Vibration damping for the Segmented Mirror Telescope

Abstract

The Segmented Mirror Telescope (SMT) at the Naval Postgraduate School (NPS) in Monterey is a next-generation deployable telescope, featuring a 3-meter 6-segment primary mirror and advanced wavefront sensing and correction capabilities. In its stowed configuration, the SMT primary mirror segments collapse into a small volume; once on location, these segments open to the full 3-meter diameter. The segments must be very accurately aligned after deployment and the segment surfaces are actively controlled using numerous small, embedded actuators. The SMT employs a passive damping system to complement the actuators and mitigate the effects of low-frequency (<40 Hz) vibration modes of the primary mirror segments. Each of the six segments has three or more modes in this bandwidth, and resonant vibration excited by acoustics or small disturbances on the structure can result in phase mismatches between adjacent segments thereby degrading image quality. The damping system consists of two tuned mass dampers (TMDs) for each of the mirror segments. An adjustable TMD with passive magnetic damping was selected to minimize sensitivity to changes in temperature; both frequency and damping characteristics can be tuned for optimal vibration mitigation. Modal testing was performed with a laser vibrometry system to characterize the SMT segments with and without the TMDs. Objectives of this test were to determine operating deflection shapes of the mirror and to quantify segment edge displacements; relative alignment of λ/4 or better was desired. The TMDs attenuated the vibration amplitudes by 80% and reduced adjacent segment phase mismatches to acceptable levels.