Top: polarimetric efficiency modeled (solid line) for the same derotator and HWP2 angles as used during the observation of TW Hydrae (squares). The polarimetric efficiencies measured for the HWP cycles of TW Hydrae are only determined relative to the other HWP cycles. We therefore scaled all data points such that the first cycle (red square, with the highest polarimetric efficiency) matches the value of the model. Bottom: residuals between the model and the polarimetric efficiencies obtained for TW Hydrae.