fects that are thus generated are also referred to as traps, as they can cause the capture of electrons from a source signal and their release at a later time during the CCD readout process, distorting the acquired images. This effect is quantified in terms of Charge Transfer Inefficiency (CTI), defined as the fraction of charge lost to traps during the transfer of the signal from a pixel to the next (see, for example Hopkinson et al. (1996)). This was identified early on in the Gaia development phase as a critical aspect of the mission. For the fainter stars in particular, CTI can cause a distortion in the observed images that strongly affects the derivation of the stars detector position and flux.
The Gaia satellite (Gaia Collaboration et al. 2016), ESA’s astrometric mission, was launched in December 2013 and collected data in its main science mode until January 2025, with the aim of creating the most detailed map of the stars of our Galaxy. The spacecraft operated in the vicinity of the second Lagrange point (L2), 1.5 million km from Earth, scanning the sky combining observations from the field of view of two telescopes, separated by a basic angle of 106 degrees, to measure the stars’ positions, distances and proper motions with an accuracy of microarcseconds. In addition to the astrometric measurements, Gaia performed photometric observations of all detected sources, lowresolution spectrophotometry with the Blue and Red Photometers (BP and RP, and more generally referred to as XP devices) as well as near-infrared medium resolution spectroscopy with the Radial-Velocity Spectrometer (RVS) for objects brighter than GRVS ∼16. Gaia’s focal plane consists of a grid of 106 scientific e2v Charge Coupled Devices (CCDs) – see Crowley et al. (2016b). The detectors have been exposed to a high-energy particle environment since launch. The characteristics of this irradiation, based on measurements using Gaia detectors in combination with data from other spacecrafts in L2, is described and analysed in Crowley (in prep). Over time, space irradiation causes permanent damage to the detectors’ silicon structure. The de-
After launch, the impact of CTI on Gaia data was found to be lower than originally anticipated by early radiation model studies, as shown in Crowley et al. (2016a). This was due to a number of factors. For instance, the timing of the launch, past solar maximum, and the low activity of the past solar cycle, proved beneficial, in terms of total irradiation dose. Another mitigating effect on CTI for Gaia has been the overall higher than expected diffuse optical background component, that reduces the impact of CTI during readout, as its signal can keep some traps permanently occupied. In particular, in the Along Scan (AL) direction, the accumulation of damage during the mission resulted in a parallel CTI that remained a factor of ∼8 below predictions. The serial CTI (sCTI) in the Across Scan (AC) direction has slowly increased over the years and in absolute terms is higher than in the parallel direction. The major cause of sCTI is the manufacturing traps. These, already present at launch, affect the serial
⋆Corresponding author: C. Pagani
e-mail: cp232@le.ac.uk
Article number, page 1 transfers to a larger degree than in the parallel direction because of the faster serial readout timing.