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Introduction

The Atacama Large Millimeter Array (ALMA) is an international project to construct an interferometer, operating between wavelengths of 300 microns and 1 centimeter at an altitude of 5 kilometers at an exceptionally dry site in Northern Chile. The project is currently a partnership between North America through the National Radio Astronomy Observatory (NRAO), Europe through the European Southern Observatory (ESO), Japan through the National Astronomical Observatory of Japan (NAOJ), and Chile. ALMA was initially planned by North America and Europe as an array of 64 12-meter antennas; Japan’s participation brought two additional receiver bands to the instrument along with a compact array comprising 4 separate 12-meter antennas and 12 7-meter antennas. The ALMA array is designed to have an angular resolution of 6 milliarcseconds at its shortest operating wavelength and a sensitivity that will enable fundamental investigations of the origin and evolution of planets, stars, and galaxies—investigations that are not possible with other instruments observing in other areas of the spectrum.

The behavior of the troposphere is a major factor in the quality of astronomical observations in the millimeter-wave region of the spectrum, because it can change both the amplitude and the phase of a celestial signal. The ALMA site, in the Atacama desert,



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The Atacama Large Millimeter Array: Implications of a Potential Descope 1 Introduction The Atacama Large Millimeter Array (ALMA) is an international project to construct an interferometer, operating between wavelengths of 300 microns and 1 centimeter at an altitude of 5 kilometers at an exceptionally dry site in Northern Chile. The project is currently a partnership between North America through the National Radio Astronomy Observatory (NRAO), Europe through the European Southern Observatory (ESO), Japan through the National Astronomical Observatory of Japan (NAOJ), and Chile. ALMA was initially planned by North America and Europe as an array of 64 12-meter antennas; Japan’s participation brought two additional receiver bands to the instrument along with a compact array comprising 4 separate 12-meter antennas and 12 7-meter antennas. The ALMA array is designed to have an angular resolution of 6 milliarcseconds at its shortest operating wavelength and a sensitivity that will enable fundamental investigations of the origin and evolution of planets, stars, and galaxies—investigations that are not possible with other instruments observing in other areas of the spectrum. The behavior of the troposphere is a major factor in the quality of astronomical observations in the millimeter-wave region of the spectrum, because it can change both the amplitude and the phase of a celestial signal. The ALMA site, in the Atacama desert,

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The Atacama Large Millimeter Array: Implications of a Potential Descope is thought to be the best accessible site in the world for millimeter and submillimeter astronomy. It is likely to attract other facilities. The NRAO-ESO agreement was for a $650 million budget, with construction shared between the two continents. Following recent increases in commodity costs, it has become necessary to contemplate a descope of the project. An investigation by the project and the ALMA Science Advisory Committee (ASAC) found that the only descope option that could lead to significant savings without catastrophic loss of scientific capability was a reduction in the number of antennas. The committee has read the ASAC reports dated September 2004 and March 2005, the relevant chapters of the ALMA Project Book, the ESO document “Science with ALMA,” and the 1999 Report of the Antenna Size Committee. It has also listened to presentations by Wayne van Citters on behalf of the National Science Foundation, Ewine van Dishoeck, representing ESO, and Jean Turner, ASAC chair. Mark Holdaway of NRAO kindly explained aspects of the ALMA imaging philosophy. As the premier instrument in the world for the exploration of planets, stars, galaxies, and the unknown for several decades, ALMA will be a long-term investment for frontier research by the U.S. astronomy community. One salient feature of the ALMA project is the array’s planned accessibility to the broader U.S. astronomy community. Previous radio and millimeter arrays have required users to be highly proficient in interferometric techniques. In contrast, ALMA is designed to be an observatory accessible to astronomers who normally observe at other wavelengths. ALMA’s unprecedented number of antennas is the key driver for this accessibility, and the potential descope will significantly curtail the number of astronomers who will be able to use the observatory. Observatory facilities have a long active life. The Very Large Array (VLA) is 24 years old and will likely operate for twice as long. ALMA is currently expected to have a 30- to 50-year productive lifetime. From this perspective, the incremental cost of com-

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The Atacama Large Millimeter Array: Implications of a Potential Descope pleting the array is a small fraction of the observatory’s lifetime cost. If the operating costs are estimated to be 10 percent per year, then reductions by 10 or 20 in the number of antennas are roughly equivalent to only 1 or 2 years of operation, i.e., roughly 2.5 percent or 5 percent of the total project cost. In addition, smaller arrays would take smaller quantities of data during ALMA’s lifetime and would be far less user friendly when high-fidelity images are required. The committee concludes that a 60-element array would be greatly superior to any current or planned comparable instrument for several decades and would revolutionize millimeter and submillimeter astronomy.

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