reinforced optical table vibration analysis

The MRO Interferometer started in 2017 a seismic measurement campaign to characterize vibration in critical areas where science instruments are to be installed. The following were the goals: to characterize vibrations in the inner BCA at floor level and on the surfaces of two optical tables; and to use these data to see how ground vibrations couple into the inner BCA optical tables. This vibration assessment has indicated that vibration is significantly amplified on the top of optical tables, and that the associated support legs require some level of structural reinforcement. A number of designs have been proposed for revision, and one based on concrete legs was selected for implementation. As a way to reduce implementation risk, the new design with reinforced legs is being implemented in the ineer BCA using one section of the metrology optical table and so a new set of vibration assessment is necessary (expected for March/2018). When the new design is approved for implementation, it will be used in all optical tables installed in the inner BCA during the year of 2018. FREQUENCY RANGE OF INTEREST: We are concerned about vibrations in roughly the 2 Hz - 250 Hz frequency range. It is anticipated that the optical table installed in the inner BCA have a first resonant frequency larger than 40Hz. DIRECTIONS: It is necessary to measure vibrations in all three orthogonal directions (x,y,z). TYPICAL MOTIONS OF INTEREST: We are worried about motions of order 20nm occurring over timescales of order 20ms. Note that 20nm is the sort of OPD difference that matters for interferometry, and 20ms is the near-infrared coherence time. SENSITIVITY REQUIRED: 20nm of motion occuring in 20ms corresponds to a velocity of 1 micron/second. MEASUREMENT CONDITIONS: As performed in previous seismic campaign, it would be good to make recordings with the HVAC running (as it would presumably be during a real observation). Other disturbances, if present, shall be logged. The overall measurement time frame suggested may take no more than a few days.