Abstract: Precise control of size and location of cathode-ray image, especially of projected image may be required for various reasons, but particularly when images from several different sources are to be matched contiguously to produce apparently continuous image, and when extreme accuracy of linear dimensions on picture is required.Edges of image normal to each scan direction are provided with markers, such as light bars. Photosensors at edge of useful picture area sense such markers. Sum of sensings of opposed photosensors is measure of displacement of picture in direction of line joining photosensors. Difference between sensings of opposed photosensors is measure of extension or contraction of picture in that direction. Sum signal is used to control centering; difference signal is used to control scan amplitude in given direction.