Image Fidelity

by Stephan W. Witz last modified Dec 03, 2014 by Gustaaf Van Moorsel

Image fidelity is a measure of the accuracy of the reconstructed sky brightness distribution. A related metric, dynamic range, is a measure of the degree to which imaging artifacts around strong sources are suppressed, which in turn implies a higher fidelity of the on-source reconstruction.

With conventional external calibration methods, even under the best observing conditions, the achieved dynamic range will rarely exceed a few hundred.  The limiting factor is most often the effective phase stability of the telescope due to atmospheric/ionospheric fluctuations, although pointing errors and changes in atmospheric opacity can also be a limiting factor.  If a good model of the sky brightness distribution exist (e.g. use of compact structures of sufficient strength, though a good model of resolved sources in the field of view may also be used), standard self-calibration can be counted on to improve the images.  At low frequencies where the dominant phase error is due to ionospheric plasma density fluctuations, more advanced techniques may be required to account for change of ionospheric phase across the field of view.  Dynamic ranges in the thousands to hundreds of thousands can be achieved using these techniques, depending on the underlying nature of the errors. With the new WIDAR correlator and its much greater bandwidths and much higher sensitivities, self-calibration methods can be extended to observations of sources with much lower flux densities than very possible with the old VLA.

The choice of image reconstruction algorithm also affects the correctness of the on-source brightness distribution. The CLEAN algorithm is most appropriate for predominantly point-source dominated fields. Extended structure is better reconstructed with multi-resolution and multi-scale algorithms. For high dynamic ranges with wide bandwidths, algorithms that model the sky spectrum as well as the average intensity can yield more accurate reconstructions.