Understanding radio polarimetry .3. Interpreting the IAU/IEEE definitions of the Stokes parameters

In two companion papers (Paper I, Hamaker et al. 1996; Paper II, Sault et al. 1996), a new theory of radio-interferometric polarimetry and its application to the calibration of interferometer arrays are presented. To complete our study of radio polarimetry, we examine here the definition of the Stokes parameters adopted by Commission 40 of the IAU (1974) and the way this definition works out in the mathematical equations. Using the formalism of Paper I, we give a simplified derivation of the frequently-cited 'black-box' formula originally derived by Morris et al. (1964). We show that their original version is in error in the sign of Stokes V, the correct sign being that given by Weller (1973) and Thompson et al. (1986).