Gaia focal plane with its 106 CCDs arranged in seven rows. Because of the rotation, stellar images slowly drift along the focal plane from left to right. All rows contain sky mapper (SM) CCDs for detecting incoming objects, eight or nine CCDs for astrometric measurements (AF1,..., AF9), and CCDs for blue (BP) and red (RP) spectrophotometry. In addition, some rows also include CCDs for the basic-angle monitor (BAM), for wavefront sensors (WFS), and for the radial-velocity spectrometer (RVS). The orientation of field angles (η,ζ) is also shown, and their origin for each of the two telescopes (yellow circles 1 and 2 for the preceding and following fields of view, respectively). The angle θ exemplifies the instantaneous orientation of the focal-plane with respect to the equatorial coordinate system (ICRS) on the sky for one of the telescopes at a random instant of time.