Design and tests of the hard X-ray polarimeter X-Calibur a b a a M. Beilicke , M.G. Baring , S. BarthelmyC, W.R. Binns , J. Buckleya, R. Cowsik , P. Dowkontta, A. Garsona, Q. Guoa, Y. Habad, M.H. Israela, H. Kuniedad, K. Leea, H. Matsumotod, T. l\1iyazawad, T. OkajimaC, J. SchnittmanC, K. Tamurad, J. Tuellerc, and a H. Krawczynski aDepartment of Physics and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, USA; bRice University, TX, USA; cGoddard Space Flight Center, MD, USA; dNagoya University, Japan; ABSTRACT X-ray polarimetry promises to give qualitatively new information about high-energy astrophysical sources, such as binary black hole systems, micro-quasars, active galactic nuclei, and gamma-ray bursts. We designed, built and tested a hard X-ray polarimeter X-Calibur to be used in the focal plane of the InFOCpS grazing incidence hard X-ray telescope. X-Calibur combines a low-Z Compton scatterer with a CZT detector assembly to measure the polarization of 10 - 80 keY X-rays making use of the fact that polarized photons Compton scatter preferentially perpendicular to the electric field orientation. X-Calibur achieves a high detection efficiency of order unity. Keywords: X-rays, polarization, black hole, InFOCuS, X-Calibur