An instrument (1) for characterizing an optical system, includes: at least one primary source (3) for emitting an illumination light beam (FE); an optical device for directing the illumination beam (FE) onto the optical system (L) to be characterized; a wave front analyzer (4) adapted for receiving a beam from the optical system (L); and a unit for processing the measure signals from the wave front analyzer (4), adapted for providing characterization information of the optical system (L). The instrument further includes a scattering member (22) substantially provided in the focal plane of the optical system (L) so as to create a secondary source generating a secondary beam flowing through the optical system (L) and further directed towards the wave front analyzer.