Event Horizon Telescope

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A collaboration and an observing array that links millimetre-wave radio telescopes across the planet into an interferometer with an effective aperture the size of the Earth. The April 2017 campaign that produced the M87* image used eight stations at six geographic sites - two on Chajnantor in Chile, two on Maunakea, and single stations in Spain, Mexico, Arizona and at the South Pole - observing at about 230 gigahertz, a wavelength of 1.3 millimetres, with the longest baselines approaching ten thousand kilometres and an angular resolution of some tens of microarcseconds. The data volume is too large to transmit: petabytes were recorded onto helium-filled hard drives and physically flown to correlators at MIT Haystack Observatory and the Max Planck Institute for Radio Astronomy in Bonn. The South Pole recordings could not leave until flights resumed at the end of the austral winter, which is why an April observation was not correlated until the following year. Four teams reconstructed images independently and with different methods before comparing results. Not a place on Earth in itself; its stations are.