; Sect. 3.3.5), (iv) low-resolution spectro-photometry using the blue and red photometers (BP and RP; Sect. 3.3.6), and (v) spectroscopy using the radial-velocity spectrometer (RVS; Sect. 3.3.7). The focal plane is depicted in Figure 4 and carries 106 charge-coupled device (CCD) detectors, arranged in a mosaic of 7 across-scan rows and 17 along-scan strips, with a
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These detectors come in three different types, which are all derived from CCD91-72 from e2v technologies Ltd: the default, broadband CCD; the blue(-enhanced) CCD; and the red(-enhanced) CCD. Each of these types has the same architecture but differ in their anti-reflection coating and applied surface-passivation process, their thickness, and the resistivity of their silicon wafer. The broadband and blue CCDs are both 16 µm thick and are manufactured from standard-resistivity silicon (100 Ωcm); they differ only in their anti-reflection coating, which is optimised for short wavelengths for the blue CCD (centred on 360 nm) and optimised to cover a broad bandpass for the broadband CCD (centred on 650 nm). The red CCD, in contrast, is based on high-resistivity silicon (1000 Ωcm), is 40 µm thick, and has an anti-reflection coating optimised for long wavelengths (centred on 750 nm). The broadband CCD is used in SM, AF, and the WFS. The blue CCD is used in BP. The red CCD is used in BAM, RP, and the RVS. The detectors (Fig. 5; Crowley et al. 2016b) are backilluminated, full-frame devices with an image area of 4500 lines along-scan and 1966 columns across-scan; each pixel is 10 µm × 30 µm in size (corresponding to 58.9 mas × 176.8 mas on the sky), balancing along-scan resolution and pixel full-well capacity (around 190 000 e−). All CCDs are operated in time-delayed integration (TDI) mode to allow collecting charges as the object images move over the CCD and transit the focal plane as a result of the spacecraft spin. The fundamental line shift period of 982.8 µs is derived from the spacecraft atomic master clock (Sect. 3.3.10); the focus of the telescopes is adjusted to ensure that the speed of the optical images over the CCD surface matches the fixed speed at which the charges are clocked inside the CCD. The 10 µm pixel in the along-scan direction is divided into four clock phases to minimise the blurring effect of the discrete clocking operation on the along-scan image quality. The integration time per CCD is 4.42 s, corresponding to the 4500 TDI lines along-scan; actually, only 4494 of these lines are light sensitive. The CCD image area is extended along-scan by a light-shielded summing well with adjacent transfer gate to the two-phase serial (readout) register, permitting TDI clocking (and along-scan binning) in parallel with register readout. The serial register ends with a non-illuminated post-scan pixel and begins with several non-illuminated pre-scan pixels that are connected to a single, low-noise output-amplifier structure, enabling across-scan binning on the high-charge-handling capacity (∼240 000 e−) output node. Total noise levels of the full detection chain vary from 3 to 5 electrons RMS per read sample (except for SM and AF1, which have values of 11 and 8 electrons RMS, respectively), depending on the CCD operating mode. The CCDs are composed of 18 stitch blocks, originating from the mask employed in the photo-lithographic production process with eight across-scan and one along-scan boundaries (Fig. 5). Each block is composed of 250 columns (and 2250 lines) except for the termination blocks, which have 108 columns. Whereas pixels inside a given stitch block are typically well-aligned, small misalignments between adjacent stitch blocks necessitate discontinuities 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