In the optical system of an electrophotographic copying apparatus embodying movable mirrors, it is of the utmost importance that these mirrors be properly aligned relatively to one another. In the present invention, two of the mirrors mounted at right angles to one another on a slidable frame which moves at half the speed of another movable mirror, are mounted on that frame for slight pivotal movement. This pivotal movement is controllable to provide adjustment of errors in the optical system and when adjustment is completed the mirrors are fixed in place.