Large aperture reflective interferometer for measuring convex spherical

Abstract

A catadioptric imaging system is used to form a converging test beam for measuring large aperture optical elements having convex spherical surfaces. A beamsplitter block divides a collimated beam of coherent monochromatic light from a laser source into a test beam and a reference beam. A focusing system receives the test beam and transforms the beam into a spherically diverging beam emanating from a point source. A beamsplitter plate partially reflects the diverging beam into a further diverging beam that impinges against a concave spherical mirror at a small angle from normal incidence. The mirror reflects the further diverging beam in the form of a converging beam having a numerical aperture at least nearly equal to numerical apertures of the test surfaces. The converging beam is also interrupted by the beamsplitter plate; but on this occasion, the beamsplitter plate partially transmits the converging beam toward a focal point coinciding with a center of curvature of the test surface. Rays of the converging beam that impinge at normal incidence to the test surface are retroreflected back to the point source. The returning test beam is recombined with the reference beam to form an interferogram exhibiting fringe distortions indicative of a departure of the test surface from desired sphericity.