This area houses the long delay line rails, used to make coarse adjustments in the starlight pathlength.

Read the transcription from the related video: "In the Basement"

The following are descriptions of locations in the panoramic image.

Location 1 - Optical switchyard
Here, starlight from the two observatories is split into multiple beams and steered in to the Fast Delay Line areas and the Combining Lab.

Location 2 - Combining Lab
This is the "back end" of the interferometer, where light from both telescopes is tracked and combined. It is carefully protected from dust and air currents that can distort the light beams.

Location 3 - Long delay line rails
These rails, spanning 128 feet of the distance between the two Keck domes, carry motorized mirror sleds that are used to make gross adjustments in the path length of incoming starlight. These adjustments correspond to pointing the telescopes to different patches upon the night sky. The sleds can be operated by astronomers in the control room or at Keck headquarters.

Location 4 - Long delay line cart
Each of these motorized carts carries three mirrors. Astronomers can adjust its position along the rails to control the length of the light path to within 1 millionth of an inch.