Observatories
LIGO operates two gravitational wave observatories in unison: the LIGO Livingston Observatory (30°33′46.42″N 90°46′27.27″W / 30.5628944°N 90.7742417°W / 30.5628944; -90.7742417) in Livingston, Louisiana, and the LIGO Hanford Observatory, on the DOE Hanford Site (coordinates of central complex: 46°27′18.52″N 119°24′27.56″W / 46.4551444°N 119.4076556°W / 46.4551444; -119.4076556), located near Richland, Washington. These sites are separated by 3,002 kilometers (1,865 miles). Since gravitational waves are expected to travel at the speed of light, this distance corresponds to a difference in gravitational wave arrival times of up to ten milliseconds. Through the use of triangulation, the difference in arrival times can determine the source of the wave in the sky.
Each observatory supports an L-shaped ultra high vacuum system, measuring 4 kilometers (2.5 miles) on each side. Up to five interferometers can be set up in each vacuum system.
At the Hanford Observatory, a second interferometer operates in parallel with the primary interferometer. This second detector is half the length at 2 kilometers (1.25 miles), and its Fabry–Pérot arm cavities have the same optical finesse and thus half the storage time. With half the storage time, the theoretical strain sensitivity is as good as the full length interferometers above 200 Hz but only half as good at low frequencies.
The LIGO Livingston Observatory houses one laser interferometer in the primary configuration. This interferometer was successfully upgraded in 2004 with an active vibration isolation system based on hydraulic actuators providing a factor of 10 isolation in the 0.1 – 5 Hz band. Seismic vibration in this band is chiefly due to microseismic waves and anthropogenic sources (traffic, logging, etc.).
The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory, as well as one half-length interferometer. Hanford has been able to retain its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.
Members of the public can tour both observatories, either by special arrangement or on regular "open house" days.
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