LVEA Test Mass
An Advanced LIGO large optic rests inside of a clean room in the rear of LIGO Hanford's Laser and Vacuum Equipment Area (LVEA). This mirror (also known as a test mass) was later installed in one of the two vacuum chambers that sit adjacent to the H1 detector's beam splitter. In operation, in combination with an identical mirror at the end of a four-kilometer detector arm, this optic will cause the laser light from LIGO's main beam to reflect several hundred times along the length of the arm. This optic and others like it that now inhabit the LIGO and Virgo gravitational wave detectors respresent the global state of the art in mirror fabrication and coating technology.
The mirror represents the final stage of a four-stage pendulum suspension (a quadruple suspension, or "quad"). The photo shows stages two, three and four of the quad -- stage one has been separated and sits on a test stand out of view. Stage three consists of an uncoated glass mass of the same size as the mirror. Two loops of metal wire suspend this "penultimate" mass from the stage two mass. The stage four mirror hangs from the penultimate mass by four thin 60cm glass fibers, two on each side. Note that the mirror is covered by a protective plastic cap. The pink tint that's visible behind the cap arises from a layer of First Contact that was applied to the optic, to be removed after the mirror's installation in the vacuum chamber.