An imager comprising a photosensitive surface; a light source which produces at least one scanning light beam; a deflector, arranged to deflect the at least one scanning light beam onto the photosensitive surface; and a sensor which measures the orientation of the deflector. The imager also comprises a controller operative to determine a placement error of said at least one scanning beam on the photosensitive surface, responsive to the orientation measurement by the sensor: and an actuator, responsive to the displacement error, and arranged to change the direction of deflection of the at least one light beam by the deflector. The sensor is configured to measure the orientation of the deflector substantially at a null in a vibrational mode of the deflector.