A Finite Element Study: Kinematic Mounts for Large Mirrors

A Finite Element Study: Kinematic Mounts for Large Mirrors

The Institute for Astronomy at Hilo is  working on a project called ‘Imaka; a ground-layer adaptive optics (GLAO) system for the 2.2-meter telescope on Mauna Kea aiming for high resolution, high sensitivity, and a large field of view using relatively large mirrors.  Due to the size and weight of large mirrors, gravity causes deflections on the mirror surface, which distorts the output image.  The design of a mount for large mirrors must be carefully considered to support the mirror and limit self-weight deflection.  Mirror surface deflections are calculated using various finite element analysis (FEA) software pacakges, i.e. COMSOL, ANSYS, Solidworks and PLOP.

70.7 deformation 3pt mount

The mount types studied are the ring and 3-point mirror mounts.  Model results will be compared with analytical solutions to gain modeling skills for kinematic mounting systems, and gain understanding of the software and the limitations of both the FEA and analytical models.  Verification of basic mounts and understanding of FEA in COMSOL and other FEA softwares will facilitate the design and analysis of the more complex mounts that will be needed to support the ‘Imaka project’s mirrors. In addition, findings from this project can help establish a foundation in designing mirror mounts for large mirrors for any given specifications. This knowledge is useful to attain as the improvement on the quality and scope of our telescopes can further investigate the future and continue studying about the past.

Previously, the project showed a 3% difference between the FEA and analytical solutions (taking into account bending and shear stresses) for the ring mount in the simply-supported and fixed configurations.  As the aspect ratio of the mirror increases, bending stress dominates the deflection and shear stress becomes negligible for the ring mount, which influences the form of the analytical solution.

simply supported ring mount multibody 70.7 deflection