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First of the Critical Vibration Isolation Tables Now Ready for Optics!

First of the Critical Vibration Isolation Tables Now Ready for Optics!

- Contributed by Fred Raab

Last month we left you with the image of the large seismic isolation downtube being placed through the roof of the clean room into (Beam Splitter Chamber) BSC8 (the vacuum chamber that holds the input mirror for the Y-arm of the 2km-long interferometer). The vibration isolation system--along with its actuation stages, drivers and electronics--has now been completed. The photographs below show the system coming together.

Figure 1. Figure 2.

In Figure 1 at left above, the downtube assembly is shown being prepared in its cleanroom. The optical table is the round structure at the bottom of the tube, from which we will hang our most sensitive mirrors. The spider-like crossbar structure at the top of the tube will rest on stacks of vibration isolators. These isolators sit on the squarish shelf (called the support table) that is surrounding (but not touching) the downtube. The support table is bolted to support tubes that hold it at the correct height inside the vacuum chamber. This entire assembly is a carefully designed ultrahigh vacuum component. Atomic vibrations can start the downtube vibrating at its resonant frequencies, so it has been designed to be as stiff as possible. This means building it stiff and light, like an airplane. Although it looks solid, it is actually built like an egg-crate--hollow with lots of cross bracing on the inside.

Then, Figure 2 at right shows assembly of the vibration isolation stacks on top of the support structure, which is now locked in place in the large BSC chamber. The worker standing on the scaffolding is preparing the constrained-layer-damped coil springs used to absorb vibrations of the ground. A mass element sitting on a layer of these springs is clearly visible. When the ground shakes at frequencies above the resonances of these mass/spring layers (a few Hz), the springs take up the shaking allowing the masses to float relatively freely. The small amount of shaking that does get through one layer is further reduced by the springs in the next layer. The springs have special shock absorbing material and structures inside the coils themselves to damp the resonant frequencies of the stack.

Next, Figure 3 at left below shows the completed stacks. The cabling (which looks like brownish straps) carries control signals for mirrors and optics that will be mounted on the optical table. This cabling needs to be carefully designed not only for electrical and vacuum quality, but also for its mechanical properties, so it does not allow vibrations to "end-run" around the stacks. Then, Figure 4 at right shows the view through the bottom of the BSC chamber. The optical table floats at the top of the photo; the metallic cylinders hanging from the table are counterweights to balance the table, making it float level on its springs. Next we will hang the folding mirror and input mirror for this interferometer.

Figure 3. Figure 4.

The total supported load inside the chamber, consisting of support structures and tubes, the stacks, downtube assembly and optics payload weigh about as much as a Chevy Suburban. With the suspension fibers and the vibration isolation layers doing their jobs, earth's vibrations will not be a factor for this interferometer at any frequencies above about 40 Hz, and the atomic vibrations in the suspension wires and the mirrors themselves will be the largest "background" motions. Over time we expect that the springs may slowly drift and the table alignment and position will need to be touched up. We have installed a set of actuators and stages that allow the table to be adjusted while the chambers are evacuated. Looking past the optical chamber in Figure 4, we can view workers in an adjoining chamber working on installation of the input optics for this interferometer. But that is a story for another day...