Radio scattering and imaging

Abstract

The problem of scattering-induced image distortion ('seeing') in radio astronomy is studied theoretically, emphasizing the strong scattering regimes. The thin-screen approximation is utilized, where a single effective phase-changing screen replaces the extended scattering medium. Length scales are appropriately redefined for an infinitely distant source to characterize incident radiation as plane wavefronts. Imaging regimes, including speckle, seeing disk, snapshot, average, and ensemble-average images are defined and identified for a number of astronomical scattering media. Strong scattering is shown to occur in all cases except in the troposphere, and very strong scattering is identified in the case of scattering by the interstellar medium. Source imaging of very high resolution is allowed by the snapshot regime when a fully redundant array and good signal/noise ratios within extremely short integration times are present. Examining the magnitude and spectrum of refractive-index fluctuations is facilitated by the average regime.