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Data Handling

Importing data
When importing data, the violetfits_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 magenta /STOKES option to the magenta@ fits_to_uvt command.

The magentaREAD UV command will read data with any polarization state(s).



Processing data
On the contrary, practically all [rgb]1,0,0IMAGER commands cannot handle more than 1 polarization state. So far, only two commands fully support polarized data: magentaUV_TIME and magentaSTOKES.

  • magentaSTOKES 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. [rgb]1,0,0IMAGER can then process the individual Stokes parameters separately.
  • For convenience (because polarized data is obviously in general bigger), the magentaUV_TIME command can be used for time averaging prior to use of the magentaSTOKES command.
  • Most other commands will flatly refuse to handle data with more than 1 polarization state (e.g. magentaUV_FILTER, magentaUV_RESAMPLE, etc...).
  • For debugging purpose, some commands like magentaUV_PREVIEW, magentaUV_MAP or magentaUV_STAT will operate with more than 1 polarization state, but will not produce meaningful results (only a subset of the data may be treated).

Some imaging strategies cannot be used directly on polarized data. Also, the automatic definition of supports (magentaMASK /THRESHOLD) only makes sense on Stokes I or parallel hand polarization states, not on Stokes Q,U,V or cross-hand states. The same applies to spectral line identifications.



PIPELINE processing The PIPELINE tools does not automatically handle polarization data. However, through judicious use of the magentaPIPELINE SELECT and magentaPIPELINE FIND commands with ad-hoc filters, imaging, including self-calibration, is possible through custom (though simple) scripts.

The script violetself-polar.ima just does that, and follows the naming conventions used by the magentaMAP_POLAR command. It contains the following commands

  CATALOG               ! No catalog, no Line emission
  ! "Split the X+Y (XX,YY,XY,YX) UVFITS file into I,Q,U,V Stokes UV tables"
  @ stokes-split &1     
  ! Setup the pipeline
  IMAGER\PIPELINE ?     
  IMAGER\PIPELINE organize        !  " Move UV tables to RAW/"
  sic mkdir UVFITS
  SIC\SYSTEM " mv *.uvfits UVFITS/"
  SIC\SYSTEM " mv RAW/&1.uvt .."  !   "Remove the X+Y UV table from RAW/"
  !
  ! Select the I image for self-calibration
  let all%filter *-I
  IMAGER\PIPELINE select
  IMAGER\PIPELINE compute
  !
  ! Apply self-calibration solution to I,Q,U,V data
  let all%filter *
  IMAGER\PIPELINE find
  IMAGER\PIPELINE select
  IMAGER\PIPELINE collect
  IMAGER\PIPELINE apply
  ! Image every thing consistently
  IMAGER\PIPELINE image
  PIPELINE SAVE



Analysing data
The magentaMAP_POLAR command allows to derive images of the Polarized intensity and/or fraction and Polarization angle, as well as display of the polarization vectors onto a background image. So far, the command only works for a single plane image.

The magentaMAP_POLAR command works through data files that follow conventional naming rules to identify them. The implicit rule is that Stokes S (where S is any of I,Q,U,V) is stored in a file of name 'NameBegin'"-N"'NameEnd', e.g. if 'NameBegin' is My_Data

    My_Data-I+C.uvt,  My_Data-U+C.lmv-clean, etc..


next up previous contents index
Next: The STOKES command Up: Polarization Handling Previous: Polarization Handling   Contents   Index
Gildas manager 2023-06-01