Mitigating Charge Transfer Inefficiency in the [ITAL]Chandra X-Ray Observatory[/ITAL] Advanced CCD Imaging Spectrometer

The ACIS front-illuminated CCDs onboard the Chandra X-ray Observatory weredamaged in the extreme environment of the Earth's radiation belts, resulting inenhanced charge transfer inefficiency (CTI). This produces a row dependence ingain, event grade, and energy resolution. We model the CTI as a function ofinput photon energy, including the effects of de-trapping (charge trailing),shielding within an event (charge in the leading pixels of the 3X3 event islandprotect the rest of the island by filling traps), and non-uniform spatialdistribution of traps. This technique cannot fully recover the degraded energyresolution, but it reduces the position dependence of gain and gradedistributions. By correcting the grade distributions as well as the eventamplitudes, we can improve the instrument's quantum efficiency. We outline ourmodel for CTI correction and discuss how the corrector can improveastrophysical results derived from ACIS data.