|| © CAMBRIDGE UNIVERSITY PRESS 1997
Radio astronomy is a child of this century, and has developed from the work of two pioneers in the 1930s to a fully developed branch of astronomy today. That development has brought with it an ambiguity, for high-frequency technology has prospered, and it is no longer clear where radio astronomy stops and infrared astronomy begins. We propose a simple distinction: radio astronomy is the study of the universe by observing electromagnetic radiation after it has been coherently amplified. When bolometers detect the radiation, the subject is infrared astronomy: the use of amplifiers that preserve the oscillatory character of the radiation - the phase information - is the mark of radio astronomy. As time passes, an increasingly large part of infrared astronomy will be subsumed into the radio astronomy domain, but there is a natural boundary, imposed by the laws of quantum mechanics. The shorter the wavelength, the noisier the amplifier, and by the time one reaches visible light, all amplifiers must become too noisy for use except for a few special cases. When a coherent amplifier is the instrument of choice, the infrared astronomers will not hesitate to use it; the further development of photon-counting infrared area detectors will make the demarcation line between the regimes even clearer.
The plan of the book is twofold: we hope that the scope and impact of radio astronomy observations wit be shown in the astrophysical discussion, and at the same time we intend to give a brief but comprehensive treatment of the elegant methods that have developed. The breadth of the subject matter necessarily limits the length of the treatment for each subject; the authors have tried, therefore, to provide recent, comprehensive references to the extent that they are available. In addition to the astronomy graduate student and those professionally committed to radio astronomy, there is a wider audience for whom this book is intended: the interested astronomers from outside the field who want to be informed of the principal ideas current in radio astronomy, and may even be thinking of carrying out radio observations that would complement other work in progress. Even though we have defined the boundary of radio astronomy for the sake of convenience, everyone is aware that the boundaries between disciplines have dwindled in importance. Radio observations would have been a baffling puzzle if the optical identifications of sources had not been made, and both radio and X-ray astronomers have long been aware of their kinship, since both study high-energy phenomena, though at the opposite ends of the spectrum. The techniques vary, but the astronomer of the future should have access to the entire electromagnetic spectrum.
We acknowledge the generosity of many colleagues who provided illustrations and other material for this book. We thank Wendy Hunter and Anne Conklin for help in the preparation of the manuscript, and Andre Fletcher for his careful reading of the text.