in a system for tracking a target which is illuminated by a laser beam , for example , apparatus is required to determine how far the target is off - axis . the apparatus includes an objective lens which concentrates light coming from the target onto a reception surface . a group of bundles of optical wave guides is disposed between the reception surface and a single photoelectric receiver . each bundle conveys light from a respective portion of the reception surface . the bundles of wave guides are of different lengths . thus , the optical signals transmitted by the various bundles are received successively by the single receiver . this avoids problems of matching the sensitivity of different receivers responsive to different portions of the reception surface .