ities in the small-scale geometric calibration of the CCDs (Lindegren et al. 2016). The maskpositioning accuracy for the individual stitch blocks also produces discontinuities in several response vectors, such as chargeinjection non-uniformity and column-response non-uniformity. At distinct positions along the 4500 TDI lines, a set of 12 special electrodes (TDI gates) are connected to their own clock driver. In normal operation, these electrodes are clocked synchronously with the other electrodes. These TDI-gate electrodes can, however, be temporarily (or permanently) held low such that charge

[Figure: Fig. 4. Schematic image of the focal plane assembly, superimposed on a real picture of the CCD support structure (with a human hand to indicate the scale), with Gaia-specific terminology indicated (e.g. CCD strip and row, TDI line and pixel column). The RVS spectrometer CCDs are displaced vertically (in the across-scan direction) to correct for a lateral optical displacement of the light beam caused by the RVS optics such that the RVS CCD rows are aligned with the astrometric and photometric CCD rows on the sky; the resulting semi-simultaneity of the astrometric, photometric, and spectroscopic transit data is advantageous for stellar variability, science alerts, spectroscopic binaries, etc. Image from de Bruijne et al. (2010a); Kohley et al. (2012), courtesy Airbus DS and Boostec Industries.]

transfer over these lines in the image area is inhibited and TDI integration time is effectively reduced to the remaining number of lines between the gate and the readout register. While the full 4500-lines integration is normally used for faint objects, TDI gates are activated for bright objects to limit image-area saturation. Available integration times are 4500, 2900, 2048, 1024, 512, 256, 128, 64, 32, 16, 8, 4, and 2 TDI lines. The choice of which gate to activate is user-defined, based on configurable look-up tables depending on the brightness of the object, the CCD, the field of view, and the across-scan pixel coordinate. Because the object brightness that is measured on board in the sky mapper (Sect. 3.3.9) has an error of a few tenths of a magnitude, a given (photometrically-constant) star, in particular when close in brightness to a gate-transition magnitude, is not always observed with the same gate on each transit. This mixing of gates is beneficial for the astrometric and photometric calibrations of the gated instruments. The Gaia CCDs are n-channel devices, i.e. built on p-type silicon wafers with n-type channel doping. Displacement damage in the silicon lattice, caused by non-ionising irradiation, creates defect centres (traps) in the channel that act as electron traps during charge transfer, leading to charge-transfer inefficiency (CTI). Under the influence of radiation, n-channel devices are susceptible to develop a variety of trap species with release-time constants varying from micro-seconds to tens of seconds. Traps, in combination with TDI operation, affect the detailed shape of the point spread function of all instruments in subtle yet significant ways through continuous trapping and de-trapping (Holl et al. 2012b; Prod’homme et al. 2012), removing charge from the leading edge and releasing it in the trailing edge of the images and spectra. The resulting systematic biases of the image centroids and the spectra will be calibrated in the on-ground data processing, for instance using a forward-modelling approach based on a charge-distortion model (CDM; Short et al. 2013).
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