Keywords: imager

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## The STOKES command

The magenta`STOKES` command operates on the current UV buffer, or on files if the magenta`/FILE` option is present. It allows to extract a UV data with visibilities for one output (pseudo-)Stokes parameter from UV data with visibilities with 1,2 or 4 (pseudo-)Stokes parameters. Besides the standard Stokes parameters I, Q, U, V, RR, LL, RL, LR (Left and Right circular), XX, YY, XY and YX (X and Y linear) which are defined in the Sky frame10, command magenta`STOKES` recognizes pseudo-Stokes values NONE, HH, VV, HV and VH which are the linear polarization states in the frame of the antennas (Horizontal and Vertical pure states).

Conversion from the H-V pseudo-Stokes polarization states to any standard Stokes parameter is made by the magenta`STOKES` command by applying the rotation due to the parallactic angle. For this, the UV data set must contain the `PARA_ANGLE` extra column. If it is not present, it can be added to the data set by command magenta`UV_ADD /FILE`. That command can also insert the Doppler correction as an extra column.

Script violet`stokes-split` can be used to split a full polarization UV table in 4 UV tables, one for each IQUV Stokes parameter, following the standard naming convention expected for final image use by magenta`MAP_POLAR`.

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