Application of modal testing to characterize ground based telescope elastic motion and its effect on image quality

Abstract

The Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) is an innovative design for a wide-field imaging facility developed at the University of Hawaii's Institute for Astronomy. The Pan-STARRS prototype telescope (PS1) is an Altitude over Azimuth telescope with an instrument rotator for its 1.4gigapixel camera. Quartus Engineering Incorporated (Quartus) performed modal survey and operational vibration testing of the Pan-STARRS prototype telescope (PS1) to assess the impact of structural resonance and star tracking (i.e. actuated telescope motion) on image quality. The telescope's modal parameters: resonant frequencies, modes shapes, and damping ratios; were measured using an impact hammer and the acceleration response gathered from components with natural modes of vibration identified in a pre-test structural analysis that that were thought to affect image quality. A baseline characterization of the PS1's elastic performance was performed and compared with real-time structural modifications to determine how an individual component's stiffness affects the telescope's overall elastic performance. Operational vibration data collected at various on-sky tracking states was then evaluated using the newly measured modal properties and legacy optical measurements to develop a relationship between elastic motion and image quality.