National Radio
Astronomy Observatory

5.2.9 Parameter stokes

The stokes parameter specifies the Stokes parameters for the resulting images. Note that forming Stokes Q and U images requires the presence of cross-hand polarizations (e.g. RL and LR for circularly polarized systems such as the VLA) in the data. Stokes V requires both parallel hands (RR and :LL) for circularly polarized systems or the cross-hands (XY and YX) for linearly polarized systems such as ALMA and ATCA.

This parameter is specified as a string of up to four letters and can indicate stokes parameters themselves, Right/Left hand polarization products, or linear polarization products (X/Y). For example,

are common choices (see the inline help of clean for a full range of possible options). The output image will have planes (along the “polarization axis”) corresponding to the chosen Stokes parameters.

If as input to deconvolution tasks such as clean, the stokes parameter includes polarization planes other than I, then choosing psfmode=’hogbom’ (§ 5.3.1.2) or psfmode=’clarkstokes’ (§ 5.3.1.3) will clean (search for components) each plane sequentially, while psfmode=’clark’ (§ 5.3.1.1) will deconvolve jointly.

Alert: As of Release 3.2, clean expects that all input polarizations are present. E.g. if you have RR and LL dual polarization data and you flagged parts of RR but not LL, clean will ignore both polarizations in slice. It is possible to split out a polarization product with split and image separately. But you will not be able to combine these part-flagged data in the uv-domain. We will remove that restriction in a future CASA release.

More information about CASA may be found at the CASA web page

Copyright © 2010 Associated Universities Inc., Washington, D.C.

This code is available under the terms of the GNU General Public Lincense