Schematic view of the STIX aspect system. Viewing the Sun through a 5 mm diameter opening in the front window, a lens in the front grid focuses an image of the solar disk onto four rows of small apertures in the rear grid. Four large photodiodes integrate the light passing through a corresponding row of apertures. When a solar limb passes over one of the apertures, the output of that diode changes by ∼20%. Such transitions provide an occasional but accurate measurement of the location of the solar image relative to the rear grid slits.