Investigation of deferred charge effects in Large Synoptic Survey Telescope ITL sensors

Adam Snyder,Aaron Roodman

doi:10.1117/1.jatis.5.4.041509

Abstract

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.

Highlights

Integration and testing of the Large Synoptic Survey Telescope (LSST) camera subcomponents is currently being performed at the SLAC National Accelerator Laboratory.[1,2] The camera focal plane consists of 189 charge-coupled devices (CCDs) arranged into 21 stand-alone raft tower modules (RTMs) that include the necessary electronic, mechanical, and thermal support components.[3]
The amount of deferred charge that occurs during readout is most often characterized by the charge transfer inefficiency (CTI), defined as the ratio of electrons not transferred between two neighboring pixels to the total electrons before the transfer, and is measured for both parallel and serial pixel transfers
The results presented here are from a single sensor that is representative of a typical Imaging Technology Laboratory (ITL) sensor; all LSST ITL sensors show similar high and low signal serial deferred charge effects

Summary

Introduction

Integration and testing of the Large Synoptic Survey Telescope (LSST) camera subcomponents is currently being performed at the SLAC National Accelerator Laboratory.[1,2] The camera focal plane consists of 189 charge-coupled devices (CCDs) arranged into 21 stand-alone raft tower modules (RTMs) that include the necessary electronic, mechanical, and thermal support components.[3]. Deferred charge at each pixel transfer may be introduced due to the fast sensor readout, due to the interplay of the characteristic time scales for self-induced drift, fringing field drift, and thermal drift compared to the pixel transfer time For these reasons, the CTI is assumed to be dominated by proportional loss (deferred charge proportional to signal) that occurs at every pixel transfer and has a weak dependence on signal. A parallel study of the overscan pixel signal values for high-signal levels showed an additional deferred charge effect that does not appear in CTI from EPER measurements.

Low-Signal Deferred Charge

High-Signal Deferred Charge

Findings

Conclusions