IRS 21 in 2005 (Wollaston). The plot shows the intensity divided by I_tot/2 as a function of angle θ (as defined in Eq. (3)). The data are obtained from mosaics. No calibration was applied. The green line marks the measured polarization angle, the blue the published angle (9.8%@14°) from Ott et al. (1999). In this plot the angle is counted in the instrument sense (negative with respect to sky).