The potentialities and present status of masers and parametric amplifiers in radio astronomy

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.