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

18.5 cmThe SA suspension and requirements on the control 109to the previousstagecrossbarmagnetsmagnetscenteringwirescenteringwiresbellowbellow moving element bladecenteringwiresto the nextstagecentralcolumnbladehinge70 cmFigure 9.5. The filter.parts: the vessel, roughly speaking a cylinder with a hole in the centre and amovable part, composed of a piece called ‘cross’ attached to a tube called ‘centralcolumn’, rigidly connected together. The ‘central column’ is free to move alongthe vertical inside the vessel, for about a centimetre. The upper wire is attachedto the vessel, while the lower is attached to the column. The two pieces areconstrained to move along the vertical by means of centring wires. The baseof each blade is rigidly attached to the outer diameter of the vessel, while the tipsare connected to the central column. When the filter is loaded the blades act as aspring, and are free to move for several millimetres, with a resonant frequencyabove 1 Hz. In order to decrease the vertical resonant frequency a magneticantispring acts between the vessel and the ‘cross’, in parallel with the blades.It is composed of an array of permanent magnets attached to the ‘cross’, placedin front of an equal array of magnets attached to the vessel; the two array aread hoc built in order to make repulsive force, proportional to the vertical offset,for a few millimetres. The effective elastic constant is the difference between theblade spring and the magnetic antispring, and each filter is tuned to oscillate at∼400 mHz. In figure 9.3 with the other noise sources, the seismic motion of theVirgo test masses is shown [14], it is possible to see that it dominates below 2 Hz,and the resonance peaks of the SA are visible.Ground motion can be of the order of hundreds of micrometres, due to tidaldeformation of the Earth’s crust and thermal effects on the surface. In order to