Resolution, pattern effects, and range of radio telescopes

Important source parameters and the characteristics of an ideal radio telescope are outlined. The resolution of a telescope antenna is given by Rayleigh´s criterion as one-half the beamwidth between first nulls. The effect of source extent on the observed antenna pattern and the inverse problem of determining the source distribution from the measured pattern are considered. The range of a radio telescope is discussed and it is shown that some types of celestial sources could be detected far beyond the celestial horizon if such did not exist. The range of the largest optical telescopes is only half the distance to the celestial horizon, and it is pointed out that observations with large radio telescopes may be vital in determining whether a celestial horizon does in fact exist. The ultimate number of celestial sources that can be resolved with any radio telescope is given by Ko´s criterion as numerically equal to the directivity of the telescope antenna.