1. An eye tracking system, comprising: a source of infrared light; optics forming an image of the source of infrared light, said image being smaller than the smallest that pupil of the eye ever becomes; a tracking mirror movable about at least one axis and directing the infrared light toward the pupil of the eye, infrared light passing through the pupil of the eye and falling on the eye's retina where it is backscattered to back light the pupil, and said tracking mirror also reflecting light from the back lighted pupil; a beamsplitter intercepting the light from the back lighted pupil and reflected by said tracking mirror; a detector positioned to receive light from said beamsplitter, said detector connected to provide a measure of decentering of the light on said detector; and a controller connected to receive an output from the detector and generating control signals controlling the rotation of said tracking mirror about said axis to maintain the image of the back lighted pupil centered on the detector.