These can be interpreted as emerging defects still in an unstable configuration, similarly to the radiation-induced defects measured on board the Euclid mission CCDs.]

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[Figure: Fig. 13: Evolution of the total density of traps for the CCD in row 3 and AF strip 6. The observed trend is caused by a gradual increase in damage caused by cosmic rays and sudden step jumps in correspondence to solar flares event, shown as the red lines for the class X8.2 flare of September 2017 and as the start of the intense period of solar activity of Spring-Summer 2024.]

In the attempt to apply the calibrated CtiPixel model for science data processing, the PSF model was used in combination with the background modelling and other auxiliary calibrations such as the bias non-uniformity corrections to estimate the science inputs, applying the CtiPixel algorithm to derive a complete model of a selected sample of observations, including serial CTI. The results of the application were only partially satisfactory, as the calibrated model reproduced qualitatively the serial CTI signature observed in the science windows but could not fully model its effects under all conditions. In particular, the model struggled to reproduce the observed CTI effects for very bright sources and in the case of high local background, where the diffuse distribution of charge over a device mitigates the losses due to traps.
of manufacturing defects in the serial register was lower in red devices at launch, while the rate of traps increase appears similar for all devices (see also Ahmed et al. (2020)). This hints to possible differences in the distribution of the impurities during the manufacturing process of thicker and thinner devices, even if both the pixels architecture and the serial readout process are common to all devices.
The fits to the calibration data sets occasionally show systematic effects in the residuals in the very first pixels of the charge trails, as mentioned previously. Effects such as an inaccurate subtraction of the CCD bias, or charge blooming from the high injected signal flux in the last pixel of the serial register seem unlikely causes of the residuals. For instance, such a blooming effect is not detected in the first pre-scan pixel. The most plausible explanation of the poor fits is an inadequacy in the CtiPixel implementation, possibly too simplistic, in modelling the full range of the trap emission timescales. Once again, the intrinsic limitation of a charge distortion modelling that allows only a small number of trap species with distinct trap parameters could be the cause of the limited success of CtiPixel to fully account for all the observed CTI effects.
The result points to possible limitations with the accuracy of the CtiPixel model and its calibration with the sole use of the dedicated serial CTI charge injection activities. It is possible, for example, that the limited number of post-scan pixels for which the charge trail flux values are recorded is insufficient to identify trap species of longer time emission constants, that could be present in the serial register and affect the readout. A second intrinsic limitation of the calibration using post-scan trails is that only the integrated effect of the damage is measured, based on which the average density of traps in the serial register can be derived; in reality, an accurate model for the treatment of serial CTI for stars and other point sources requires the estimate of the density of traps in each pixel, to account for its spatial nonuniformity (both of the initial manufacturing defects and of the traps accumulating due to radiation damage). The assessment of the accuracy of the CtiPixel model, when applied to images affected by radiation damage, could have been hindered by other systematic effects that were not fully calibrated at the time science validation was attempted. This is for example the case of the brighter/fatter effect, that can result in image residuals of the same order as the ones caused by serial CTI. Furthermore, it is worth noting how the PSF model does not actually represent the purely linear part of the signal, as real science observations, that present a certain level of serial CTI depending on their signal level and the specific AC position on the detector, are used for its calibration. The attempted science tests were therefore not ideally suited to reproduce the exact scenario for which CtiPixel was designed and developed.
The CtiPixel model was developed with the main goal of characterising the defects responsible for the effects of serial CTI in Gaia CCDs and to measure the evolution of radiation damage at L2. Trying to apply the model, calibrated using engineering data, to science observations is a challenging endeavour for a number of reasons. For instance, the PSF needs to be sampled fully in 2D for all predicted sources. Processing of science data would require predicting the scene of each science window to account for any other signal that might affect the history of the traps and their occupancy level, including background variations. The scene outside the PSF core would inevitably be poorly characterized, as the mid-to-far PSF wings are not well-covered by calibration data. This would be problematic in the serial CTI context, where the wings in the Across Scan direction are most important. A proper application of the serial CTI model would also need to account for other CCD response effects, such as the column response non uniformity, when realising the predicted scene. The readout sequencing of science data would be critical in the application of the CTI model, but this aspect might not have been well-constrained by the calibrations using only engineering sequences.
Article number, page 13 was concluded that the integration of the CtiPixel task into the Gaia’s Image Parameter Determination software (IPD)