Mitigating charge transfer inefficiency in the Chandra X-Ray Observatory advanced CCD imaging spectrometer

The ACIS front-illuminated CCDs on board the Chandra X-Ray Observatory were damaged in the extreme environment of the Earth's radiation belts, resulti ng in enhanced charge transfer inefficiency (CTI). This produces a row depe ndence in gain, event grade, and energy resolution. We model the CTI as a f unction of input photon energy, including the effects of detrapping (charge trailing), shielding within an event (charge in the leading pixels of the 3 x 3 event island protects the rest of the island by filling traps), and n onuniform spatial distribution of traps. This technique cannot fully recove r the degraded energy resolution but it reduces the position dependence of gain and grade distributions. By correcting the grade distributions as well as the event amplitudes, we can improve the instrument's quantum efficienc y. We outline our model for CTI correction and discuss how the corrector ca n improve astrophysical results derived from ACIS data.