  Radio astronomical imaging using aperture synthesis telescopes requires
deconvolution of the point spread function as well as calibration of the
instrumental characteristics (primary beam) and foreground
(ionospheric/atmospheric) effects. These effects vary in time and also across
the field of view, resulting in directionally-dependent (DD), time-varying
gains. The primary beam will deviate from the theoretical estimate in real
cases at levels that will limit the dynamic range of images if left
uncorrected. Ionospheric electron density variations cause time and position
variable refraction of sources. At low frequencies and sufficiently high
dynamic range this will also defocus the images producing error patterns that
vary with position and also with frequency due to the chromatic aberration of
synthesis telescopes. Superposition of such residual sidelobes can lead to
spurious spectral signals. Field-based ionospheric calibration as well as
"peeling" calibration of strong sources leads to images with higher dynamic
range and lower spurious signals but will be limited by sensitivity on the
necessary short-time scales. The results are improved images although some
artifacts remain.

Instrumentation and Methods for Astrophysics