The Evanescent Wave Coronagraph (EvWaCo) is a coronagraph that utilizes the principle of Frustrated Total Internal Reflection (FTIR) to simultaneously collect both the starlight and the companion light by using a focal plane mask composed of a convex diopter and a prism placed in contact. The mask exhibits an achromatic behavior, and its size can be varied by adjusting the pressure at the contact area. The National Astronomical Research Institute of Thailand (NARIT) is developing a prototype to demonstrate on-sky the performance of EvWaCo. This prototype will be installed at the Thailand National Telescope (TNT). In this paper, the mechanical design of the EvWaCo prototype is documented. The mechanical requirements of this prototype include a maximum weight equal to 180 kg, a maximum deformation of 120 μm, and an average deformation of 100 μm for every optical component. To achieve this, the structural parts are designed to achieve the high directional stiffness, and the passive thermal compensation is conceptualized for athermalization. Then, the lightweight, high-performance materials are selected. The Finite Element Analysis (FEA) method is used to simulate the performance of the prototype under the realistic conditions. The prototype performs with an average deformation of 43 ± 15 μm and a maximum deformation of 63 ± 18 μm at the average thermal condition of ΔT = 13.6 ⁰C. The instrument performs with an average deformation of 67 ± 16 μm and a maximum deformation of 92 ± 19 μm at the worst thermal condition of ΔT = 25 ⁰C. This instrument design weights 175.7 kg.
Lightweight, aluminum, freeform prototype mirrors have been designed and fabricated by a Thai led team, with UK support, for intended applications within the Thai Space Consortium (TSC) satellite series. The project motivation was to explore the different design strategies and fabrication steps enabled by both conventional (mill, drill, and lathe) and additive (3D printing) manufacture of the prototype substrates. Single Point Diamond Turning was used to convert the substrates into mirrors and optical metrology was used to evaluate the different mirror surfaces. The prototype criteria originated from the TSC-1 satellite tertiary mirror, which is designed to minimize the effect of Seidel aberrations before the beam enters the hyperspectral imager. To converge upon the prototype designs, Finite Element Analysis (FEA) was used to evaluate the different physical conditions experienced by the prototypes during manufacture and how these influence the optical performance. The selected designs satisfied the mass and surface displacement criteria of the prototype and were adapted to either the conventional or additive manufacturing process. This paper will present the prototype design process, substrate manufacture, optical fabrication, and an interferometric evaluation of the optical surfaces comparing the conventional and additive manufacturing processes.
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