Median telescope polarisation for α Cen A from 2017 inN_R-band measured for both Stokes parameters – Q and U – as a function of the telescope parallactic angle θ_par. The plots show the measurements for camera 1 (+) and camera 2 (◇). The measurements have been fit with a model function (solid lines) resulting in a measurement of the telescope polarisation amplitude p_tel and phase δ_tel for both cameras.