5.8 Polarization

No convention has been formally and universally adopted within FITS to describe polarization. The convention most widely used is the one developed by NRAO that is supported in AIPS. This convention has also been adopted in a proposed standard FITS form for interferometry intended to replace Random Groups.

When values of the Stokes parameters are mapped, an axis with the CTYPEn value 'STOKES  ' is used to describe which Stokes or correlator parameters are displayed in the images. The values and the parameters they denote are shown in Table 5.3.

 

-8 -7 -6 -5 -4 -3 -2 -1 1 2 3 4

YX XY YY XX LR RL LL RR I Q U V

X and Y are linear polarizations with fixed orientation relative to the antenna. In the interferometry proposal, X is horizontal and Y vertical. R and L refer to right and left circular polarization. The convention has been suggested that -2 be used to refer to left circularly polarized images and -1 to right circularly polarized images. The definitions of circular polarization follow the IEEE convention; right circular polarization corresponds to the electric vector of an approaching wave rotating clockwise. This convention should be noted in the comments; use of the opposite convention is not unknown.