Atacama Large Millimeter

Sixty-six antennas on a 5,000-metre desert plateau in Chile, linked by a supercomputer into one telescope that sees cold gas and dust at millimetre wavelengths. It imaged planets forming and helped capture the first picture of a black hole.

Three continents, one array

ALMA is a partnership among the European Southern Observatory, the US National Science Foundation with Canada, and the National Astronomical Observatory of Japan with partners in East Asia, in cooperation with the Republic of Chile. The Chajnantor plateau, at about 5,000 metres in the Atacama Desert, is among the driest places on Earth, an advantage because water vapour absorbs the millimetre waves ALMA observes. Science observations began in 2011, with first images released on 3 October, and the array was inaugurated on 13 March 2013.

The array has 54 antennas of 12 metres and 12 of 7 metres. The antennas can be moved among configurations up to 16 kilometres across.

Rings, gaps and a shadow

In 2014 ALMA imaged the young star HL Tauri and showed the disk of gas and dust around it carved into rings and gaps, a sign of planets forming, an image that made headlines around the world. In 2019 ALMA was part of the Event Horizon Telescope, the global network of radio telescopes that produced the first picture of the shadow of a black hole. ALMA's sensitivity was crucial because it served as the anchor of the array.

Thijs de Graauw led the project as ALMA's director in its construction phase.

An observatory at altitude

Staff work at the array only briefly. The operations support facility is at 2,900 metres, where people sleep and prepare, and those at the array carry oxygen. Antennas are moved by two giant transporters. Data from every observing run are processed in a pipeline and archived, and proposals are open to astronomers worldwide, judged by merit. Data become public after a proprietary period of twelve months.

An array that changes its shape

ALMA's antennas are moved between configurations by transporters, so the same array can map a wide field at low resolution or a small one at high resolution. The array is under constant development: a wideband sensitivity upgrade will double the bandwidth of the receivers and increase the speed of the correlator. Operating at 5,000 metres is a challenge for the staff, who work in shifts and carry oxygen, and for the electronics, which must be cooled to a few kelvin. The array's success shows that a global partnership can build and run a single facility, provided each partner's contribution is clear and the decision-making is shared.

Instruments and technologies

  • 12-metre antennas Fifty-four antennas with a surface accurate to 25 micrometres, to focus millimetre waves.
  • Correlator A supercomputer performing about 17 quadrillion operations per second, combining the signals of every pair of antennas.
  • Receiver bands Cryogenically cooled receivers covering ten frequency bands from 35 to 950 GHz.

The people

  • Thijs de Graauw Astronomer who directed ALMA's construction.

Timeline

  • 2011 Science observations begin; first images released on 3 October.
  • 2013 Inaugurated on 13 March; the 66th antenna is delivered later that year.
  • 2014 Image of the HL Tauri disk shows rings and gaps.
  • 2019 Contributes to the Event Horizon Telescope's first image of a black hole.

Key lessons for observation today

  • Partnership across agencies works with one array Three regional partners share one instrument and one archive. Federated funding and unified operations can coexist.
  • A good site is a scientific asset The dry plateau made the instrument possible. Siting study is part of the science case.
  • Processing is part of the instrument The correlator creates the telescope from separate antennas. Data processing is inseparable from sensing.

Sources

  • Wikipedia, Atacama Large Millimeter Array
  • NAOJ, ALMA inauguration ceremony (2013)
  • Leiden Observatory, In memoriam Thijs de Graauw 1942–2026