ALMA

ALMA links 66 antennas on a plateau at 5,000 meters into one telescope for millimeter light

The Atacama Large Millimeter/submillimeter Array (ALMA) is an interferometer of 66 antennas that observes at wavelengths from 3.6 to 0.32 mm (31 to 1,000 GHz), through 10 receiver bands. At these wavelengths the sky shows cold dust and molecular gas, where stars and planets form. In 2014 ALMA imaged concentric rings in the planet-forming disk around HL Tauri. It is the anchor station of the Event Horizon Telescope, which published the first direct image of a black hole in 2019. It also took part in the claimed detection of phosphine on Venus, which remains disputed.

ALMA is a partnership of ESO for Europe; NSF, through NRAO, with the National Research Council of Canada for North America; and NAOJ, under NINS, with Academia Sinica of Taiwan for East Asia, in cooperation with the Republic of Chile; South Korea is also named as a partner. It cost about US$1.4 billion. Ground was broken in November 2003, the first images came on 3 October 2011, the observatory was inaugurated on 13 March 2013, and the 66th antenna was accepted on 23 September 2013.

The antennas stand at the Array Operations Site (AOS) on the Chajnantor plateau in the Antofagasta Region of Chile, at 5,058.7 m. The Operations Support Facility (OSF) is at 2,900 m, 35 km from San Pedro de Atacama. See the ALMA listing, radio telescopes around the world and more observatories in Chile.

Three industrial teams built ALMA's antennas, each surface true to 25 micrometers

ALMA has 54 antennas of 12 m and 12 of 7 m. Europe supplied 25 of the 12 m antennas through the AEM Consortium: Thales Alenia Space as lead, with its units in France and Italy, European Industrial Engineering (EIE) of Italy and MT Mechatronics of Germany. It was ESO's largest contract at the time, and the last European antenna arrived on 1 October 2013. The 25 North American antennas, 110 metric tons each, came from General Dynamics SATCOM Technologies, formerly Vertex RSI, which also built their assembly hangar. Mitsubishi Electric (MELCO) built Japan's 16 antennas, four of 12 m and 12 of 7 m: the Atacama Compact Array, renamed the Morita Array in 2013 for its designer, Koh-ichiro Morita.

Every surface is accurate to 25 micrometers, and every antenna points to 0.6 arcseconds. The array reconfigures from baselines of 150 m out to 16 km: two 28-wheel transporters from Scheuerle Fahrzeugfabrik of Germany, 130 metric tons each, carry the antennas between pads.

Ten receiver bands built on three continents share a 4 K cryostat in every antenna

Each antenna carries a front end; the project required 70. Its largest single element is the cryostat, built by Rutherford Appleton Laboratory (RAL) in the UK, which cools the receivers down to 4 K. The receiver cartridges plug into it band by band. NAOJ of Japan built Bands 4, 8 and 10 and co-led Band 1 with ASIAA of Taiwan. NRC Herzberg in Canada built Band 3, NRAO in the US built Band 6, IRAM in France built Band 7, NOVA in the Netherlands built Band 9, and Onsala Space Observatory GARD in Sweden built Band 5 with NOVA. NRAO integrated the North American front ends in Charlottesville, Virginia.

Band 2 (67 to 116 GHz), the last ALMA band, is led by ESO with NOVA, Chalmers University and Onsala GARD, INAF of Italy and NAOJ, which makes the optics. Pre-production cartridges on three antennas had first fringes by August 2023, and the band is expected for science in 2026 or 2027.

Stirling Cryogenics of the Netherlands supplied the liquid nitrogen generator at the OSF for the receiver calibration loads. The maker of the cryocoolers that bring the cryostats to 4 K is not publicly named.

The ALMA correlator runs 17 quadrillion operations a second at 5,000 meters

Each antenna digitizes its signal and sends it over fiber, on the Digital Transmission System at about 120 Gb/s, to the correlator in the AOS Technical Building at 5,000 m. NRAO led the design and build of the 64-antenna correlator, with an ESO contribution of 550 digital filter boards from the University of Bordeaux. Completed in December 2012, it has more than 134 million processors and runs up to 17 quadrillion operations a second. A separate correlator built by Fujitsu and delivered by NAOJ serves the 16 antennas of the Morita Array.

Timing: two hydrogen masers keep the array in step

Hydrogen masers give the absolute time reference for the array and for very long baseline interferometry (VLBI). MIT Haystack Observatory installed a second active maser in November 2023. The maker of the masers is not publicly named.

The Wideband Sensitivity Upgrade replaces the whole digital chain

The Wideband Sensitivity Upgrade (WSU) doubles, then quadruples, the correlated bandwidth. It replaces the digitizers, the Digital Transmission System and the correlator, and upgrades most receivers. Its new correlator, ATAC, approved at $36 million in February 2023 for about six years of work, will produce 200 times more data per second at first and 400 times eventually.

NRC Herzberg in Canada designs the ATAC hardware and firmware on TALON-DX boards; a Canadian industry partner that is not publicly named builds the hardware. NRAO leads the WSU and writes the correlator software, and MIT Haystack adds the VLBI and phased-array mode that keeps ALMA in the Event Horizon Telescope. NAOJ and NRAO are taking the Digital Transmission System from about 120 Gb/s to 1.6 Tb/s over up to 80 km. The digitizers are due in 2028; ATAC and the Total Power GPU Spectrometer (TPGS) in 2029; and the first WSU capabilities, Bands 2, 6 and 8 at twice the bandwidth on at least 36 retrofitted antennas, by 2030.

ALMA data grow from half a terabyte a day to more than 40 after the upgrade

ALMA produces about 0.5 TB a day today, rising to more than 40 TB a day after the WSU; the archive is about 2 PB. ALMA Regional Centre nodes help maintain the data archive.

Deep learning is still in development at ALMA. An ESO development study, DeepFocus, chains a convolutional autoencoder (Blobs Finder) to find sources, a recurrent network (DeepGRU) for spectra, and residual networks (ResNets) for morphology and flux, and runs at least 150 times faster than tCLEAN, the standard imaging task in CASA. The study also tests RESOLVE, a Bayesian imaging method. Its partners are ESO, the Max Planck Institute for Astrophysics (MPA) in Garching, the University of Naples Federico II and the University of Florence. It is not part of the production pipeline. The TPGS in the WSU is GPU compute for spectra, not machine learning.

ALMA public tours are suspended; visits continue only for educational, research and media groups

The general public visit program is currently suspended. Visits continue for educational, research and media groups only; check the ALMA visits page for when public tours return.

When the program ran, tours were free on Saturdays and Sundays, by bus from Tumisa Street in San Pedro de Atacama at 9:00 am, back by 1:00 pm, with registration required and no children under five. Tours went to the OSF at 2,900 m; the AOS at 5,000 m is not open to visitors.

Sources

1. Wikipedia: Atacama Large Millimeter Array
2. ESO: ALMA receiver bands
3. ESO: ALMA antennas
4. ESO: visiting ALMA
5. ESO: the ALMA correlator
6. ESO: final European ALMA antenna delivered
7. NRAO: final North American ALMA antenna delivered
8. arXiv: the ALMA front-end cryostats
9. MIT Haystack: ALMA hydrogen maser
10. NAOJ: new ALMA receivers
11. ALMA Observatory: new correlator approved
12. ALMA Wideband Sensitivity Upgrade (UK workshop, 2026)
13. ALMA Observatory: visits
14. NRC: a new brain for the ALMA telescope
15. ESO: ALMA development study on deep-learning imaging
16. ALMA Observatory: Band 2 receivers
17. Stirling Cryogenics: ALMA reference