  The Swift X-ray Telescope (XRT) focal plane camera is a front-illuminated MOS
CCD, providing a spectral response kernel of 144 eV FWHM at 6.5 keV. We
describe the CCD calibration program based on celestial and on-board
calibration sources, relevant in-flight experiences, and developments in the
CCD response model. We illustrate how the revised response model describes the
calibration sources well. Loss of temperature control motivated a laboratory
program to re-optimize the CCD substrate voltage, we describe the small changes
in the CCD response that would result from use of a substrate voltage of 6V.
