Vieregg, A. G.Ade, P. A. R.Aikin, R.Bischoff, C.Bock, J. J.Bonetti, J. A.Bradford, K. J.Brevik, J. A.Dowell, C. D.Duband, L.Filippini, J. P.Fliescher, S.Golwala, S. R.Gordon, M. S.Halpern, M.Hilton, G.Hristov, V. V.Irwin, K.Kernasovskiy, S.Kovac, JohnKuo, C. L.Leitch, E.Lueker, M.Montroy, T.Netterfield, C. B.Nguyen, H. T.O, R.Ogburn, R. W.Pryke, C.Ruhl, J. E.Runyan, M.Schwarz, R.Sheehy, C.Staniszewski, Z.Sudiwala, R.Teply, G.Tolan, J.Turner, A. D.Wilson, P.Wong, C. L.2016-03-032016-03-03Vieregg, A. G., P. A. R. Ade, R. Aikin, C. Bischoff, J. J. Bock, J. A. Bonetti, K. J. Bradford et al. 2012. Optical Characterization of the Keck Array Polarimeter at the South Pole. Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VI edited by Wayne S. Holland, Jonas Zmuidzinas, Proceedings of SPIE 8452: 845226. dx.doi.org/10.1117/12.9266390277-786Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:25682777The Keck Array (Spud) is a set of microwave polarimeters that observes from the South Pole at degree angular scales in search of a signature of Inflation imprinted as B-mode polarization in the Cosmic Microwave Background (CMB). The first three Keck Array receivers were deployed during the 2010-2011 Austral summer, followed by two new receivers in the 2011-2012 summer season, completing the full five-receiver array. All five receivers are currently observing at 150 GHz. The Keck Array employs the field-proven Bicep/Bicep2 strategy of using small, cold, on-axis refractive optics, providing excellent control of systematics while maintaining a large field of view. This design allows for full characterization of far-field optical performance using microwave sources on the ground. We describe our efforts to characterize the main beam shape and beam shape mismatch between co-located orthogonally-polarized detector pairs, and discuss the implications of measured differential beam parameters on temperature to polarization leakage in CMB analysis.en-USCosmic Microwave BackgroundpolarizationInflationThe Keck ArrayConference Paper2016-03-0310.1117/12.926639