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Fig. 8.


a1, a2, and a3: QIPS images of three HWP cycles with θder = 169.7° ,162.0°, and 154.1°. These images are created with the Uϕ minimization method (Sect. 6.1). The butterfly patterns clearly rotate in clockwise direction and the signal decreases from left to right. b: when we use the assumption of azimuthal polarization, we can find the correct ϕ0 for each HWP cycle and compute the final r2-scaled Qϕ (b1) and Uϕ (b2) images (Sect. 6.1). Because we used the assumption that we know the angle of linear polarization at each point, we cannot claim to have determined the AoLP in this image. c: when we apply the correction method to the Q and U images, we retrieve the best approximation of the incident Q and U images (Sect. 6.2). The displayed Q images (for the same θder as in panel a) contain the ideal orientation of the butterflies and are corrected for the reduced polarimetric efficiency. However, the latter does not improve the signal-to-noise ratio (illustrated by the increased noise in panel c3). d1 and d2: IRDAP-corrected Qϕ and Uϕ image. Because we no longer need the assumption that we know the AoLP a priori, we can use Q and U after the correction method to determine the AoLP in our final reduction.

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