as possible to the tower cylinder, the baffles inside the towers at the junction of the big links have the 1150 600 mm2 with a shape shown in green 34: in the centre a big plate hole having a radius equal to 1150 120 mm2 5 times the beam waist, and on each side three plates installed as shown in the side view in 34. The tower has 4 such devices, covering most of the cylinder (Figures 33 and 35), the input towers have 2, the end towers have one and the recycling tower has one. Figure 35. Picture of the 4 glass baffle sets inside the beam splitter tower, taken from below with a fish-eye lens before mirror installation. ~ some 10-25/Hz. The resulting noise from the central area is reduced to 6. Mirror suspension 6.1 Seismic isolation Seismic motion, always present on ground, even in absence of earthquakes, is a very powerful noise source for gravitational wave detectors. The typical linear spectral density of seismic displacement in all three space coordinates can be approximated, in the frequency range of Virgo, by: Hz-1/2, measured in m as a function of the frequency and of the parameter , which ranges f 10-6 10-9 m Hz3/2, typically between and depending on the geographical location. From the previous equation, it out that the seismic motion reaching the test masses (the mirrors) 1012, has to be reduced by at least a factor in order to have an acceptable sensitivity in the 10 Hz region. performing devices have been developed in order to suspend the Virgo mirrors, strongly reducing the induced motion; those devices have been called: Superattenuators 6.2 Superattenuator working principle The SA design is based on a simple idea. If the suspension point of a simple pendulum moves in the (f0), horizontal plane at a frequency f higher than the pendulum normal mode the oscillation is transmitted to (f0/f)2. the suspended mass with attenuation proportional to The wire length connecting the suspension l point to the mass sets the pendulum normal mode frequency according to the equation: . Therefore a device suspending a mirror based on the working principle of a simple pendulum, represents a good isolation system against horizontal seismic noise at frequency f>>f0.
as possible to the tower cylinder, the baffles inside the towers at the junction of the big links have the shape shown in green 34: in the centre a big plate 1150 600 mm2 with a hole having a radius equal to 5 times the beam waist, and on each side three 1150 120 mm2 plates installed as shown in the side view in 34. The tower has 4 such devices, covering most of the cylinder (Figures 33 and 35), the input towers have 2, the end towers have one and the recycling tower has one. Figure 35. Picture of the 4 glass baffle sets inside the beam splitter tower, taken from below with a fish-eye lens before mirror installation. The resulting noise from the central area is reduced to ~ some 10-25/Hz. 6. Mirror suspension 6.1 Seismic isolation Seismic motion, always present on ground, even in absence of earthquakes, is a very powerful noise source for gravitational wave detectors. The typical linear spectral density of seismic displacement in all three space coordinates can be approximated, in the frequency range of Virgo, by: measured in m Hz-1/2, as a function of the frequency f and of the parameter , which ranges typically between 10-6 and 10-9 m Hz3/2, depending on the geographical location. From the previous equation, it out that the seismic motion reaching the test masses (the mirrors) has to be reduced by at least a factor 1012, in order to have an acceptable sensitivity in the 10 Hz region. performing devices have been developed in order to suspend the Virgo mirrors, strongly reducing the induced motion; those devices have been called: Superattenuators 6.2 Superattenuator working principle The SA design is based on a simple idea. If the suspension point of a simple pendulum moves in the horizontal plane at a frequency f higher than the pendulum normal mode (f0), the oscillation is transmitted to the suspended mass with attenuation proportional to (f0/f)2. The wire length l connecting the suspension point to the mass sets the pendulum normal mode frequency according to the equation: . Therefore a device suspending a mirror based on the working principle of a simple pendulum, represents a good isolation system against horizontal seismic noise at frequency f>>f0.