X-CALIBUR IS A BALLOON BORNE HARD X-RAY POLARIMETRY MISSION COMBINING A GRAZING INCIDENCE 8M FOCAL LENGTH X-RAY MIRROR WITH A HARD X-RAY SCATTERING POLARIMETER MADE OF A SCATTERING ELEMENT SURROUNDED BY AN ASSEMBLY OF CADMIUM ZINC TELLURIDE (CZT) DETECTORS. THE COMBINATION OF A FOCUSING MULTILAYER MIRROR WITH AND AN ACTIVELY SHIELDED POLARIMETER GIVES X-CALIBUR EXCELLENT SENSITIVITY AND ENERGY RESOLUTIONS OVER THE BROAD ENERGY RANGE FROM 20 KEV TO 60 KEV. AFTER A CONVENTIONAL 1-DAY TEST FLIGHT FROM FORT SUMNER WE APPLY HERE FOR SUPPLEMENTARY FUNDING FOR A DECEMBER 2018-JANUARY 2019 LONG DURATION BALLOON (LDB) FLIGHT. THE LDB FLIGHT WILL BE LAUNCHED FROM MCMURDO (ANTARCTICA) AND WILL WILL ALLOW US TO MEASURE THE 20-60 KEV X-RAY POLARIZATION PROPERTIES OF FLARING XRAY BINARIES THE SUPERGIANT X-RAY BINARIES VELA X-1 GX 301-2 AND 4U 1700-377 AND THE PROTOTYPICAL RADIO GALAXY CENTAURUS A. THE LDB FLIGHT WILL TAKE ADVANTAGE OF AN OPTIMIZED POLARIMETER AND SHIELDING DESIGN WHICH IMPROVES THE OVERALL SIGNAL-TO-NOISE RATIO BY A FACTOR OF THREE COMPARED TO THE ORIGINAL DESIGN.