Refocusing a Radio Telescope to Image Sources Within the Nearfield of the Antenna Array

摘要

A theory is developed from three forms of the Fraunhofer approximation that describes a method for a refocusing a radio telescope to image sources that are within the nearfield of the antenna array. In the first section, the Fraunhofer approximations of the first, second, and third kinds are derived and studied in applications to the propagation of a scalar field amplitude from sources to detectors. In the second section, these approximations are applied to the propagation of the cross-spectral density function of coherence theory. Important differences between the conditions for validity and the mathematical structure of these three approximations are described. In the final section, a theoretical model is developed to describe the operation of a radio telescope using the theory developed in the first two sections. This model is used to develop a method for refocusing a radio telescope to image sources that are within the nearfield of the antenna array. Two methods for refocusing the telescope are described that would require no modification to most existing telescopes but would require some modifications to the data processing software. Keywords: Radio astronomy; Coherency theory; Radio telescope; Fraunhofer; Nearfield. (RH)