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

A few words about the Low Frequency Facility 111different stages of the SA. The last part of the SA is composed of steering filters,which support four coils, the marionetta, a cross shaped piece of steel, whichsupports four magnets, positioned in the form of four coils, see figure 9.6. In thisway the mirror can translate along the beam direction and along the vertical, rotatearound the vertical axis and around the horizontal axis perpendicular to the beam.The test-mass (the interferometer mirror) and the reference mass are attached aspendulums to the marionetta [15]. Magnets and coils produce forces between thereference mass and the mirror.The control has two main parts: the damping and the mirror control. Thedamping is a local control, which reduces the large motion of the SA, due to theresonance frequencies below 1 Hz: on top of the inverted pendulum feedbacksare produced using accelerometer signals and electromagnetic actuators [16]. Theother control uses the interferometer signal itself and it is aimed to keep alignedthe whole Virgo interferometer in its working point. The actuators, couples ofmagnet-coil, are divided per frequency band: below 100 mHz the feedback goesto the IP actuators, in the intermediate band to the marionetta, and above to thereference mass. This kind of control, called hierarchical is necessary for the hugedisplacement range required and the necessity not to induce electronic noise atthe mirror level.9.3 A few words about the Low Frequency FacilityMuch work is being done around interferometers at the moment, but the plannedsensitivity is not enough to guarantee the detection of events. A large community[17] is working hard to improve the sensitivity of the antennae. LIGO plansto make an upgrade in five years from now. The effort is mainly concentratedon improving the suspension and keeping as low as possible the thermal noiseof pendulum and mirror substrate. With the aim of reducing thermal noise,several methods have been proposed, like cryogenic cooling, choice of low lossmaterial for the test mass, as zaffire or silicon, and novel cancellation techniques.Figure 9.3 shows that, below 600 Hz, Virgo is entirely dominated by the thermalnoise of the suspension. The improvement below 600 Hz implies changes inthe suspension. In order to demonstrate that a new choice, once applied to theantenna, makes a real improvement in the sensitivity of the whole antenna it isnecessary to make tests in conditions as close as possible to the real one. TheLow Frequency Facility (LFF) [9, 19], has been conceived in order to measurethe noise of test masses attached to the SA, with a sensitivity at 10 Hz close tothe Virgo one: 10 −18 m/ √ Hz. The goal is to take a measurement of the thermalnoise of the Virgo suspension, while the system is under active control of the lowfrequencymotion. In this way, it will be possible to characterize the suspensionin an environment close to the real one.The use of a Fabry–Perot cavity as a displacement transducer relies on theexcellent frequency stabilization of the laser beam. A frequency stabilization, for