apparatus arrangement can also be used to measure other problems that exist in the optical path referring to fig5 a there is shown a test apparatus 50 that is formed by adding a planar reference plate 48 to the interferometer system described earlier components shown outside the dashed box are those parts of the interferometer that provide incident collimated light that is directed toward the test surface and read back the test and reference light signals in conventional use a mount 54 is added to the interferometer components provided for temporarily seating reference plate 48 in position as part of this test setup reference surface 48 slightly wedge shaped as shown in the enlarged view of fig6 a is positioned between the aspheric lens element of fizeau optics 18 and the source of collimated light from upstream optics in the interferometer the wedge shape provides a slight tilt to a surface 51 the first surface that receives incident collimated light so that this surface is oblique to the incident light and deflects unwanted back reflection from this surface away from the signal path any detected back reflection from this surface could interfere with the signals of interest for measurement of aspheric surface 42 using test apparatus 50.