopes and in the radiofrequency region out to the X-ray region, of a wide variety of antenna systems based on the for galactic and extragalactic research we must keep in principles of interferometry and of aperture synthesis mind the Lyman continuum of the interstellar hydrogen (M. Ryle) which have yielded better and better angular atoms (Sect. 10.2). Their absorption sets in strongly at resolution, as well as the improving detection sensitiv• A.= 91.2 nm and allows no "viewing" until we reach ity, obtained especially through low-noise amplifiers. the X-ray region beyond about A.= 1 nm. Using long-baseline interferometry with transcontinen• At the beginning of "space astronomy", immediately tal base lengths, an angular resolution of better than after the end of the 2nd World War, observations were 10-4 seconds of arc has now been attained, by far made using rockets which were developed in Germany exceeding the precision of optical observations. during the war, and later with stabilized research rock• Since about 1970, radio-astronomical observations ets: in 1946, H. Friedman and his group obtained the have been extended to the millimeter and recently to first ultraviolet spectrum, and in 1948, T. R. Bumight the submillimeter ranges, in particular as a result of recorded the first X -ray image of the Sun. Following the progress in amplifier technology. launching of the first artificial satellite Sputnik 1 ( 1957), Observations outside the atmospheric envelope of a rapid development in the investigation of the Solar the Earth with rockets and especially with satellites System and in astronomical observation using satellites and space probes have allowed astronomical observa• and space probes has occurred, which we have to some tions to extend to all the spectral regions which would extent already described in Sect. 2.5. otherwise be completely absorbed by the Earth's atmo• From the 1960's on, beginning with the Orbiting sphere: the medium and far infrared between 20 and Solar Observatory (OSO) and Orbiting Astronomical 350 iJ-m, which is absorbed by atmospheric water vapor Observatory (OAO), satellites with sufficient positional and oxygen bands; the far ultraviolet past the trans• stability (::::; 1") to permit longer series of observations mission limit of atmospheric ozone at A. = 285 nm; the of the Sun and other cosmic sources in the ultraviolet, contiguous Lyman region, where absorption is mainly X-ray, and gamma-ray regions became available. I The Development of Astronomical Observation Methods

100 In the ultraviolet, from 1979 and for some years with larger satellite telescopes in 1995/98 with ISO, and thereafter, the IUE satellite with its 0.45 m telescope from 2003 with the Spitzer Space Telescope. carried out successful observations. In the extreme ultra• For the mm and sub-mm wavelength ranges, the violet region below the Lyman edge at 91.2 nm, where COBE satellite was launched in 1989 and has been suc• observations are blocked in many directions through cessfully employed for the investigation of the cosmic absorption by interstellar hydrogen, the EUVE satellite background radiation. was operating from 1992 to 2003. Observations from space also offer clear advantages In 1962, (nonsolar) X-ray astronomy began with the in the optical spectral region, owing to the fact that the accidental discovery of the strong source Sco X -1 by angular resolution is no longer limited by air motion. R. Giacconi and coworkers using a rocket-carried in• The 2.4 m Hubble Space Telescope launched in 1990 strument; in 1970, the first X-ray satellite, UHURU, has offered (since the repair of an imaging error in its began its survey of the skies. Larger X-ray satellite• mirror system in 1993) an enormous improvement in borne telescopes became available in 1978 with the resolution and detection limits for optical astronomy, Einstein Observatory, in 1990 (ROSAT), and from 1999 and with it a plethora of exciting new observations, on (Chandra and XMM Newton). especially in the area of extragalactic astronomy. In the 1970's, gamma-ray astronomy also attained To end our introductory overview, we shall men• a sufficient angular resolution to be able to resolve indi• tion several astronomical observation methods which vidual cosmic sources. Following the COS-B satellite, are not based on the detection of electromagnetic radia• launched in 1975, since 1991 the Compton Observa• tion. In 1912, V. F. Hess had already discovered cosmic tory CGRO has carried out detailed observations in the radiation in the course of a balloon ascent; today, it gamma region. Astronomy with observation of high• represents an important research area of high-energy as• energy photons has thus developed within a few years tronomy. While the secondary particles resulting from into one of the most interesting areas of research, mak• interactions of the primary energetic protons and heav• ing use of an arsenal of new and unusual instruments. ier nuclei in the upper atmosphere can be investigated In the infrared region, the generally weak sources on the ground, the primary radiations themselves must must be detected against a background of thermal ra• be observed from outside the atmosphere. diation from the instrument itself and from the Earth's The search for gravitational waves from cosmic atmosphere. The development of cooled semiconductor sources has thus far remained unsuccessful. detectors since the 1960's has produced a rapid im• The interesting area of neutrino astronomy, whose provement in our ability to observe infrared sources early development was due to R. Davis Jr. (1964 ), is at from stratospheric balloons and aircraft, as well as from present still limited to the Sun and to unusual events the first infrared satellite observatory, IRAS, launched such as the explosion of the supernova SN 1987 A in in 1983. Infrared astronomy could again be carried out the nearby Large Magellanic Cloud.