Figure 1. Schematic diagram of the experiment. View is from above. The radiation from a blackbody radiation soure (BB) is modulated by a rotary chopper (CH). The gold-coated focusing mirror (FM) images the aperture of the blackbody onto the focal plane of the polarimeter (P). Three rays are shown. The angle of incidence of the radiation at the aluminum mirror (AM) was varied by rotating this mirror and physically moving the polarimeter. Both AM and P were mounted on rotary milling tables, and the polarimeter mount had two additional translational degrees of freedom. As described in section III, we were able to measure the polarization introduced by the reflection at AM.