(ed.). Gravitational waves (IOP, 2001)(422s).

108 The Earth-based large interferometer VirgoFigure 9.4. Scheme of the Virgo interferometer.cavity; ten vertical vacuum towers, up to 11 m high, 2 m in diameter, containingthe different SAs. Figure 9.4 shows the Virgo scheme. The construction of Virgois planned in two main steps: the realization of the central interferometer by late2000, and the realization of the 3 km arm full interferometer by late 2002.9.2 The SA suspension and requirements on the controlRoughly speaking, above a few hertz, the seismic noise is a function of thefrequency ν and goes as ν −2 . In our site the measured spectrum is about 10 −6 –10 −7 /ν 2 m/ √ Hz, in all degrees of freedom; this implies a factor of at least10 10 of the noise reduction at 10 Hz. An harmonic oscillator, with resonantfrequency ν 0 acts as a low pass filter if N oscillators are connected in cascade,and for frequencies above the resonant frequencies, the displacement noise of theattachment point is transmitted to the test-mass with a transfer function going as∼1/ν 2N . Each SA is a multistage pendulum [13], acting as a multipendulum inall six degrees of freedom. The basic tool is a piece called a mechanical filter. Itis essentially a rigid steel cylinder (70 cm in diameter, 18 cm in height and with amass of about 130 kg), with a thin steel wire. In this way the horizontal oscillatorin two degrees of freedom and the torsion spring are made. The attachment pointsof the two wires are connected close to the centre of mass of the filter: in this waythe pitch spring is formed. The vertical string is a mechanical one, a triangularcurled piece of a special steel (MARAGING). The SA blades are designed tohold about 50 kg, at rest they are bent and when loaded act as a spring andbecome flat. Each filter of the SA is equipped with a number of blades from12–4, in function of the load. Each filter of the SA is composed of two main