Histograms of two distributions used to estimate the uncertainty due to an imperfect setting of the retarder waveplate (see text) and their best fits with Gaussian distributions. Blue lines correspond to the data obtained without order-separating filter, and red lines to the data obtained with GG435 filter. Best fits were obtained using data with | P_Q | ≥ 0.5% and | P_U | ≥ 0.5%, respectively, as explained in the text.