changing the tilt of a mirror in the reflective subsystem although angle correction in this example can be achieved by placing both wedged plates in one leg of the interferometer chromatic errors change in optical path difference with wavelength can be minimized in some applications by placing one of the plates in each leg in the previous example the corrective elements were used to redirect a ray in angle fig4 a and 4 b illustrate that a corrective element 412 in fig4 a 414 in fig4 b can be used to correct for angle errors in a reflective subsystem in which a ray undergoes a specular reflection angle of reflection equal to negative of angle of incidence fig4 a or for shear errors in a retroreflective subsystem fig4 b shear error is the lateral displacement between two parallel rays one having traveled a first path through the interferometer and the other having traveled a second path a retroreflective system is one in which a reflected ray always returns parallel to the incident ray in these cases the correction can be achieved by rotating the wedged corrective elements about an axis approximately normal to one of the element surfaces for the retroreflective system the magnitude of correction can also be changed by translating the corrective element in a direction approximately normal to one of the element surfaces in fig4 a and 4 b the uncorrected optical path is depicted as a dotted line the corrected path is shown as a solid line fig4 c depicts another corrective element according to the present invention which can be used to correct for shear errors in a retroreflective subsystem it consists of one or more refractive plates 416 with the two surfaces approximately parallel shear correction in two dimensions can be obtained by tipping.