Overview of Difmap polarization mapping. Mapping polarization data in Difmap. ----------------------------------- Difmap has limited support for polarization mapping. The main limitation is that there is currently no way to combine two maps of different polarizations to form a percentage polarization map or other combinations. Such post-processing of images must be done in other programs such as AIPS. Also the only polarization calibration performed by Difmap is self-calibration. All absolute calibration must be performed outside of Difmap, say in AIPS. While it is only possible to make a map of one polarization at a time in Difmap, self-calibration corrections and editing applied while making one map are always applied to all other polarizations. This is a necessary feature, designed to ensure that polarization calibration and consistent UV coverage over all polarizations is preserved. So, if self-calibration is required, all polarizations can simultaneously be self-calibrated while making a single map of the strongest polarization. The same is true of the scope of self-calibration corrections wrt spectral-line data. The polarizations recorded in the originating UV FITS file can be displayed with the 'header' command. If an observation is recorded with multiple polarizations, the 'select' command must be used to select the polarization to be processed. This selection affects what is displayed in all plotting commands, and forms the data upon which all processing is based until the 'select' command is again called upon to change the selection. The selected polarization may either be one of these polarizations, or be the name of a polarization that can be derived from them. The names of recognised polarizations are: I Q U V RR LL RL LR XX YY XY YX The following is a list of polarizations that may be contructed from other polarizations: Name Derived from: Derived as: I RR and LL (RR + LL)/2 Q RL and LR (RL + LR)/2 U RL and LR i(LR - RL)/2 V RR and LL (RR - LL)/2 If you need to save one polarization map before making a second of a different polarization, you will have to save as a FITS file, for post-processing in another package, with the 'wmap' command.