Development of nanolaminate thin-shell mirrors

The space science community has identified a need for ultra-light weight, large aperture optical systems that are capable of producing high-resolution images of low contrast. Current mirror technologies are limited due either to not being scalable to larger sizes at reasonable masses, or to lack of surface finish, dimensional stability in a space environment or long fabrication times. This paper will discuss the development of thin-shell, nano-laminate mirror substrates that are capable of being electro-actively figured. This technology has the potential to substantially reduce the cost of space based optics by allowing replication of ultra-lightweight primary mirrors from a master precision tool. Precision master tools have been shown to be used multiple times with repeatable surface quality results with less than one week fabrication times for the primary optical mirror substrate. Current development has developed a series of 0.25 and 0.5 meter spherical nanolaminate mirrors that are less than 0.5 kg/mz areal density before electroactive components are mounted, and a target of less than 2.0 kg/m with control elements. This paper will provide an overview of nanolaminate materials for optical mirrors, modeling of their behavior under figure control and experiments conducted to validate precision control. 1.0 INTRODUCTION In order to meet the NASA goal of large (hundred of square meters), lightweight optics to enable future missions like Terrestrial Planet Finder, there is a need to change the paradigm regarding how large aperture optical structures are manufactured for space applications. The impact of this work is to develop a new class of enabling optical mirror substrates by breaking away from conventional optical materials. Current space optics are designed for ground-based testing in a 1 g environment using conventional materials. The most common is ultra-low thermal expansion glass (example Zerodur or ULE). This is the base technology for the Hubble Space Telescope. This approach has resulted in large, expensive, and heavy high-precision primary optics. Since Hubble, the 1-2 meter-class optics state-of-the-art has advanced to the current generation of light-weighted, non-actuated mirrors, generally fabricated from bonded or fused glass with complex core structures, with areal densities of 30-80 kg/m2. Other programs are advancing the state of the art of large space optics. These include the Advanced Mirror System Demonstrator (AMSD), a joint NASA-DOD program developing lightweighted glass and beryllium mirrors (target areal density target of 15-20 kg/m2). The next step is to develop lightweight optics that have integrated actuation and without a reaction structure. The nanolaminate mirror program has developed a series of 0.25 and 0.5 meter spherical nanolaminate mirrors that are less than 0.5 kg/m2 areal density before electroactive components are mounted, and a target of less than 2.0 kg/m with control elements. The continued development of nano-laminate mirrors will break the current paradigm of heavy, costly and long fabrication time primary optical mirrors for telescopes. This technology is directly applicable to both spherical and aspheric mirrors. Both would use the same fabrication process; where for non-spherical optics the only requirement would be a master aspheric precision tool. This technology is scaleable from the current 0.25 and 0.5 meter spherical mirrors currently being developed. In order to fabricate larger primary apertures, there would need to be a capitol investment for larger vacuum and deposition facilities. The practical limit for this technology is limited to several meters for the master tool. For tools greater than two meters, alternatives to Zerodur or ULE glass would have to be considered. Likely candidate materials include optics grade Silicon Carbide.