Impact of Charge Transfer Inefficiency on transit light-curves: A correction strategy for PLATO

TL;DR

This paper presents a correction strategy for charge transfer inefficiency (CTI) effects on transit light-curves in the PLATO mission, ensuring accurate exoplanet measurements despite radiation damage to CCDs.

Contribution

The authors developed a novel calibration and correction method for CTI in CCDs, restoring transit depths within the mission's accuracy requirements.

Findings

CTI causes up to 4% bias in transit depth measurements.

The correction reduces bias to 0.06%, meeting PLATO's accuracy goals.

The method effectively compensates for spatial and temporal variations in CTI.

Abstract

PLATO is designed to detect Earth-sized exoplanets around solar-type stars and to measure their radii with accuracy better than \(2\%\) via the transit method. Charge transfer inefficiency (CTI), a by-product of radiation damage to CCDs, can jeopardise this accuracy and therefore must be corrected. We assessed and quantified the impact of CTI on transit-depth measurements and developed a correction strategy that restores CTI-biased depths within the accuracy budget. Using a calibration dataset generated with PLATOSim to simulate a realistic stellar field, we modelled the parallel overscan signal as a sum of exponential decays and used least-squares fitting to infer the number of trap species and initial estimates for the release times (\(\tau_{r,k}\)). Smearing was modelled with an exponential-plus-constant function and removed on a column-wise basis. We modelled the spatial variation in trap density with a quadratic polynomial in radial distance from the focal-plane center. The polynomial coefficients (\(a_{p,k}\)), the well-fill power index (\(\beta\)), and the release times (\(\tau_{r,k}\)) were adjusted via an iterative application of the extended pixel edge response (EPER) method combined with a CTI correction algorithm, yielding the final calibration model. In the worst-case scenario (8-year mission, high-CTI zone), CTI induced a bias of about \(4\%\) in measured transit depth, reduced to a residual of \(0.06\%\) after correction - well within PLATO's accuracy requirements. From the calibrated parameters, we derived a correction scheme that brought the photometric measurements within PLATO's noise budget, ensuring that the mission's precision requirements are met.