Atacama Large Millimeter Array

Atacama Large Millimeter/submillimeter Array
The ALMA logo
Organization	Multinational
Location	Llano de Chajnantor Observatory Atacama Desert, Chile
Coordinates	23°01′9.42″S 67°45′11.44″W / 23.0192833°S 67.7531778°W / -23.0192833; -67.7531778
Altitude	5,058.7 m
Weather	365 clear nights/year
Telescope style	50 to 64 identical 12 m reflectors connected by fibre-optic cables
Website	Official NRAO ALMA site Official UK site Official ESO site Official NAOJ site

The Atacama Large Millimeter/submillimeter Array (ALMA) is an international astronomy project that consists of an astronomical interferometer formed from an array of radio telescopes, located at Llano de Chajnantor Observatory in the Atacama desert in northern Chile. The telescope is expected to revolutionize modern astronomy by providing an insight on star formation in the early universe and imaging local star and planet formation in great detail. With a cost in excess of one billion US dollars, it is the most ambitious ground-based telescope currently under construction.

Overview

The first two ALMA antennas linked together as an interferometer.

The telescopes and their receivers are capable of detecting sub-millimeter and millimeter wavelengths. The array will have much higher sensitivity and higher resolution than existing sub-millimeter telescopes such as the single-dish James Clerk Maxwell Telescope or existing interferometer networks such as the Submillimeter Array or the IRAM Plateau de Bure facility.

By moving the antennas at regular intervals, the resolution of the array and the size of object that can be imaged will be altered, producing a "zoom-lens" capability, similar to that employed at the VLA site in New Mexico.

The high sensitivity is mainly achieved through the large numbers of telescopes that will make up the array. While sixty-four 12 metres (39 ft) dishes were originally envisaged, it is now possible that there will ultimately only be fifty built.

The American and European partners have each placed orders for twenty-five antennas, with options for an additional seven. Japan is also contributing antennas in the form of the Atacama Compact Array (ACA) which will also be located at the ALMA site.

By using smaller antennas than ALMA (twelve 7 m and four 12 m dishes are planned) larger fields of view can be imaged at a given frequency. The ability to move them closer together also results in the possibility to image sources of larger angular extent. The ACA will work together with ALMA in order to enhance the latter's wide-field imaging capability.

Controversy

ALMA prototype-antennas at the ALMA test facility.

The moon high above Cerro Chajnantor at sunset.

The original plan was for the project to employ a single antenna design^[1]. The intent of testing the three prototype antennas was to compete the designs to down-select (choose the best). Three factors conspired to alter the plan:

• The size of the antenna production contract is substantial.
• It was politically expedient (or necessary) for each of the major partners to realize some non-scientific, economic return on their investment (see realpolitik).
• No arrangement was found to break up the antenna production into parts that could be parceled to the partners.

The evaluation of the prototypes yielded each partner finding technical justification to favor their own prototype. Since there was no clear winner, the plan was changed to allow different antenna designs^[2]. Each partner is now funding production of a portion of the antennas via associated contractors. This is a workable solution which allows the project to complete without any significant descope (erosion) of the science capabilities. The disadvantages with this solution are:

• The complexity of combining the electronic signals from different antenna designs is significantly increased.
• The overall costs of funding multiple contractors with smaller contracts increases (see Economies of Scale).
• The additional overhead of integrating different antenna designs and managing multiple contractors likely results in an overall schedule delay.

A positive result of the new plan is some reduction of risks. Because some productive science can be accomplished with partial completion of the array, both technical and schedule risks are reduced.

Funding

ALMA was initially a 50-50 collaboration between the European Southern Observatory (ESO) and the North American partners. The array has been extended with the help of the new Japanese, Taiwanese, Spanish and Chilean partners.^[3] ALMA is the largest and most expensive ground-based astronomical project currently under construction (current cost estimate is US$1.3 billion).

Partners

• European Southern Observatory and the European Regional Support Center
• Spanish Ministry of Science and Technology
• National Science Foundation via the National Radio Astronomy Observatory and the North American ALMA Science Center
• National Research Council of Canada
• National Astronomical Observatory of Japan (NAOJ) under the National Institutes of Natural Sciencs (NINS)
• ALMA-Taiwan at the Academia Sinica Institute of Astronomy & Astrophysics (ASIAA)
• Republic of Chile

Assembly

One of the antennas under construction.

The complex will be built primarily by European, U.S., Japanese and Canadian companies (including General Dynamics) and universities. Three prototype antennas have undergone evaluation at the Very Large Array site in New Mexico since 2002.

General Dynamics C4 Systems signed a contract with Associated Universities, Inc. to provide twenty-five to thirty-two 12m antennas.^[4] Alcatel Alenia Space, a consortium of manufacturers from France, Italy, and Germany, has been signed up to provide twenty-five of the antennas,^[5] the largest-ever European industrial contract. The first antenna will be delivered in 2007, and the rest at about one per month, finishing in 2011.

Progress at the ALMA Site - a panoramic view.

Transporting antennas to the site

A view across the plains of Chajnantor with the ALMA construction site at the centre.

Transporting the 115 tonne antennas from the base camp at 2900 m altitude to the site at 5000 m presents enormous problems. The solution chosen is to use two custom 28-wheel self-loading heavy haulers. The vehicles are made by Scheuerle Fahrzeugfabrik in Germany and each is 10 m wide, 20 m long and 6 m high, weighing 130 tonnes. They are powered by twin 500 kW diesel engines.

The transporters, which feature a driver's seat designed to accommodate an oxygen tank to aid breathing the thin high-altitude air, can pick up the antennas and place them precisely at the site. The first vehicle was completed and tested in July 2007.^[6] Both transporters were delivered to the ALMA Operations Support Facility (OSF) in Chile on February 15, 2008.

On July 7, 2008 an ALMA transporter for the first time moved an antenna, moving it from inside the antenna assembly building (Site Erection Facility) to a pad outside the building for testing (holographic surface measurements). The antenna was of the North American VertexRSI design.^[7]

Antenna Transport

The 130-ton ALMA antenna transporter "Otto" during its naming ceremony.			Image of telescope in transit at the Site Erection Facility.
	A worker inspects a transporter.			Moonrise above the 12 metre wide access road to the ALMA High Site.

General information

ALMA construction and operations are led on behalf of North America by the National Radio Astronomy Observatory (NRAO). NRAO is managed by Associated Universities, Inc (AUI). ALMA construction and operations are led on behalf of Europe by ESO and Japan by the National Astronomical Observatory of Japan (NAOJ). Administration of the ALMA site in Chile is lead by ESO.

Project detail

A starry night at the ALMA site.

• 50 to 64 antennas of 12 m diameter located at an elevation of 5,000 m at Llano de Chajnantor Observatory, enhanced by a compact array of 4 x 12 m and 12 x 7 m antennas (consortium currently considering to build 50 or 64[1][2])
• Imaging instrument in all atmospheric windows between 350 μm and 10 mm
• Array configurations from approximately 150 m to 14 km
• Spatial resolution of 10 milliarcseconds, 10 times better than the Very Large Array (VLA) and the Hubble Space Telescope
• The ability to image sources arcminutes to degrees across at one arcsecond resolution
• Velocity resolution under 50 m/s
• Faster and more flexible imaging instrument than the Very Large Array
• Largest and most sensitive instrument in the world at millimeter and submillimeter wavelengths
• Point source detection sensitivity 20 times better than the Very Large Array

Project timeline

See also

Looking over to the ALMA site from APEX.

• Atacama Pathfinder Experiment (APEX), single dish sub-millimeter telescope built on a modified ALMA prototype antenna
• Atacama Submillimeter Telescope Experiment
• CARMA a sensitive millimeter-wave array operated by a consortium including Caltech, University of California Berkeley, University of Illinois, University of Maryland and University of Chicago
• Cosmic Background Imager a 13 element interferometer operating in Llano de Chajnantor since 1999.
• IRAM 30 Meter Telescope (Pico Veleta, Spain), the largest millimeter telescope in the world, operated by IRAM
• James Clerk Maxwell Telescope The most sensitive existing sub-millimeter telescope
• Plateau de Bure Interferometer, one of the most successful existing millimeter-wave arrays, operated by IRAM
• List of observatories

References

External links

• Official site
• Official Chilean ALMA site
  • Webcam showing array construction work at the Operations Support Facility further down the mountain
• NRAO ALMA site
• UK ALMA site
• ESO ALMA site
• 'Alma' sets new heights for astronomy, BBC News, 7 August 2006.
• Huge Observatory in Andes Takes Shape, Space.com, 7 February 2009.
• Podcast Interview with the European ALMA Instrument Scientist Robert Laing