1. In a radiant energy tracking device sensitive to radiation of scanned targets the combination of;Optical focusing means having an optical axis for receiving modulated signal pulses reflected from a target and forming a target image at a focal point,Means for nutating said image in the focal plane about said optical axis,Detector means comprising four surfaces, each surface occupying a quadrant in the circle encompassed by said image as it nutates about said optical axis,Said detector means providing an output pulse for each of said quadrants, the pulse width being indicative of the duration of said target image on each said surface, andSumming means responsive to said output signals for providing signals proportional to the deviation of said target from said optical axis.