Ground Calibration of the IXPE Optics and Telescope

The Imaging X-ray Polarization Explorer (IXPE) is a NASA Small Explorer mission to measure the polarization of astrophysical objects in the X-ray (2-8 keV) band set for launch in 2021. The calibration of three three flight optics took place in June-Aug 2020 at the NASA/MSFC Stray Light Test Facility (SLTF), a 100-meter vacuum beam line. One flight telescope (optic + detector: Mirror Module Assembly [MMA] + Detector Unit [DU]) and the flight spare were also calibrated at SLTF. We describe the calibration program and present results, including the measurement of the PSF and effective area of the MMAs at several energies, and the response of the telescope to incident polarized X-rays, including measurement of the Detector Modulation Factor (the response of the telescope to 100% polarized X-rays) and the telescope spurious modulation (response to an unpolarized X-ray source).