Special features of Newton-type fringe formation in a diffraction interferometer

An interferometer with a Fresnel zone plate located in the center of curvature of a concave mirror was studied. Attention was paid to the unique features of the interference field, which has a special point at which the path difference is equal to zero, thereby allowing for the observation of Newton-type fringes in white and quasi-monochromatic light. The conditions necessary for reducing the instrumental error to values less than (lambda)/20 were determined. Methods for suppressing noise and destructive interference patterns were also found. Metrological tests were carried out, and they proved the possibility of using this interferometer for industrial testing of spherical and parabolic mirrors.