On the use of Bowen compensators for polarimetry

For high precision polarimetry it is very desirable to observe with a telescope that is free of instrumental polarization. Yet it is often inevitable to introduce optical devices which generate crosstalk between the Stokes Q, U components and the V component due to phase retardations upon reflection at mirror surfaces. We show that Bowen compensators are well suited to compensate for these phase changes. They consist of 2 rotatable lambda/8 phase retarder plates, and it can be shown that they act on polarized light either by a rotation of the axes of the polarization ellipse followed by a prescribed, desired phase change or by a phase change followed by rotation of the ellipse. We present applications of Bowen compensators in the Gregory Coude Telescope at the Observatorio del Teide/Tenerife.