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

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 ((τ_{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 ((β)), and the release times ((τ_{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.

💡 Research Summary

The paper addresses a critical systematic error for the PLATO (PLAnetary Transits and Oscillations of stars) mission: charge transfer inefficiency (CTI) caused by radiation damage to the CCD detectors. PLATO aims to detect Earth‑size exoplanets around solar‑type stars and to measure planet‑to‑star radius ratios with better than 2 % accuracy, a requirement that translates into a stringent noise budget for transit depth measurements. The authors first quantify how CTI can bias transit depths, showing that in a worst‑case scenario (an 8‑year mission in a high‑CTI zone) the measured depth can be reduced by about 4 %.

To mitigate this, they develop a comprehensive calibration and correction pipeline. Using a realistic stellar field simulated with PLATOSim, they model the parallel overscan signal as a sum of exponential decays, fitting for the number of trap species and initial release times (τ₍r,k₎). Smearing is removed column‑wise with an exponential‑plus‑constant function. Crucially, they incorporate spatial variations in trap density across each CCD: the radiation map supplied by OHB System AG indicates that trap density scales roughly with the square of the radial distance from the focal‑plane centre. This is captured by a quadratic polynomial aₚₖ·R_norm² for each trap species k, where R_norm is the normalised radial coordinate.

Trap densities are scaled from reference values (Prod’homme et al. 2016) according to mission duration and the local radiation fluence, then distributed using the radial model to produce a cumulative trap count cntₖ