Dynamical modeling and resonance frequency analysis of 3.6m optical telescope pier

Abstract

The pier is an important building block of large optical telescopes. There are several sources of vibrations all around the optical telescope. A large height makes it vulnerable to low frequency oscillations, both as surface waves, as well as bulk oscillations which may be transferred to the optical system. It mainly happens through the pier, since it is directly connected to the telescope. In order to avoid enhanced transfer of energy between pier and optical system, it is necessary to ensure that the resonance frequency of the pier and the telescope fixtures are fairly separated. The as-built structure of telescope pier was simulated using FEM analysis for finding the resonating modes. Various test procedures were defined and on-site testing was done using 3C geophones and piezoelectric sensors to closely observe response of the pier. The main mode in low frequency regime was found to occur at approximately 22±2 Hz by both testing and simulations. Theoretical supports are also provided for observed modes. A proof of concept was demonstrated for impulse response based resonance frequency determination. It was also demonstrated that a low-cost piezoelectric sensor based test-bench can be used for finding the resonating modes, compared to expensive 3C geophones.