A device and method for subaperture stray light detection and diagnosis. A test light beam is generated. Stray light is detected. Based on the detected stray light, potential paths that light may have taken to arrive at the detection surface are determined. A testing device comprises a test light beam source whereby the cross sectional area of the test light beam is made less than the cross sectional area of the system aperture. A relative lateral positioning stage and an angular beam directing stage launch the test light beam into the aperture. A detector and a data processing system produce a data set relating the stray light to the location and directional angles of the test light beam to identify the sources of stray light. A light trap and test light beam delivery system are provided.