Radio astronomy is now about 65 years old, but is far from retiring. Karl Jansky made the first detection of cosmic static in 1932, which he correctly identified with emission from our own Milky Way. A few years later Grote Reber made the first rough map of the northern sky at metre wavelengths, demonstrating the concentration of emission towards the Galactic Plane. During World War II the Sun was discovered as the second cosmic radio source. It was not until the late 1940s that the angular resolution was improved sufficiently to allow the first extragalactic sources be identified: Centaurus A (NGC 5128) and Virgo A (M 87). Interestingly, the term radio astronomy was first used only in 1948 ([Haynes et al. (1996)], p.453, item 2). During the 1950s it became obvious that not only were relativistic electrons responsible for the emission, but also that radio galaxies were reservoirs of unprecedented amounts of energy. Even more impressive radio luminosities were derived once the quasars at ever-higher redshifts were found to be the counterparts of many radio sources. In the 1950s radio astronomers also began to map the distribution of neutral hydrogen in our Galaxy and find further evidence for its spiral structure.
Radio astronomy provided crucial observational data for cosmology from early on, initially based on counts of sources and on their (extremely isotropic) distribution on the sky, and since 1965 with the discovery and precise measurement of the cosmic microwave background (CMB). Only now are the deepest large-area surveys of discrete radio sources beginning to provide evidence for anisotropies in the source distribution, and such surveys continue to be vital for finding the most distant objects in the Universe and studying their physical environment as it was billions of years ago. If this were not enough, today's radio astronomy not only provides the highest angular resolution achieved in astronomy (fractions of a milliarcecond, or mas), but it also rivals the astrometric precision of optical astronomy (~ 2 mas; [Sovers et al. (1998)]). The relative positions of neighbouring sources can even be measured to a precision of a few micro-arcsec (µas), which allows detection of relative motions of ~ 20 µas per year. This is comparable to the angular ``velocity'' of the growth of human fingernails as seen from the distance of the Moon.
The ``radio window'' of the electromagnetic spectrum for observations from the ground is limited at lower frequencies mainly by the ionosphere, making observations below ~ 30MHz difficult near maxima of solar activity. While Reber was able to measure the emission from the Galactic Centre at 0.9MHz from southern Tasmania during solar minimum in 1995, observations below about 1MHz are generally only possible from space. The most complete knowledge of the radio sky has been achieved in the frequency range between 300 ( = 1m) and 5000MHz ( = 6cm). At higher frequencies both meteorological conditions as well as receiver sensitivity become problems, and we have good data in this range only for the strongest sources in the sky. Beyond about 1000GHz ( 0.3mm) we reach the far infrared. Like the optical astronomers, who named their wavebands with certain letters (e.g. U, B, V, R, I, ...), radio astronomers took over the system introduced by radio engineers. Jargon like P-, L-, S-, C-, X-, U-, K- or Q-band can still be found in modern literature and stands for radio bands near 0.33, 1.4, 2.3, 4.9, 8.4, 15, 23 and 40GHz (see [Reference Data for Radio Engineers, 1975]). The [CRAF Handbook for Radio Astronomy (1997)] gives a detailed description of the allocation and use of the various frequency bands allocated to astronomers (excluding the letter codes).
Unlike optical astronomers with their photographic plates, radio astronomers have used electronic equipment from the outset. Given that they had nothing like the ``finding charts'' used in optical astronomy to orient themselves in the radio sky, they were used to working with maps showing coordinates, which were rarely seen in optical research papers. Nevertheless, the display and description of radio maps in older literature shows some rare features. Probably due to the recording devices like analogue charts used up to the early 1980s, the terms ``following'' and ``preceding'', were frequently used rather than ``east'' and ``west''. Thus, e.g. ``Nf'' stands for ``NE'', or ``Sp'' for ``SW''. Sometimes the aspect ratio of radio maps was deliberately changed from being equi-angular, just to make the telescope beam appear round ([Graham (1970)]). Neither were radio astronomers at the forefront of archiving their results and offering publicly available databases. Happily all this has changed dramatically during the past decade, and the present report hopes to give a convincing taste of this.
As these lectures are aimed at professional astronomers, I do not discuss services explicitly dedicated to amateurs. I leave it here with a mention of the well-organised WWW site of the ``Society of Amateur Radio Astronomers'' (SARA; http://www.bambi.net/sara.html). Note that in all addresses on the World-Wide-Web (WWW) mentioned here (the so-called ``URL''s) I shall omit the leading characters ``http://'' unless other strings like ``ftp://'' need to be specified. The URLs listed have only been verified to be correct as of May 1998.