Atmospheric Limitations to the Angular Resolution of Aperture Synthesis Radio Telescopes

Abstract

The technique of aperture synthesis has reached a stage where it is technologically feasible to construct a radio telescope several tens of kilometres in extent. The angular resolution of maps made with such an instrument on the surface of the Earth would, however, be limited by refraction in the troposphere and the ionosphere. Tropospheric irregularities having a scale size of about 1 km set a limit to the operation of large synthesis telescopes at the higher frequencies whilst ionospheric irregularities having a scale size of about 100 km become a hindrance at the lower frequencies. The permissible observing wavelengths and baselines are further limited by a systematic refraction due to the curvature of the Earth. A numerical simulation of the effects of the irregularities on the synthesized response of a typical instrument, and a consideration of the effects of systematic refraction, show that the limiting resolution of a telescope in middle latitudes is in the range $$0.1-{1}^{\prime{}\prime}$$ arc. In principle the effects of refraction could be reduced by determining the atmospheric parameters along the ray paths throughout the period of observation and applying suitable corrections. The effects of systematic refraction could be greatly reduced by this procedure, but it would in practice be extremely difficult to achieve an accuracy sufficient to correct for the effects of irregularities.