The 106 detectors that form the Gaia focal plane are categorized by their function and properties, with the majority of them (62) identified as Astrometric Field (AF) CCDs, dedicated to the measurement of the position of the stars detected by the spacecraft Star Mapper (SM) devices – for a full description of the Gaia focal plane see Kohley et al. (2014).

All detectors have image section dimensions of 4500 by 1966 pixels in the parallel and serial readout directions, with each pixel the physical size of 10 × 30 µm.

The main difference between the devices employed in the focal plane is their thickness.

For the AF and Blue Photometers (BP) CCDs the thickness is 16 µm, while for the red variants employed in the RP and RVS detectors a higher value of 40 µm enhances their sensitivity for sources emitting mainly at longer wavelengths.
2.1.

CCD operations The impact of radiation damage on the images and spectra acquired by a CCD depends both on the characteristics of the device and on how it is operated, for example on the timings of the readout process and the CCD temperature.

A critical aspect for Gaia is the way its images are acquired, exposing the CCDs continuously while they are clocked in the parallel direction.

This technique is known as ‘Time Delayed Integration’ (or TDI) mode, in which the detectors are clocked in the parallel direction with a period of 0.9828 millisecond (corresponding to a clocking frequency of 1.017 kHz), matching the scanning rate of the stars over the detectors.

Therefore, the signal from a transiting source is accumulated during the readout, in contrast to the typical CCD acquisition technique of exposing the camera at a fixed sky position for a determined amount of time followed by the readout of the frame.

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The parallel transfer rate in TDI mode is rather long compared to that in more common CCD imaging.

Furthermore, this is in stark contrast to the two-phase readout serial clocking frequency, which is extremely fast (10 MHz corresponding to charge transfer times of 0.1 µs in the serial register) except when samples are being read, yielding longer transfer times of order 10 µs upstream in the serial register.

The fast–slow serial scan is further complicated by a readout suspension around the four–phase parallel clocking.

This is because the video chain electronic offsets would be otherwise hugely disturbed during readout sampling by the voltage swings commanding those parallel phases — residual offset disturbances on resumption of the serial readout are presented in Hambly et al. (2018) where these aspects are discussed in greater detail.

At each readout pause, the dwell time in the serial register, during which charge is confined under a pixel before being moved to the succeeding one is 80.4 µs for AF devices, and 76.8 µs for XP CCDs.

Hence the serial scan consists mainly of about 2000 transfers of 0.1 µs, within which are scattered several tens of dwell times of 10 µs in contiguous groups (depending on instrument mode, allocation of science windows and of sacrificial ‘braking’ samples used to further sample away the offset disturbances) plus three dwells of around 80 µs (one readout pause having taken place before the serial scan starts).

There are three other important characteristics of the Gaia’s CCDs that influence the impact of irradiation on its measurements.

Gaia’s devices were designed and manufactured with pixels presenting a supplementary buried channel (SBC; Seabroke et al. 2013), that limits the volume within a pixel where the charge signal from a source is enclosed during readout, thereby reducing parallel CTI effects (the serial register has no SBC).

The second is the charge injection functionality of the CCDs, in which a controlled amount of charge is injected at the top of the devices at specific intervals.

During observations taken in normal science mode, a block of four contiguous lines are injected with approximately ten thousand electrons of charge per pixel every 2000 (AF mode) or 5000 (XP mode) lines.

When these blocks of injected lines are readout they temporarily fill the defects of the entire CCDs with a fraction of their charge, mitigating the effects of CTI.

Finally, of particular importance for CTI effects is the detectors’ temperature.

On Gaia, the CCDs nominal operating temperature is 163 K, with some variations of several degrees observed over the focal plane.

The temperature has been found to be stable over time, albeit with low–level seasonal variations dependent on heliocentric distance in addition to abrupt variations in short intervals during specific mission phases such as the decontamination events and other disturbances to power dissipation in the focal plane array caused by operational changes (see Section 6.2).
2.2.