Polarized Beam Patterns from a Multi-Moded Feed For Observations of the Cosmic Microwave Background

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We measure linearly polarized beam patterns for a multi-moded concentrator and compare the results to a simple model based on geometric optics. We convolve the measured co-polar and cross-polar beams with simulated maps of CMB polarization to estimate the amplitude of the systematic error resulting from the cross-polar beam response. The un-corrected error signal has typical amplitude of 3 nK, corresponding to inflationary B-mode amplitude r 10⁻³. Convolving the measured cross-polar beam pattern with maps of the CMB E-mode polarization provides a template for correcting the cross-polar response, reducing it to negligible levels.