measured as On– Board Mission Time for initial 2–trap species calibrations fitting every device and epoch independently at an assumed constant focal plane operating temperature. The top panel is for all blue (AF and BP) devices, the bottom panel for RP devices.]

[Figure: Fig. 4: Gaia Focal Plane Array temperature variation over the course of the ten year mission at three different positions: (blue) row 1 SM; (red) row 4 AF5; (green) row 7 RVS. The large variations seen later in the timeline are caused by a series of end-oflife engineering tests.]

As mentioned earlier, differences of 5 degrees over the focal plane can introduce uncertainties in the estimated release time values of 30%, for a fast trap species like the ones that dominate CTI in the serial register, while a change of 1 degree over the course of the mission for a specific device can results in estimated trap emission time constants errors of almost 10%. In later implementations of CtiPixel and in the final calibrations the temperature of each device and its evolution during the epochs has been explicitly defined in the model. This choice also allows us to fit and directly derive the fundamental trap species energy level parameter Et, instead of the release timescale τe. Additional attempts to improve the fits by allowing a per– species charge volume confinement parameter β were limited in success. After some experimentation in configuration we settled on a three trap species model including per–species β but with the additional constraint of fitting for a single set of Et, α and β common across all epochs on a device–by–device basis. The motivation was that for a given species we expect those trap parameters to be constant while the trap density ρ is free to evolve with time. Figures 5 to 7 illustrate the results for the 3 trap species fits. Improvements were evident in the RMS and goodness–of– fit histograms while the time evolution of the robustly estimated Gaussian–equivalent unit–weight calibration residual (in comparison with the same for the 2 trap, individual epoch calibrations shown in Figure 3) was flattened significantly. In Figure 8 we show the time evolution in mean trap density for the 3 individual trap species modelled in the blue and red variant devices. We see clear evidence for strong increase in the density of the higher energy level traps (i.e. those with the longer timescales) with time, while the lowest energy level trap species remains relatively constant in both cases. In order to investigate the robustness of our results we subdivided the measurement data into two sets covering the mission timeline, taking every odd or even epoch in the time–ordered sequence to provide two independent input data sets of the same size (13 epochs) covering the same overlapping time range. Figure 9 shows y = x comparison plots for those trap parameters fixed across the epochs, which we expect to be the same within the uncertainties propagated through the weighted fits from the measurements, for the three trap species. The agreement is good for the dominant first species, fair for the second and somewhat less so for the third, although for many devices the agreement is reasonable across all trap types. This gives us some confi-
species of significantly lower characteristic emission timescale, as radiation damage increased with mission time. Adding the third trap species to the fit improves the fit results, while the increased number of free parameters inevitably leads to higher levels of correlation and might render their physical interpretation more uncertain (this can be mitigated as further described below). In the initial calibrations, the CCD temperature was assumed as fixed at the nominal focal plane value of 163 K. This simplification was not justified on close inspection of the FPA thermal measurements included in the spacecraft telemetry. A detailed investigation into the thermal properties of the focal plane revealed variation of many tenths of a degree over time for a specific device, as well as differences of up to five degrees between CCDs. Figure 4 shows focal plane array temperature telemetry for the entire mission time line. Three thermistors are positioned on the FPA: with reference to the FPA device schematic in Figure 1 of Crowley et al. (2016b), these are located on the lower left (row 1 SM), centrally (row 4 AF5) where the FPA is warmest, and on the upper right (row 7 RVS). The plot shows several interesting features:
– annual variation as solar irradiance changes with the Earth– Sun orbital distance;
– a secular change from cooler to warmer temperatures from the start of the mission to the end;
– very short time scale, large amplitude disturbances when operational changes cause large fluctuation in power dissipation – e.g. the step change near the end of mission when Proximity Electronics Module 4 failed on row 3 with the FPA subsequently stabilising at an operating temperature lowered by 0.3 K; and
– quantisation at the level of 0.05 K per ADU in the on–board analogue–to–digital converters reading the thermistor signals.
– large temperature variations induced by the mission end-oflife engineering tests.
The available telemetry was used to create an FPA temperature interpolation map by device position and mission time with the simplifying assumption of reflection symmetry to create a 2d temperature model from the limited positional coverage afforded by the FPA thermistors. The incorrect assumption of a fixed temperature introduces significant scattering in the values derived for the fits of the parameter τe, the characteristic emission time of a trap species, Article number, page 8 very first pixels of the release curve following the injected signal block.