Ray paths produced by an incoming beam and a ghost beam formed by the detector and one of the optical surfaces. The incoming beam is backreflected from the detector, travels back almost the same way, experiences ghost reflection and then travels back forming a different path. Top panel: ray paths produced by the filter, middle panel: those produced by the rear surface of the O3/L5 lens, and bottom panel: those produced by the front surface of the O3/L5 lens.