Counting sheep on the Chajnantor plateau

This Picture of the Week shows a beautiful flock of sheep, grazing in the Chilean Andes. No, wait, wait, wait. There's no grass to feed on here, and they are at an altitude of around 5000 metres. Ok, no, they are not sheep. Let’s start again. This Picture of the Week shows the beautiful antennas of the Atacama Large Millimeter/submillimeter Array (ALMA), operated by ESO and its international partners in the Chilean Andes. Ok, that’s better!

As you can see, ALMA’s 66 white antennas are not the same size: most of them are 12-metre antennas, and only twelve are 7-metre antennas (see the smaller ones over to the left of the ‘flock’?). They work together as an interferometer, acting like a single big telescope. ALMA observes the light emitted from some of the coldest objects in the Universe, such as the most distant galaxies.

These antennas can be physically moved and arranged in different configurations, allowing ALMA to view the Universe with different levels of detail. The telescope can in fact probe both the broad structure of an astronomical source and its very finest details. The former requires the antennas to be close to each other, like in this image, whereas for the latter they need to be several kilometres apart. Each antenna is almost 15 metres high and weighs over 100 tons, so moving them across the desert and positioning them on concrete docking pads with millimetre precision is anything but simple — much trickier than herding sheep. To this end, two enormous transporters provided by ESO are used: yellow, 20 metres long and with 28 tyres each, they have been named Otto and Lore.