Measuring transmission and forces from observatory equipment vibration

Abstract

We describe measurements of both the vibration forces imparted by various types of observatory equipment. and the transmission of these forces through the soil, foundations and telescope pier. These are key uncertainties both in understanding how to mitigate vibration at existing observatories and for developing a vibration budget in the design of future observatories uch as the Thirty Meter Telescope. Typical vibration surveys have measured only the resulting motion (acceleration); however. this depends on both the source and the system being excited (for example, isolating equipment results in less force being t ransmitted, but greater motion of the equipment itself). Instead, here we (a) apply a known force input to the pier from a shaker and measure the response at different locations, and (b) use isolator properties combined with measured acceleration to infer the forces applied by various equipment directly. The soil foundation and pier transmission can then be combined with a finite element model based vibration transmission analysis to estimate the optical consequences. Estimates of plausible source levels supports the development of a vibration budget for TMT that allocates allowable forces to the sources of vibration: this is described in a companion paper.

Management of equipment vibration for extremely large telescopes

“…At the Subaru telescope, in coordination with TMT engineers, an assessment was made of the transmission of ground vibration from facilities. 8 LSST performed a thorough review that describes the various options available for vibration mitigation. 9 This collaborative paper documents the efforts of all three observatories to mitigate equipment-induced vibrations within their own telescope.…”

Equipment vibration budget for the TMT

“…Note t hat all off-telescope sources affect the optical response through motion of t he pier; the propagation of forces from the enclosure or facilities building is not included in the Finite Element Model (FEM) but is captured by an estimated attenuation factor anchored from data taken at Subaru Observatory. 2 2. Azimuth and Elevation cable wrnps; these are modeled as torques about drive axes.…”

“…Our basic approach is to (i) use the t elescope finite element model to evaluate the opt ical sensitivity to forces applied at different locations and at different frequencies, (ii) use this sensitivity analysis to place requirements on source equipment to ensure that the error budget is met, and (iii) evaluate potential vibration sources to determine what steps (e.g., isolation) would be required to meet these requirements, or alternatively assess whether the AO error budget allocation to vibration should be increased. Vv'e intend to complement this model-based approach with transfer-function measurements made using calibrated sources as the observat ory is built; initial measurements at Subaru Observatory are described in [2]. Section 2 describes the modeling of optical sensitivity; preliminary results can also be found in [3] .…”

Vibration budget for observatory equipment

“…We intend to complement this model-based approach with transfer-function measurements made using calibrated sources as the observatory is built; initial measurements at Subaru Observatory are described in Ref. 9. Improved estimates for force levels from different sources will be used to reallocate force budgets to achieve the most cost-effective solution.…”