Measurements of submillimeter polarization induced by oblique reflection from aluminum alloy

We have measured the linear polarization induced in a beam of submillimeter radiation when it is obliquely reflected by a flat mirror made of aluminum alloy. For angles of incidence in the range 15°–45°, we measured induced polarizations in the range 0.05%–0.25%. Our measurements are within a factor of 2 of theoretical predictions. We conclude that astronomical telescopes that incorporate oblique reflections from good conductors will not introduce spurious polarizations large enough to cause significant problems for submillimeter polarimetric observations.