Abstract: Optical detectors are arranged along the focal length of a spherical mirr Signals passed by a lens and reflected by the mirror impinge on the detectors as the system scans the horizon. A reticle means is placed between the mirror and the detector to cause the optical system to discriminate against large radiating areas in the field of view of the lens. The reticle means may be straight or zig-zag opaque lines. The cross-sectional area of the lines and the area therebetween is such that radiation from point sources will impinge the detector in only one area at a time while radiation from large sources will overlap a plurality of areas. The output frequency from the detectors is indicative of the type of the radiation source.