(via APOD: 2012 April 21 - 3 ATs)
They look like R2D2, but are the ATs for Panaral, so they’re AT-AT-AT, I guess?
Anyway, “AT” stands for Auxiliary Telescope, 1.8 meter movable telescopes designed to be used for interferometry in tandem with the 8 m Very Large Telescope units. Interferometry allows for very high resolution observations without having to build quite so monolithically large a mirror in the telescope. You use the observations from all the various ‘scopes to reduce interference from the atmosphere (by compensating for the effects of the air) and collect as much light as possible. It requires the light to all be added together and collected in underground tunnels with mirrors.
Pretty nifty “little” droids, if you ask me.
Image Credit & Copyright: Yuri Beletsky (ESO)