Abstract: 1. In an optical tracking system, lens means defining an optical axis, radiant energy responsive means spaced from said lens means along said optical axis, a pair of radiant energy directing means disposed to be traversed by radiant energy passing along said optical axis from said lens means to said radiant energy responsive means, drive means for simultaneously rotating said radiant energy directing means whereby to rotate a beam of received radiant energy and cause the same to nutate about the optical axis and to trace a substantially circular image upon said radiant energy responsive means, and means for angularly adjusting one of said radiant energy directing means relative to the other to modify the radius of said circular image.