Stress mirror polishing for future large lightweight mirrors: design using shape optimization

Abstract

This study proposes a new way to manufacture large lightweight aspherics for space telescopes using Stress Mirror Polishing (SMP). This technique is well known to allow reaching high quality optical surfaces in a minimum time period, thanks to a spherical full-size polishing tool. To obtain the correct surface' aspheric shape, it is necessary to define precisely the thickness distribution of the mirror to be deformed, according to the manufacturing parameters. We first introduce active optics and Stress Mirror Polishing techniques, and then, we describe the process to obtain the appropriate thickness mirror distribution allowing to generate the required aspheric shape during polishing phase. Shape optimization procedure using PYTHON programing and NASTRAN optimization solver using Finite Element Model (FEM) is developed and discussed in order to assist this process. The main result of this paper is the ability of the shape optimization process to support SMP technique to generate a peculiar aspherical shape from a spherical optical surface thanks to a thickness distribution reshaping. This paper is primarily focused on a theoretical framework with numerical simulations as the first step before the manufacturing of a demonstrator. This "two-steps" approach was successfully used for previous projects.

Off-axis parabolas super polished under stress: the case of the Roman Space Telescope coronagraphic instrument mirrors

“…The majority of the OAPs are in the specification for the RST CGI; however, OAP1 and OAP2 show higher residuals due to their larger clear aperture and the border effect of the warping function. Further investigations are on-going, for example: using the shape optimisation method developed by S. Lemared et al (2020) [37] that aims to improve the thickness distribution; accommodating the RST CGI optical design to incorporate the surface errors; or, taking the advantage of finishing techniques, such as MRF and IBF to remove surface errors.…”