Polarisation Handling

Models to the self-calibration tasks should generally be Stokes parameters (I, Q, U or V). The biggest difference between selfcal and gpscal is in the handling of polarisation and dual feeds.

Though selfcal can handle multiple pointings and frequency ranges, it is quite simplistic in its approach to polarisations.

• selfcal assumes a single gain for each antenna. If the antenna has dual feeds, they are assumed to have equal gains. This may be a reasonable assumption if phase errors are the dominant ones, and if the XY phase is constant and has been well determined. It is somewhat less likely that the amplitude gain errors will be closely matched between the feeds.

• Generally selfcal is unable to convert between raw polarisations and Stokes parameters. This means that the visibility dataset must contain a correlation which matches the Stokes or polarisation type of each model. For example, if selfcal is given a Stokes Q model, then the dataset being calibrated must contain Stokes Q visibilities.

• selfcal generally assumes that a source is unpolarised. In this case it treats parallel-hand polarisation parameters (XX, YY, RR and LL) as if they were measures of Stokes I. It will also refuse to accept models of Stokes Q, U and V. Giving options=polarized will allow selfcal to accept Stokes Q, U and V models, but it will no longer assume that parallel-hand correlations are measures of Stokes I.

• selfcal will accept multiple input models which represent the data at different polarisations, different pointing centres and different frequency or velocity ranges. As the gains are assumed to be independent of these, all the information is used in a simultaneous solution.

Task gpscal is intended for telescopes with dual linear feeds, such as the ATCA. Indeed, it will fail for other sorts of data.

• gpscal solves for the two feeds either totally independently, or with the constraint that the XY phase of an antenna must be constant with time.
• gpscal converts between Stokes parameters and raw polarisations. It can take multiple input models for different Stokes parameters. However, if a model of a particular Stokes parameter is not given, it is assumed to be zero. For example, if you give it models of I, Q and U, but not V, then the source is assumed to have no circular polarisation.
• gpscal uses the antenna polarisation leakage table if this information is present. If it is not present, leakages are assumed to be zero.
• Unlike selfcal , gpscal cannot take models from multiple pointings or from different frequency ranges. All models must correspond to the same pointing and same frequency range.