Null corrector design for white light scatterplate interferometry on a large conic surface

We present a method for designing and testing a null corrector for use with scatterplate interferometry on a large conic mirror. The null corrector in a scatterplate interferometer must maintain OPD of less than 1/2 wave over a finite field size for optimal fringe visibility. Our design uses an aspheric diamond-turned mirror (DTM) to exactly cancel out the spherical aberration of the surface under test. The DTM has the additional benefit of being useable in other types of interferometers for testing of the conic surface in a null condition. Low power refractive elements correct field aberrations over the finite aperture of the scatterplate. The null corrector can be certified using another smaller DTM or a computer generated hologram (CGH). This design has the advantages of being small in size, less expensive than designs using spherical surfaces (due to the small size of the null-correcting mirror), useable with other interferometers, and easy to align.