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The European Space Agency Gaia satellite was launched into orbit around L2 inDecember 2013. This ambitious mission has strict requirements on residualsystematic errors resulting from instrumental corrections in order to meet adesign goal of sub-10 microarcsecond astrometry. During the design and buildphase of the science instruments, various critical calibrations were studied indetail to ensure that this goal could be met in orbit. In particular, it wasdetermined that the video-chain offsets on the analogue side of theanalogue-to-digital conversion electronics exhibited instabilities that couldnot be mitigated fully by modifications to the flight hardware. We provide adetailed description of the behaviour of the electronic offset levels onmicrosecond timescales, identifying various systematic effects that are knowncollectively as offset non-uniformities. The effects manifest themselves astransient perturbations on the gross zero-point electronic offset level that isroutinely monitored as part of the overall calibration process. Using in-orbitspecial calibration sequences along with simple parametric models, we show howthe effects can be calibrated, and how these calibrations are applied to thescience data. While the calibration part of the process is relativelystraightforward, the application of the calibrations during science dataprocessing requires a detailed on-ground reconstruction of the readout timingof each charge-coupled device (CCD) sample on each device in order to predictcorrectly the highly time-dependent nature of the corrections. We demonstratethe effectiveness of our offset non-uniformity models in mitigating the effectsin Gaia data. We demonstrate for all CCDs and operating instrument and modes onboard Gaia that the video-chain noise-limited performance is recovered in thevast majority of science samples.

Gaia Data Release 2. Calibration and mitigation of electronic offset effects in the data