Y Arm End Station 2
At the left, viewers can see the LIGO Hanford Y arm beam tube entering the end station, proceding through a cryogenic pump and continuing to the right, ultimately to a large vacuum chamber that's obscured from view by a pump cart and racks of electronics. On the concrete pedestal adjacent to the beam tube's entry point, a large structure sits wrapped in sterile plastic. This is a large piece of vacuum-rated equipment (hence the need for a clean protective wrap) scheduled for installation at a time after these photos were taken.
Inside the clean room that shelters the cryopump, a large pyramid-shaped metal pier rises to a height slightly above the beam tube. This optical lever pier houses a laser in the top encasement that sends a small beam onto the surface of the test mass downstream at an angle. The beam refects onto a sensor on the opposite side of the beam tube, provide a signal that LIGO uses to control the angular position of the mirror.