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Representation of polarimetry data in WCS
I've been writing polarimetric FITS files for some time now with no regards for how others do things, but I'm now revising lots of old code and thought I might take the time to properly standardize it. But first I need to actually be sure I understand what the standard is! WCS Paper I defines a 'STOKES' axis convention for storing polarimetric data, which uses a series of integers to denote the various Stokes parameters and/or other forms of polarimetry data. WCS Paper III defines the -TAB convention for representing nonlinear coordinates via table lookup. It seems like it should be fairly straightforward to combine these two forms to easily handle arbitrary arangements of polarimetry data. Paper III says that the CTYPEn keyword for a -TAB axis should follow the 4-3 format, using the 4-letter axis type. So then the proper value to use is 'STOK-TAB', correct? Next question. Table 7 of WCS Paper I presents the mapping between integer codes and polarimetry measurements such as Stokes Q or right-circular polarization. But there are a few possible polarimetry measurements which are left out of this table, in particular polarized intensity (P = sqrt(Q^2+U^2+V^2). Is there any convention on an integer code to denote that with? Thanks, - Marshall |
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