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Polarization/Stokes Handling (stokes and select)

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 first approach, used by the ATCA, allows all polarization parameters to be measured simultaneously. The second forces a `time-sharing' approach, where the feed or correlator must be switched between measuring one polarization to another. With the four simultaneous measurements, it is possible to convert a visibility from raw polarization parameter to a Stokes parameter. With the time-shared mode, this is not directly possible. These two different scenarios mean that two very different suites of software are needed to obtain Stokes parameters.

In Miriad, each different polarization/Stokes measurement is treated as a separate visibility (this differs from the AIPS approach - which does not really support the time-shared approach to polarization measurement). 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 the previous section. 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

• For Stokes parameters: i, q, u, v
• For circular polarization parameters: rr, ll, rl, lr
• For linear polarization parameters: xx, yy, xy and yx

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 xx, yy, xy and yx polarizations must be present for I and V to be returned.

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:

• Mnemonic ii returns Stokes-I given the assumption that the source is unpolarised. For example Miriad would not return any data if you requested stokes=i and the dataset contains only, say, xx (xx is not Stokes-I for a polarised source). How if you requested stokes=ii in this situation, the xx data would be passed through, because you have told Miriad that the source is unpolarised. Many tasks use stokes=ii as their default.
• Mnemonic qq and uu returns Stokes-Q and Stokes-U given the assumption that the parallactic angle is 0. These quantities are rarely of astronomical interest, but they are useful to investigate some instrumental effects.

The select mechanism is usually used for time-shared polarization measurements, whereas the stokes mechanism is usually used where there are simultaneous measurements. However this is not a hard and fast rule. The two approaches generally cannot be used at the same time. Some tasks will prohibit the use of the select approach altogether (they will give error messages if you attempt to), whereas others are more lenient. The rules which determine whether you can or cannot use the select approach are quite arcane and can be file dependent. Check the documentation for each task, especially if both select and stokes parameters are present. If polarization selection is not allowed, then increment selection will also be prohibited (due to some arcane quirk).