Mount-induced deflections in eight-inch flat mirrors at the Navy Prototype Optical Interferometer

The preservation of mirror surface quality and figure are of paramount importance at the Navy Prototype Optical Interferometer. There are on the order of 108 eight-inch optical flats mounted in the interferometer's optical train, 102 of which are permanently mounted inside the 9000 cubic foot vacuum feed system. The flats are specified for manufacture at λ/20 peak-to-valley surface variation (λ = 633 nm) over a 7.2 inch clear aperture. Silver coating with a dielectric overcoat is subsequently applied to the reflecting surface. The objective when mounting the mirror is to preserve the surface quality and figure of the coated flats as much as possible. Surface deflections occur due to pressure points inherent in the mount. The mount consists of a modified commercially available tangent-arm gimbaled-type structure. In order to minimize the mounting effects and allow for a wider thermal operational range, modifications were made to the primary mirror cell in the following areas: edge support region, front face tabs, rear face loaders, and diameter. In this paper we describe the detailed cell modifications, a finite element analysis (FEA) of the mounted flat, the free-standing and as-mounted surface figure of a typical eight-inch diameter flat as measured with a phase-shifting interferometer, the resulting mount-induced deflections, a comparison between the measured and FEA model, and conclusions.