Seismic vibrations of the ground are billions of times larger than the distance
variations induced by gravitational waves. Therefore, avoiding spurious motions
of the optical components is essential. The seismic isolation is achieved
through a chain of suspended seismic filters made of triangular cantilever
blade springs. The springs provide the vertical isolation while the compound
pendulum provides isolation against horizontal motions. To further reduce
the seismic disturbances, this chain is attached to an actively stabilized
platform which compensates for very low frequency and large amplitude oscillations.
It also provides a first stage of position control down to about one micron.
A second stage of position control is achieved at the end of the suspension chain by a marionetta from which the mirror and a recoil mass are suspended by extremely fine wires. The ultimate mirror position control is obtained through very small forces generated between the mirror and the recoil mass by sets of electro-magnetic or electrostatic actuators.
In the end, the environment of the VIRGO optics is much quieter than what is achievable in a spacecraft orbiting the earth.