Layout of CCDs in Gaia’s focal plane. Star images move from right to left in the diagram, as indicated in the lower part of the drawing by the nominal paths of two images, one in the preceding FoV (PFoV) and one in the following FoV (FFoV). The along-scan (AL) and across-scan (AC) directions are indicated in the top left corner. To the right, one of the CCDs is shown magnified, with the fiducial observation lines indicated for selected gates (g). Also indicated is the AC pixel coordinate μ, running from 13.5 to 1979.5 across the image area of each CCD. The skymappers (SM1, SM2) provide source image detection and FoV discrimination, but their measurements are not used in the astrometric solution. The astrometric field (AF1–AF9) provides accurate AL measurements and (for two-dimensional windows) AC positions. Other CCDs are used for the blue and red photometers (BP, RP), the radial-velocity spectrometer (RVS), wavefront sensing (WFS), and basic-angle monitoring (BAM). One of the CCD strips (AF3) illustrates the system for labelling individual CCDs by strip and row index. The origin of the field angles (η, ζ) is at different physical locations on the CCDs in the two fields. (Adapted from Lindegren et al. 2012.)