Investigation of Deferred Charge Effects in LSST ITL Sensors

The traditional characterization of charge transfer inefficiency (CTI) in charge-coupled devices (CCDs) can suffer from a number of deficiencies: CTI is often only calculated for a limited number of signal levels, CTI is calculated from a limited number of pixels, and the sources of CTI are usually assumed to occur at every pixel-to-pixel transfer. A number of serial CTI effects have been identified during preliminary testing of CCDs developed by Imaging Technology Laboratory (ITL) for use in the Large Synoptic Survey Telescope (LSST) camera focal plane that motivate additional study beyond the traditional CTI characterization. This study describes a more detailed examination of the serial deferred charge effects in order to fully characterize the deferred charge measured in the serial overscan pixels of these sensors. The results indicate that in addition to proportional CTI loss that occurs at each pixel transfer, ITL CCDs have additional contributions to the deferred charge measured in serial overscan pixels, likely caused by fixed CTI loss due to charge trapping, and an electronic offset drift at high signal.