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Radio astronomical imaging using aperture synthesis telescopes requiresdeconvolution of the point spread function as well as calibration of theinstrumental characteristics (primary beam) and foreground(ionospheric/atmospheric) effects. These effects vary in time and also acrossthe field of view, resulting in directionally-dependent (DD), time-varyinggains. The primary beam will deviate from the theoretical estimate in realcases at levels that will limit the dynamic range of images if leftuncorrected. Ionospheric electron density variations cause time and positionvariable refraction of sources. At low frequencies and sufficiently highdynamic range this will also defocus the images producing error patterns thatvary with position and also with frequency due to the chromatic aberration ofsynthesis telescopes. Superposition of such residual sidelobes can lead tospurious spectral signals. Field-based ionospheric calibration as well as"peeling" calibration of strong sources leads to images with higher dynamicrange and lower spurious signals but will be limited by sensitivity on thenecessary short-time scales. The results are improved images although someartifacts remain.

High Dynamic-Range Radio-Interferometric Images at 327 MHz