A light beam is detected/localized by multisector detector—quad-cell, or 5+ sectors handling plural beams. Preferences: Beams focus to diffraction limit on the detector, which reveals origin direction by null-balance—shifting spots to a central sector junction, and measuring shifts to reach there. One or more MEMS reflectors, and control system with programmed processor(s), sequence the spot toward center: following a normal to an intersector boundary; then along the boundary. One afocal optic amplifies MEMS deflections; another sends beams to imaging optics. After it's known which sector received a spot, and the beam shifts, source direction is reported. The system can respond toward that (or a related) direction. It can illuminate objects, generating beams reflectively. Optics define an FOR in which to search; other optics define an FOV (narrower), for imaging spots onto the detector. The FOR:FOV angular ratio is on order of ten—roughly 180:20°, or 120:10°.