[fitsbits] Representation of polarimetry data in WCS

[Marshall Perrin]

Hello all,

I've been writing polarimetric FITS files for some time now with no  
regards for how others do things, but I'm presently revising my 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