Fringe-fitting in CASA

© (c) Des Small, 2016

Averaging over larger time and frequency intervals allows radio astronomers to observe fainter objects. In VLBI this is particularly challenging: due to the long baselines, small instrumental errors cause the phases to wind much faster with time and frequency than in traditional radio astronomy. To achieve phase stability, a fringe-fitting algorithm is used to calculate phase delays (as function of frequency) and phase rates (as function of time).

Until now, this has generally been done in the AIPS software package; the lack of a fringe-fitting task in CASA has made it unsuitable for VLBI data processing. As part of the BlackHoleCam and SKA-NL Roadmap projects, JIVE is working to add full VLBI capability to CASA, including fringe-fitting.

A prototype of the fringe fitter code is now available and supporting functionality for applying the delay and rate solutions is being implemented in CASA.

Fringe fitting is usually a two step process. A bright fringe-finder source is used to determine the overall instrumental delays and apply these to the entire observation. In the second step, a fainter phase-reference is used to correct the residual variations in delay, and determine the phase rate. Phase referencing is a standard VLBI technique in which observations alternate between the target and a nearby phase reference source. The corrections derived for the phase reference source are applied to the target.

The first image shows the phases as function of frequency for the phase reference source after applying the instrumental delay, but *before* applying the residual delay solutions over the entire frequency band. The second image shows the phases *after* the residual delay is applied. As expected, the phases are successfully flattened across the full width of the frequency range and on all baselines.

The fringe-fitter prototype is now fully functional, and will be implemented in CASA soon, as the new task "fringecal".