Apparatus for compensating for optical error in a rotative mirror

An optical scanning apparatus comprising:

• a polygonal radiation deflecting device for scanning radiation across a surface and means mounting said device for rotation about an axis substantially parallel to a plurality of radiation deflecting facets on said device, said plurality of facets being substantially congruent and the angles between each pair of adjacent facets being substantially equal;
• means for compensating for optical errors in the angular relationship of said facets;
• said compensating means comprising radiation deflecting means positioned intermediate a source of radiation and said surface for deflecting the radiation to be deflected by said facets, at least part of said deflecting means being movable in a first direction such that movement in said first direction would cause said radiation to move along said surface in a direction parallel to the direction of scan;
• wherein at least part of said deflecting means is movable in a second direction such that movement in said second direction would cause said radiation to move along said surface in a direction perpendicular to the direction of scan and wherein each of at least some of said signals in said set is proportional to the error in the angle between a facet and said axis of said device;
• electro-mechanical transducer means operatively associated with said deflecting means for moving said deflecting means in response to electrical signals applied to said transducer means;
• and
• means for generating a set of signals synchronized to the rotation of said polygonal deflecting device, each of at least some of said signals said set being proportional to the error in the angle between a pair of facets.