Maintenance of the CCF components relies mainly on observing the emission lines of the internal calibration source and adjusting the coefficient in several time periods (the same intervals as the CTI) since launch.
Further analysis is made by checking spectral residuals near fluorescent background lines and absorption edges.
Note that from issue 12 of these CCFs onwards all quadratic terms have been set to zero, i.e. a linear relationship is assumed between the charge detected from an event and the energy of the detected X-ray hence :
