ABSTRACT
Karl Jansky detected radio signals serendipitously from an extraterrestrial source, marking the beginning of the field of radio astronomy. The field of radio astronomy has advanced significantly since those early days, and observations at radio wavelengths today play a vital role in the study of astronomical objects ranging from solar system bodies to distant quasars. The full range of electromagnetic radiation is enormous, spanning wavelengths from the very shortest gamma-ray radiation, through the x-ray, ultraviolet, optical and infrared, and ending with the very long radio wavelengths. The techniques used for filtering, dispersing and detecting light at optical and radio wavelengths can be quite different. The collecting area of most radio telescopes is determined by the size of their primary reflector. Radio astronomers never adopted the practice of using magnitudes for flux density, but instead use real physical units of flux density.
