Contribution of various sources of uncertainties to the cross-spectra leakage, for 143 HM1 × 143 HM2 (upper panels) and 143 HM1 × 217 HM2 (lower panels). The beam errors are based on Monte Carlo simulations of planet observations, mimicking the actual beam measurements. The gain calibration error assumes an independent relative uncertainty of 0.1% for each detector. The PE (between 0 and 1 by definition) is assumed to be known with an absolute error of a few tenths of a percent, as determined on pre-flight measurements (Rosset et al. 2010), while the polarization angles are assumed to be known with a precision of 1° for PSBs and 5° for SWBs.