Raw polarization spectra often have a PA that is a slow function of wavelength due to the slight chromicity of the waveplate. Two commands are provided in order to calibrate this effect (FITPA and PACALIB). The FITPA command fits a polynomial to the difference spectrum of the observed PA spectrum (usually a spectrum taken through a 100% polarizing filter) and the actual PA. Care must be taken that the observed PA spectrum does not wrap-around (i.e. flip from to ) as this will spoil the PA fit. This is not checked for by polmap, but may be fixed with a call to ROTPA prior to the fit. The PACALIB command applies this fit to the current arrays.