The VIRGO suspensions

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Abstract
The VIRGO suspensions are chains of passive mechanical filters designed to isolate the interferometer mirrors from seismic noise starting from a few Hz. In order to reduce the low-frequency swing of the mirror along the beam, an active control system, acting at the level of the suspension point, damps the main resonant modes of the system (all below 2.5 Hz). Another control loop, at the level of the optical payload, makes use of a digital camera monitoring the mirror position in all six degrees of freedom. Its main goal is to decrease the rms angular displacements of the mirror, on a time scale of several minutes, down to less than 1 μrad. All the seven suspensions of the VIRGO central interferometer are presently in operation, while the assembly of the last two, for the terminal mirrors, is in progress. The design and performance of the system are described in this paper.