Optomechanical simulations of an advanced stitching interferometer set-up for large freeform optics

Abstract

The Physikalisch-Technische Bundesanstalt is currently developing a form measurement system for optical surfaces with diameters up to 1.5 metres, and local radii of curvature larger than 10 m. Moreover, the surfaces can have a radial symmetrical form, or a freeform shape. This contribution presents an optomechanical simulation of the measurement system to give hints about the achievable accuracy. The simulated system uses a Fizeau interferometer with an aperture of 100 mm for subaperture stitching interferometry. An advanced subaperture stitching method (called angle- and distance-assisted subaperture stitching, ADASS) is used to reconstruct the absolute topography of a virtual specimen. The topographies of the single subaperture measurements are positioned in space by using the angular position of the interferometer and the distance to the specimen. Thereafter the remaining height difference of the neighbouring subtopographies in their overlap region is corrected. The resulting surface form is fitted to the Zernike polynomials. In the simulation, a parabolic surface with a radius of curvature of 70 m was used as the virtual specimen. The reconstructed absolute surface form has a deviation of 418 nm (RMS) from the input surface over a diameter of 1.5 m.