The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer of 66 radio telescopes in the Atacama Desert of northern Chile, which observe electromagnetic radiation at millimeter and submillimeter wavelengths. The array has been constructed on the 5,000 m (16,000 ft) elevation Chajnantor plateau – near the Llano de Chajnantor Observatory and the Atacama Pathfinder Experiment. This location was chosen for its high elevation and low humidity, factors which are crucial to reduce noise and decrease signal attenuation due to Earth’s atmosphere. ALMA is expected to provide insight on star birth during the early Stelliferous era and detailed imaging of local star and planet formation.
ALMA is an international partnership among Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile. Costing about US$1.4 billion, it is the most expensive ground-based telescope in operation. ALMA began scientific observations in the second half of 2011 and the first images were released to the press on 3 October 2011. The array has been fully operational since March 2013.
The initial ALMA array is composed of 66 high-precision antennas, and operates at wavelengths of 3.6 to 0.32 millimeters (31 to 1000 GHz). The array has much higher sensitivity and higher resolution than earlier submillimeter telescopes such as the single-dish James Clerk Maxwell Telescope or existing interferometer networks such as the Submillimeter Array or the Institut de Radio Astronomie Millimétrique (IRAM) Plateau de Bure facility.
The antennas can be moved across the desert plateau over distances from 150 m to 16 km, which will give ALMA a powerful variable “zoom”, similar in its concept to that employed at the centimetre-wavelength Very Large Array (VLA) site in New Mexico, United States.
The high sensitivity is mainly achieved through the large numbers of antenna dishes that will make up the array.
The telescopes were provided by the European, North American and East Asian partners of ALMA. The American and European partners each provided twenty-five 12-meter diameter antennas, that compose the main array. The participating East Asian countries are contributing 16 antennas (four 12-meter diameter and twelve 7-meter diameter antennas) in the form of the Atacama Compact Array (ACA), which is part of the enhanced ALMA.
The complex was built primarily by European, U.S., Japanese, and Canadian companies and universities. Three prototype antennas have undergone evaluation at the Very Large Array since 2002.
General Dynamics C4 Systems and its SATCOM Technologies division was contracted by Associated Universities, Inc. to provide twenty-five of the 12 m antennas, while European manufacturer Thales Alenia Space provided the other twenty-five principal antennas (in the largest-ever European industrial contract in ground-based astronomy). Japan constructed 16 Antennas. The first antenna was delivered in 2008, the last in 2011.
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences of Japan (NINS) in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA. Its current director since February 2018 is Sean Dougherty.
What is ALMA?
Half of all light in the universe is in millimeter-wavelength light between the far infrared and radio waves. ALMA can detect this light, which is emitted by cool objects and distant objects. It’s possible thanks to the telescope’s location at 16,400 feet in the driest desert on Earth, and because of the incredible precision of its 66 antennas.
All telescopes are limited in their angular resolution by the ratio of their aperture to the wavelength they observe, explained Michael Thornburn, head of the ALMA department of engineering. ALMA is an aperture synthesis telescope.
“We cannot make a single aperture 15 kilometers across, so we do it in pieces,” he said. “The signals from individual dishes are combined to build up the image from a single large aperture.”
Radio signals from distant cosmic sources arrive at each dish at ever-so-slightly different times, and these are combined with the signals from every other antenna. This technique, interferometry, allows ALMA to operate like a single huge dish with an adaptable radius.
In a carefully choreographed ballet, each dish moves in unison with the others to change the telescope’s observing area. Along with moving in place, giant transporter trucks, specially designed for the dishes, can pick them up and cart them across the Chajnantor Plateau to one of 192 concrete pads. At their greatest distance apart–16 kilometers–ALMA’s angular resolution will be equivalent to the Hubble Space Telescope, Peck said.
ALMA is observing sources that are 10 times weaker than those observed with other arrays, explained Pierre Cox, ALMA’s incoming director. This is key to ALMA’s capability for observing phenomena like star formation, he said.
“Future observations should allow us to detect dark matter substructure and shed light on its nature,” he added.
There’s much more to learn about how ALMA works, and why astronomers are so excited about it–stay tuned for more dispatches from the Atacama.