13.1 Polarized Data Handling

Practically all IMAGER commands cannot handle more than 1 polarization state. So far, only two commands fully support polarized data: UV_TIME and STOKES.

• STOKES is the primary command that allows to derive or extract a UV data with only one Stokes parameter from a UV data set with several polarization states. IMAGER can then process the individual Stokes parameters separately.
• For convenience (because polarized data is obviously in general bigger), the UV_TIME command can be used for time averaging prior to use of the STOKES command.
• Most other commands will flatly refuse to handle data with more than 1 polarization state (e.g. UV_FILTER, UV_RESAMPLE, etc...).
• For debugging purpose, some commands like UV_PREVIEW, UV_MAP or UV_STAT will operate with more than 1 polarization state, but will not produce meaningful results (only a subset of the data may be treated).

Specific treatment is thus required when handling data with partial or complete polarization information.

• From UVFITS format by fits_to_uvt script
  When importing data, the fits_to_uvt script assumes by default the data is unpolarized and produces the pseudo-polarisation state "None" from the UVFITS file, by a properly weighted combination of the two parallel hand states if more than one state is present. The weighting is based on the relative noise in both polarization states.
  Full polarization information can be preserved by adding the /STOKES option to the @ fits_to_uvt command.

• By command READ UV
  READ UV command will read data with any polarization state(s). The handling depends on whether 4 states are present (in which case they are preserved), or only 2 (HH+VV or XX+YY, or RR+LL), in which case the 2 polarization data are merged into one state representing unpolarized emission, unless they are preserved by explicitely adding keyword STOKES. See HELP READ UV for details.

The resulting multi-polarization UV Tables should be split into tables with one polarization for further processing. This is done by command STOKES. To simplify data handling, we strongly recommend following strict naming conventions, as defined in the next section.