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It is well known that high-time-resolution imaging of the optical seeing disk reveals the existence of speckles inside the envelope. Since analogs of seeing disks occur in many different fields, one might expect that analogs of speckles also exist. Using radio-astronomical techniques, we have studied one such example in depth: interplanetary scintillation. By using a radio-interferometric array, we measured directly the coherence of the radiation after scattering by the solar wind and confirmed the existence of fine-scale structure similar to speckles. In order to understand further the behavior of the coherence function after scattering, we adapted and extended the theory previously developed by Cornwell and Napier [in Proceedings of the National Radio Astronomy Observatory Workshop on Radio Astronomy in Space ( National Radio Astronomy Observatory, Greenbank, W. Va., 1986); Radio Sci. 23, 739 ( 1988)] for the case of coherence measurements at the focal plane of a single-reflector radio telescope. We develop and demonstrate a test for coherence of the scattered radiation, and we discuss the prospects for high-angular-resolution imaging of objects. This treatment also leads to new insights into the effect of spatial coherence of emitters on interferometric measurements.
© 1989 Optical Society of America
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