EvWaCo Prototype Athermal Mechanical Design

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 mu m, and an average deformation of 100 mu m 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 um and a maximum deformation of 63 +/- 18 um at the average thermal condition of Delta T = 13.6 degrees C. The instrument performs with an average deformation of 67 +/- 16 um and a maximum deformation of 92 +/- 19 um at the worst thermal condition of Delta T = 25 degrees C. This instrument design weights 175.7 kg.