ALMA telescopes at dusk. Crescent moon in the sky over the telescopes of the Atacama Large Millimetre Array (ALMA) in the Atacama Desert, Chile, at dusk. These large radio telescope antennas are located at an altitude of 5000 metres on the Chajnantor Plateau in the Chilean Andes. They have a diameter of 12 metres, while 12 smaller antennas with a diameter of 7 metres make up the ALMA Compact Array (ACA). Data from the separate antennas is combined by interferometry, making it one of the most powerful telescopes in the world.