Spatially constrained direction-dependent calibration.

Direction-dependent calibration of widefield radio interferometers estimates the systematic errors, along with multiple directions in the sky. This is necessary because with most systematic errors, which are caused by effects such as the ionosphere or the receiver beam shape, there is a significant spatial variation. Fortunately, there is some deterministic behaviour of these variations in most situations. We enforce this underlying smooth spatial behaviour of systematic errors as an additional constraint on to spectrally constrained direction-dependent calibration. Using both analysis and simulations, we show that this additional spatial constraint improves the performance of multifrequency direction-dependent calibration. [ABSTRACT FROM AUTHOR]