On Identifying Natural Frequencies in the JAST80 Telescope and Validation With a Simplified 3-D Model

Abstract

The instrumentation of professional telescopes is very sensitive to vibrations. These vibrations, caused by wind, terrain, or its actuators, affect the quality of the images taken. If vibrations present frequency components coinciding with the natural frequencies associated with the proper modes of the telescope mount, the vibratory response is amplified and cause a decrease in image quality below the required standards. The goal of our study is to identify the natural frequencies of the german-equatorial mount of the JAST80 telescope of the Observatorio Astrofísico de Javalambre (OAJ) and to validate a simplified Finite Element Model (FEM). In addition, the interaction of the vibrations produced by the actuators in a right ascension (RA) movement with the natural frequencies obtained by means of a Finite Element Analysis (FEA) is analyzed. To excite the structure of the telescope, the response of it in free vibration after an unexpected emergency stop of an RA movement was used. A setup of three accelerometers allowed us to compare the excitation frequencies of the structure with those obtained with the FEA. Natural frequency values obtained through the FEM showed deviations below 12% with respect to the experimental values, thus validating the simplified 3D numerical model of the telescope mount for further analysis of its dynamic behavior. On the other hand, the analysis of the measured vibratory indicated that the natural frequencies of the mount for a RA movement in normal operation were not excited by the actions produced by the actuator and therefore the quality of the images taken is not compromised. Finally, the possibility of using the measurement of the vibrational response of the mount in emergency stop conditions to monitor its structural integrity is pointed out.