Modal analysis of an optical test system for metre-scale optic

The next generation of earth based large telescopes, such the European Extremely Large Telescope (E-ELT) targeted for 2024, will boost the space exploration and the search for other life in the universe. The primary mirror of the E-ELT will reach up to 39 metres of aperture. It will be composed by hundreds of 1.5 m corner to corner hexagonal mirror segments. The surface from of these segments need to be manufactured at nanometre level. The bottleneck on the manufacturing process of these segments is the figuring. Indeed, the iterative measurements and correction is extremely time consuming. To address the measurement issue, the Precision Engineering Institute of Cranfield University has developed a unique Optical Test System (OTS) for metre-scale optics. It is a four metre structure that uses the latest technology in laser interferometry to assess the surface topography of the optical component. This result is used to feed the de-convolution algorithm used for moving the plasma tool that etch the surface. The motion system of on the figuring machine enable to precisely remove atomic layer of material on well-defined regions of the surface. This thesis explores both the characterization and the scaling up process of the optical test system (OTS) designed for the figuring process mentioned above. In order to do so, a finite element model was created using ANSYS Mechanical APDL and the numerical modal analysis was performed. Then, the structure was tested through an experimental modal analysis (EMA), using the software LMS Test.LAB. The modal parameters of the structure (natural frequencies, damping and mode shapes) were obtained. A correlation with less than 5% discrepancy was achieved between the modelling and numerical approaches. This small discrepancy was achieved after numerous model refinement iterations. However, this correlation enabled to validate the numerical model. Based on this broad set of results, a transmissibility study between the points of interest (test piece and the interferometer) was carried out. Results showed the influence that different modes may have on the performance of the OTS. Finally, recommendations were elaborated for addressing the scaling up of the OTS that is made of bolted aluminium extrusion profiles. Reinforcing the base structure and stiffening of joints are the two main recommendations for scaling up the OTS