The Gemini Planet Imager (GPi) is an instrument that will mount to either of two nominally identical Telescopes,
Gemini North in Hawaii and Gemini South in Chile, to perform direct imaging and spectroscopy of extra-solar planets.
This 2,000-kg instrument has stringent mass, center-of-gravity, flexure, and power constraints. The Flexure Sensitive
Structure (FSS) supports the main opto-mechanical sub-systems of the GPi which work in series to process and analyse
the telescope optical beam.
The opto-mechanical sub-systems within the FSS are sensitive to mechanical vibrations, and passive damping strategies
were considered to mitigate image jitter. Based on analysis with the system finite element model (FEM) of the GPi, an
array of 1-kg tuned mass dampers (TMDs) was identified as an efficient approach to damp the first two FSS flexural
modes which are the main sources of jitter. It is estimated that 5% of critical damping can be added to each of these
modes with the addition of 23 kg of TMD mass. This estimate is based on installing TMD units on the FSS structural
members. TMD mass can be reduced by nearly 50% if the units can be installed on the opto-mechanical sub-systems
within the FSS with the highest modal displacements.
This paper describes the structural design and vibration response of the FSS, modal test results, and plans for
implementation of the TMDs. Modal measurements of the FSS structure were made to validate the FEM and to assess
the viability of TMDs for reducing jitter. The test configuration differed from the operational one because some
payloads were not present and the structure was mounted to a flexible base. However, this test was valuable for
understanding the primary modes that will be addressed with the TMDs and measuring the effective mass of these