[fitsbits] Stokes parameters coordinate system

[Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC]]

Folks:

WCS Paper I describes a convention for Stokes parameters.  I’ve been asked how to best represent the coordinate system that these Stokes parameters are expressed in, specifically the Q and U parameters.  Is there a standard practice of how this should be done?  It appears that some space-based instruments define Q and U relative to instrument (pixel) coordinates.  However, the comment was made that this is not really practical for a ground-based alt-az telescope due to field rotation.  Another possible way to encode the Stokes parameters would be relative to the real-world coordinates of the data.

Suppose that one had a three dimensional cube with the following axis definitions:

CTYPE1 = ‘RA---TAN’
CTYPE2 = ‘DEC--TAN’
CTYPE3 = ‘Stokes’

And that the PC matrix had cross terms between axes 1 and 2, i.e. image rotation.  Is there a convention for how the Q and U values should be interpreted?  For example, if Q/I=1, would you interpret that as polarization aligned along the first pixel direction, or along the RA direction as defined by the PC rotation matrix?

Is there a better way to distinguish between these cases other than putting a comment into the header?

Thank you,

Bill Thompson

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