  The Swift X-ray Telescope focal plane camera is a front-illuminated MOS CCD,
providing a spectral response kernel of 135 eV FWHM at 5.9 keV as measured
before launch. 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. Comparison of observed spectra with
models folded through the instrument response produces negative residuals
around and below the Oxygen edge. We discuss several possible causes for such
residuals. Traps created by proton damage on the CCD increase the charge
transfer inefficiency (CTI) over time. We describe the evolution of the CTI
since the launch and its effect on the CCD spectral resolution and the gain.
