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Fig. 4.


Left: polarimetric efficiency as a function of derotator angle for all broadband filters listed in Table 1. The green squares show the H-band polarimetric efficiencies measured for TW Hydrae scaled to the model curves (Sect. 5.3). The green dashed curve shows the model polarimetric efficiency for the range 0° ≤θder ≤ 90° in H band. This filter shows the strongest asymmetry around θder = 90°, which is caused by the non-ideal retardance of HWP2. Right: model polarization angle offset plotted against θder for the same filters (solid and dashed lines; as before, the dashed line covers the range 0° ≤θder ≤ 90°). In the ideal case, the polarization angle offset would remain 0°. However, there is a clear dependency on θder for H and Ks band, and to a lesser extent, for Y band, while the J band remains close to ideal. The polarization angle offsets measured as ϕ0 for TW Hydrae are plotted as green squares. The deviation of the on-sky data from the model curves is caused by the crosstalk contribution of other optical components.

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