|Origins of the ALMA Project|
Based on a text by Paul A. Vanden Bout, Nov. 2004
ALMA is a worldwide project; the synthesis of early visions of astronomers in its three partner communities: Europe, North America and Japan.
ALMA is a fusion of ideas, with its roots in three astronomical projects: the Millimeter Array (MMA) of the United States, the Large Southern Array (LSA) of Europe, and the Large Millimeter Array (LMA) of Japan.
The origins of the Millimeter Array (MMA) are found in the pioneering science of the NRAO 36-Foot Telescope (later known as the 12-Meter Telescope), soon followed by the 4.9m telescopes at the University of Texas and Aerospace Corporation, the 14m telescope at the Five Colleges Radio Astronomical Observatory, and the 7m telescope at AT&T Bell Labs. The millimeter interferometers of the University of California (Berkeley) at the Hat Creek Observatory (later the Berkeley-Maryland- Illinois Association, or BIMA) and the California Institute of Technology at the Owens Valley Radio Observatory demonstrated the power that comes with high angular resolution for studying the sources found with the single dishes. The experience of using a powerful, flexible array that was provided by NRAO’s Very Large Array (VLA) at longer wavelengths was also very influential. Indeed, the prime characteristic of the MMA was the ability to obtain rapid high-quality images at 230GHz, that is, the MMA was to be a millimeter version of the VLA. The science targets of the MMA included the same broad range of topics seen at the VLA: (NSF) in July, 1990, called for an array of 40 antennas of 8-meter diameter, with four receiver bands covering the atmospheric windows from 30–350 GHz, configurable in four arrays of size 70–3000 m. The proposal discussed two possible sites for the MMA, both in the southwestern United States. Studies of the atmospheric transparency and phase stability at these sites led to similar studies on Mauna Kea, in Hawaii. Extensive atmospheric monitoring was also conducted there.
Concerns with the limited size of the area available to the MMA on Mauna Kea and with potential environmental problems prompted a search for potential sites in Chile. From April 1994, many highelevation sites were visited. Finally, the site retained for the MMA was formally proposed in 1996: it was the Chajnantor plateau.
Large Southern Array
As in the United States, a broad science program in millimeter wavelength astronomy had developed in Europe, centered around the two telescopes of IRAM, a 30m single dish and an interferometer of three (now six) 15m antennas, the 14m telescope of the Onsala Space Observatory (OSO), and the submillimeter capability of the 15m James Clerk Maxwell Telescope. The group at the University of Bordeaux were pioneers in millimeter wavelength interferometry. The first concept for a millimeter interferometer in the Southern Hemisphere came in the late 1980’s out of OSO following the success of the Swedish-ESO Submillimeter Telescope (SEST), and called for an array of 10 antennas of diameter 8m to be located near ESO’s Very Large Telescope on Cerro Paranal in northern Chile. An array in the Southern Hemisphere became the hallmark of the European array for scientific reasons (the Galactic Center, Magellanic Clouds, etc.) and because ESO, the natural organization for a European astronomical project, had its telescopes there.
A collecting area of at least 10 times that of the IRAM Interferometer on the Plateau de Bure, was incorporated in the thinking for the Large Southern Array (LSA). It’s concept proposal (1995) called for a 10000 m2 collecting area provided by 50 antennas of 16m diameter or 100 antennas of 11m diameter. The LSA was to work at frequencies of 350 GHz and below and be equipped with state-of-the-art receivers and signal correlator. To obtain angular resolution of 0.1'' at a wavelength of 2.6 mm, configurations of size ~10km were contemplated. Because its highest operating frequency was 350 GHz, the LSA did not require a site as high as that picked for the MMA, and sites at lower elevations of 3300m and 3750m were studied.
Large Millimeter Submillimeter Array
|The Nobeyama Radio Observatory. Credit: NRO|
|Site testing instruments and containers at the 4800 m site of Pampa la Bola, Region II, Chile. Two containers equipped with solar panels and one of the element antennas of the radio seeing monitor can be seen in the background. Three weather stations under cross calibration are seen in the foreground. Credit: NAOJ|
In Japan, plans for a large millimeter wavelength array grew naturally out of a desire to expand the Nobeyama Millimeter Array. The Large Millimeter Array (LMA) was discussed in 1983, just following the dedication of the NRO, and in its first form expanded the five 10m diameter antennas of the NRO interferometer to 30, working to a maximum frequency of 230 GHz on baselines up to 1 km. It was decided in 1987 to expand the concept to 50 antennas of diameter 10 m working at frequencies of 35–500 GHz with the possibility of going to submillimeter frequencies, in configurations of size 20–2000 m. Japanese university groups established a small, automated submillimeter telescope on Mt. Fuji and a small millimeter telescope in Chile. The site of the NRO precluded observations in the submillimeter, and sites on Mauna Kea and in North Africa and Chile were considered as possibilities. Serious site studies in Chile began in 1992 with a survey of 20 possibilities. As the quality of the sites in Chile became apparent, consideration of Mauna Kea was dropped and the prospects of observing in the submillimeter band became the focus of the LMA program, leading to the change in project name to the Large Millimeter/ Submillimeter Array (LMSA). In 1995 a memorandum of understanding between the NAOJ and NRAO was signed whereby the two groups agreed to work cooperatively on site studies. Sites at Pampa la Bola and Rio Frio received intensive study, with Pampa la Bola to the north-east of Llano de Chajnantor site becoming the site of choice in 1997; the importance of 10 km baselines to a combined MMA+LMSA had become clear in a workshop held in Tokyo on submillimeter astronomy at 10 milliarcseconds resolution. The Pampa la Bola site showed excellent phase stability and is now the location for a long arm of ALMA antenna pads that stretches northeast of the Llano de Chajnantor. Protoplanetary disks and high-z galaxies were considered to be the main scientific targets of the project. The frequency range of the LMSA was expanded to include 650 and 900 GHz bands in the submilllimeter.
|This image is an artist rendering of the ALMA Array, in it's extended configuration. In this configuration, the distance between the 2 farthest antennas is 16 kms. Credit: ALMA (ESO/NAOJ/NRAO)|
The ALMA main array has a large collecting area of 5650 m2 provided by 50 antennas of 12-m diameter. ALMA is very flexible. Its antennas can be placed in configurations with sizes from 150 m. to 16 km, providing a range of angular resolution of nearly a factor of 1000 at fixed observing frequency.
It has the potential to cover all ten frequency bands from 30–950 GHz where the earth’s atmosphere is reasonably transparent, with an initial set of receivers that covers the four of these bands. It has a powerful and flexible signal correlator that can process 2016 baselines with 16 GHz of bandwidth per antenna. In addition, it has been agreed that ALMA include the Atacama Compact Array (ACA), an array of 12 antennas of 7-m diameter (with 4 additional 12- m diameter antennas for single dish observations and calibration purposes), equipped with the same receivers as the large array, and equipped with its own signal correlator of similar power as the large array, as well as three additional receiver bands for all antennas.
|This image is an artist rendering of the ALMA Array, in it's compact configuration. In this configuration, all the antennas fit in a 250-meter diameter circle. Credit: ALMA (ESO/NAOJ/NRAO)|
The ACA provides data on spatial frequencies between the compact configuration of the large array and a single 12 m antenna. ALMA with the ACA and additional receiver bands is known as "Enhanced ALMA". These specifications are those necessary to meet scientific requirements. They also reflect the union of visions for the three earlier conceptual projects now realized in ALMA.
Finally, few of the participants in the MMA, LSA, and LMSA projects were entirely comfortable with the prospect of building three separate large millimeter/ submillimeter arrays in Chile.
The major breakthrough occured with the signing of a resolution between ESO and NRAO on June 26, 1997, whereby the two parties agreed to pursue a common project that merged the MMA and LSA into what would eventually be named ALMA. The merged array combined the sensitivity of the LSA with the frequency coverage and superior site of the MMA. The merger was made official in June 1999 with the signing of the Phase 1 ALMA Agreement. ESO and NRAO worked together in technical, science, and management groups to define and organize a joint project between the two observatories with participation by Canada and Spain (which didn’t belong to ESO at that time).
A flurry of resolutions and agreements ensued, including the choice of “Atacama Large Millimeter Array”, or ALMA, for the name of the new array in March of 1999. This effort culminated in the signing of the ALMA Agreement on February 25, 2003, between the North American and European parties.
This effort culminated in the signing of the ALMA Agreement on February 25, 2003, between the North American and European parties.
Following mutual discussions over several years, the ALMA Project received a proposal from the NAOJ whereby Japan would provide the ACA and three additional receiver bands for the large array, to form Enhanced ALMA. Further discussions between ALMA and the NAOJ led to the signing of a high-level agreement on September 14, 2004, that makes Japan an official participant in Enhanced ALMA.
Three decrees issued by the Republic of Chile and the signing of two contracts marked the completion of the last stage in the long process to ensure use of the ALMA site in the long term -50 years. The outcome of this process means: AUI is established in Chile with the same rights and privileges as ESO; AUI is awarded exploration mining rights for the site; the Chilean Government establishes a scientific reserve including the site; ESO is awarded a permit to create a new observation site (APEX); approval of an environmental impact study; land is purchased to set up operations at a medium elevation (OSF); concession and easement agreements for the site and access road; approval of an area reserved for coordination around the site to protect against radiofrequency interference; an agreement with the regional government to support cultural, educational and production activities; an agreement with CONICYT giving Chilean astronomers observation time at ALMA; and fostering astronomy in Chile.
More than 14 government agencies in Chile were involved in the negotiations.
Assuming all three partners are able to meet their commitments, it was decided that the final project would be cost-shared 37.5% / 37.5% / 25% between North America, Europe, and Japan, respectively. The observing time, after a 10% share for Chile, would be shared accordingly. WhenEnhanced ALMA, to be known as the Atacama Large Millimeter/submillimeter Array, with the same acronym ALMA, would come into full operation it would truly be a world millimeter/submillimeter array.