Andrew Cooper put together this video of the two Keck telescopes in action on top of Mauna Kea in Hawai'i. There are some beautiful time-lapse shots like in the pic above, but the reason to watch this video is for all the behind-the-scenes footage showing what telescopes like this have going on behind the mirror.

The whole video is worth watching just for the artistry of it (and the music), but there are some specific scenes that could use some extra explanation. Specifically, the "delay lines" moving along the tracks like little trains at 0:28, 0:39, and elsewhere. Many telescopes are starting to use systems like these to combine multiple discrete mirrors into a much larger imaging device (like the ESA's Very Large Telescope), so let's just go over how it works.

Part of what makes the Keck telescopes so effective is that they can work together and pretend to be one much larger telescope. The technique that they use to do this is called interferometry, and it involves combining the light beams from both telescopes to form one single image. In order to do this, the light from each telescope has to travel the exact same distance to the detector, and as each telescope moves continually to track the sky, this is a tricky thing to do. So, between the two telescopes is a "beam train," which uses mirrors and lenses to transport the beams of light, and included in that beam train are delay lines, which you'll see moving back and forth in the video: they look kinda like train cars.

Inside each car are mirrors that bounce the beam of light back and forth down the beam train, "delaying" it to compensate for the distances between the two telescopes. As the telescope moves, so do the carriages, making sure that the distance that the light has to travel remains exactly the same. There are two lines: the long delay lines, for coarse adjustments, and the fast delay lines, for final focusing. Generally, the long delay lines are set at the start of an observation, while the fast delay lines are used to compensate for continuous movement. It's not easy to do this for two telescopes, to say nothing of four, but it allows astronomers to make observations that would otherwise take a much larger mirror.

Other highlights include CNC milling of a spectrograph slitmask at 0:55 (which is used to block out light to get better measurements of what stars are made of), and 1:22, which shows how different instruments can be mounted onto the telescope using a sort of roundhouse and turntable type thing. The whole vid's down below, and it's only three minutes long, so take a minute (three minutes) and check it out.

Vimeo, via Neatorama