High spatial resolution solar polarimetry with interference filters

Abstract

A new type of two-dimensional polarimeter is used to measure the center-to-limb variation of the scattering induced polarization in a narrow continuum window up to the extreme limb. The polarimeter is set on the Tenerife Gregory Coude telescope's hour axis, where the two folding flat mirrors cancel their polarizing effects for zero solar declination at the equinox. The short CCD exposure of only 5 ms allows high spatial resolution images in polarized light. A beam switching technique together with an integration parallel to the solar limb over 20", yields a high polarimetric accuracy with an rms noise of 2 x 10(-4). Our results for a continuum window at 4506-4508 Angstrom agree with model calculations down to limb distances of 0."32 (i.e. cos theta <0.025).