Schematic illustrating the logic behind the digital shift-vector map employed in the IPA. Each rectangle shown represents the acceptance zone of a viable shutter (identified by its MSA coordinates) in shift-vector space, accessible to the associated target (red for target A and blue for target B) within a given search box. The (green) area of overlap of these acceptance zone areas in shift-vector space contains the shift vector value required from the nominal pointing position to simultaneously observe these two targets in a single MOS observation.