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

A novel null corrector design for use with a white light scatterplate interferometer on a large conic surface is presented. In this design, an aspheric diamond-turned mirror (DTM), hereafter called the null-correcting mirror (NCM), exactly cancels out the spherical aberration of the mirror under test. Low-power refractive elements correct field aberrations over the finite aperture of the scatterplate. The null corrector maintains a phase difference between the test and reference beams of less than 1/2 wave over the finite field size of the scatterplate for optimal fringe visibility. The null corrector can be certified using another smaller DTM and/or a computer-generated hologram. The design has the significant advantages of being small in size, less expensive than designs using spherical surfaces (due to the small size of the NCM), and useable with other interferometers. We include discussions on the calculation of the surface profile for the NCM and certifying mirror, field correction, and achromatization.