half-rectangular field of view

This term is defined in ECSS-E-ST-60-20C Rev. 1 as
angular region around the Boresight Reference Frame (BRF) frame Z-axis, specified by the angular excursions around the BRF X- and Y-axes between the BRF Z-axis and the appropriate rectangle edge, within which a star produces an image on the Detector array that is then used by the star sensor