Atacama Large Millimeter/submillimeter Array

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The sixty-six-antenna millimetre and submillimetre array on the Llano de Chajnantor in northern Chile, at about five thousand metres, where the air is dry enough for observations at 1.3 millimetres to reach the ground at all. For Event Horizon Telescope observations the array is phased - its antennas combined into a single very large effective dish - which contributes the bulk of the array's sensitivity and turns marginal baselines into usable ones. The upgrade that made ALMA capable of phased VLBI is generally regarded as the step that made imaging a black hole shadow practical rather than merely conceivable, because sensitivity on the long baselines to Spain, Hawaii and the South Pole was the binding constraint.