Image Fidelity

by Stephan W. Witz last modified Apr 29, 2013 by Stephan Witz

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 atmospheric phase stability, although pointing errors and changes in atmospheric opacity can also be a limiting factor.  If the target source contains compact structures of sufficient strength (depending on the band, bandwidth, atmospheric coherence time, and source complexity), self-calibration can be counted on to improve the images.  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.