Schematic illustration of how the field angles (η,ζ) are defined in terms of the CCD layout in Fig. 3. For simplicity only the projections of two CCDs, AF8_3 and AF8_4, into the Scanning Reference System (SRS) are shown (not to scale). The field angles have their origins at the respective viewing direction fP or fF (Fig. 2), which are defined in relation to the nominal centres of the CCDs (crosses); the actual configuration of the detectors is described by fiducial observation lines according to Eq. (15). The dashed curve shows the apparent path of a stellar image across the two fields of view. Its intersections with the observation lines define the instants of observations.