The effects of charge transfer inefficiency (CTI) on galaxy shape measurements

TL;DR

This study investigates how charge transfer inefficiency in CCDs affects galaxy shape measurements crucial for weak gravitational lensing, predicting significant biases over time that can be mitigated with software and hardware improvements.

Contribution

The paper provides a simulation-based analysis of CTI effects on galaxy shape measurements, quantifies bias growth over time, and discusses mitigation strategies for future dark energy missions.

Findings

Charge traps with specific time constants impact galaxy shape measurements.

Bias in galaxy shape measurement increases at 2.65e-4 per year at L2.

Software mitigation can reduce CTI effects by a factor of 10.

Abstract

(Abridged) We examine the effects of charge transfer inefficiency (CTI) during CCD readout on galaxy shape measurements required by studies of weak gravitational lensing. We simulate a CCD readout with CTI such as that caused by charged particle radiation damage. We verify our simulations on data from laboratory-irradiated CCDs. Only charge traps with time constants of the same order as the time between row transfers during readout affect galaxy shape measurements. We characterize the effects of CTI on various galaxy populations. We baseline our study around p-channel CCDs that have been shown to have charge transfer efficiency up to an order of magnitude better than several models of n-channel CCDs designed for space applications. We predict that for galaxies furthest from the readout registers, bias in the measurement of galaxy shapes, Delta(e), will increase at a rate of 2.65 +/- 0.02 x 10^(-4) per year at L2 for accumulated radiation exposure averaged over the solar cycle. If uncorrected, this will consume the entire shape measurement error budget of a dark energy mission within about 4 years. Software mitigation techniques demonstrated elsewhere can reduce this by a factor of ~10, bringing the effect well below mission requirements. CCDs with higher CTI than the ones we studeied may not meet the requirements of future dark energy missions. We discuss ways in which hardware could be designed to further minimize the impact of CTI.