the low frequency range (10-200 Hz), the sensitivity of Virgo will initially
be limited by the mechanical «thermal noise», i.e. by the fact that
each vibration mode of the mirrors and of the mirror suspensions is permanently
excited randomly, with an energy which is proportional to the absolute temperature
T. If these modes are very weakly damped, their energies are concentrated in
very narrow lines around each resonance frequency, and do not spoil the sensitivity
at other frequencies. But if a resonance is damped, then the line gets broader,
and the thermal energy generates a noise which can dominate the whole spectrum.
Thus, it is extremely important to minimize all sources of mechanical damping,
by selecting the materials and the right way to connect all the components to
avoid friction. Further improvement will require cooling the interferometer
to very low temperatures, close to T=20° Kelvin (-253 °C) in order to
minimize the thermal energy.
With the very high optical power accumulated in the various resonant cavities,
any defect or contamination of the optical surfaces may generate undesirable
effects such as diffused light, absorption of light and deformation of the optical
element and even permanent damages.
Special care must be taken at all stages of the manufacturing of the optical
elements and above all during their integration to prevent contamination. From
their delivery at the site all mechanical components entering in the ultra high
vacuum part are cleaned in a special automatic machine and kept in the most
extreme cleanliness conditions during the whole assembly and integration phase.
Optical components are integrated with even more care.