The visibility datasets possibly contain correlations for different polarizations and Stokes parameters. There are two basic ways that polarization characteristics can be measured: firstly, two orthogonal feeds can be present on the antennas, and the four different polarization correlations from a baseline can be taken and recorded. Alternatively only one feed may be present, which can be rotated, or there are not enough correlators to calculate all four polarization correlations simultaneously. The last approach, used by the SMA allows all polarization parameters to be measured in a sequence.
In Miriad each different polarization/Stokes measurement is treated as a separate visibility. So if a baseline measures four simultaneous polarization correlations, then four `visibilities' will be produced for this baseline, per integration interval. In this case, each visibility will be tagged (with a u-v variable pol) to indicate the polarization type.
There are two ways the user can determine which polarizations he or she wants to process, either with the select parameter, or with the stokes parameter.
The select approach selects visibilities purely on the basis of their polarization or Stokes parameter. It is a normal part of the u-v selection process, as described in Section 2.3.2. The select mechanism cannot convert from raw polarization parameters to Stokes parameters. It just selects visibilities, in the file, based on their polarization. The general form is:
select=polarization(a,b,c,...)where a,b,c etc can be one of the mnemonics
Using the stokes approach is superficially similar, in that it causes only certain polarization/Stokes parameters to be processed. However, with the stokes approach, the software can perform conversion between raw polarizations and Stokes parameters if required. It also insists that all the requested polarization/Stokes parameters are calculable, at a given time, before it will allow any of them to be processed. For example, if the user requests Stokes I and V, and the dataset contains linear polarization data, then all four of rr, ll, rl and lr polarizations must be present for I and V to be returned in the case of circular polarization.
The general form of the stokes approach is:
stokes=a,b,c,...where a,b,c etc, can be one of the mnemonics given above (i, q, u, v, rr, ll etc).
The stokes can also take the values ii, qq and uu:
stokes=iand the dataset contains only, say, xx (xx is not Stokes-I for a polarized source). How if you requested
stokes=iiin this situation, the xx data would be passed through, because you have told Miriad that the source is unpolarised. Many tasks use
stokes=iias their default.