[fitswcs] Polarization codes

[Mark Calabretta]

On Thu 2008/03/13 19:56:05 -0000, Paddy Leahy wrote
in a message to: Doug Tody <dtody at nrao.edu>
and copied to: FITSWCS <fitswcs at nrao.edu>

Dear Paddy,

>Simple example: A common move in polarimetry is to convert (I,Q,U) 
>triplets into (Angle, Fractional polarization, I) triplets which are 
>easier to interpret (e.g. they map nicely to an HSV display). This 

This is a very nice use-case, but ironically it illustrates my point
about unrepresentable STOKES axes.

Stokes-I has STOKES code 1 and codes -1 to -8 and 2 to 4 are already
defined, and also 8 and 9 informally).  Therefore, the simplest way to
make it possible to store the said triple on one axis would be to give
"Angle" STOKES code 6, which forces "Fractional polarization" to have
code 11.  If, as originally suggested, the new codes started at 10 (or
100), then with "Angle" at 10 (or 100), "Fraction" would have to be
at 19 (or 199).

So you could force it to work for one triple, but then no doubt it
would be broken for most others.

Regards, Mark