RVS focal plane. For the complete Gaia focal plane, see Gaia Collaboration (2016, their Fig. 4). Left panel: 12 CCDs of the RVS focal plane laid out in three strips (we use the standard Gaia nomenclature and refer to columns as “strips”) and four rows. The exposure time is fixed at 4.4 seconds per CCD (in TDI mode). The star images move in the along-scan (AL) direction (as indicated by horizontal arrows). During each transit, the star crosses all three CCDs on the row, and a spectrum is acquired in each of the three corresponding observational windows centred on the star. If the star has onboard G_RVS ≤ 7, the telemetered window is 2D, of size 1260- or 1296-AL by 10-AC pixels (upper right panel). For fainter stars, the 10-AC pixels are summed during read-out to produce a 1D window (lower right panel). The FoV of both Gaia telescopes are projected onto the focal plane. The orientation of the field angles η and ζ is indicated in the lower left corner. The origin of the axes is in the astrometric focal plane and is different for the two FoVs.