Abstract :
The paper reviews the potentialities and present status of maser and parametric radiometer systems in the whole field of radio astronomy, including radar astronomy. A short historical account of the development of the maser is followed by an outline of the objectives of radio-astronomical research, and the radiation mechanisms which arise. After an account of the requirements of the radio astronomer, a detailed discussion is presented of the limitations and relative merits of masers and parametric amplifiers in this field of research. The paper includes a compilation of data on existing and planned observatory installations. This data has been gathered from published work and from replies to a questionnaire which was widely circulated by the author; it embraces, it is hoped, most of the currently available information. Finally, there is a brief summary of some of the achievements in astronomy which have resulted directly from the developments and applications of these low-noise devices. A few notes are added, in conclusion, as pointers to the likely trends in the near future.
