Readout, Sensing, and Control

Suspending the mirrors is one of the most crucial tasks in gravitational wave interferometer technology. The performance of the suspensions must provide the required attenuation of seismic noise and reduction of thermal noise, two fundamental limits to the sensitivity of any gravitational wave detector. Moreover, the suspension system must be equipped with sensors and actuators which are used to actively control some relevant degrees of freedom, so to be able to keep the interferometer at its working point (i.e., "locked"). In the first part of this chapter we deal with the basic principles behind the super attenuator chains developed in Virgo to reduce the seismic noise. In the second part, we illustrate the techniques to suspend the mirror reducing the thermal noise in the detection bandwidth, according to the theory illustrated in Chap. 8.