J ALMAThe Atacama Large Mil lime te rSubm il timet er Array

In the quest to better understand the nature of molecular clouds and what drives the chemistry of the universe, an array of 64 dishes, each 12 m in diameter capable of operating from 30 to 1,950 MHz (wavelength range from 9.6 to 0.3 mm) is to be constructed at the heady altitude of 5,100 m (16,500 ft) on the Chajnan-tor plain of the Chilean Andes in the District of San Pedro de Atacama. Figure 15.4 shows an artist's impression of what it will look like. At that altitude there is little water vapor in the atmosphere overhead, which would otherwise absorb millimeter radio waves, and there is also much less oxygen to breathe. The telescopes will be spread over 15 km of the plains. ALMA has its own science center far away at a low altitude, at the NRAO headquarters in Charlottesvile, VA. Very few technical people will be on site, given that the array will be remotely controlled.

ALMA will cost something like S60 million by the time it comes into full operation in 2012. Already 7 years of funding by the US National Science Foundation has allowed the science center to be built, people to be hired, and telescopes and receivers to be designed and constructed. Any project of this scope is only possible with international cooperation and here we find the NRAO and the US National Science Foundation NSF as well as the National Research Council of

FIGURE 15.4. Artist's conception of ihe antennas planned for the ALMA. (Image courtesy of NRAO/AUI and ESO.)

Canada, working with the European Southern Observatory (ESO), Spain, and the National institutes of Natural Science in Japan, and. of course Chile, which gets 10% of the observing time. The Japanese collaborators are contributing 25% of the cost ($80 million). In July 2005 a contract for the first 32 dishes was signed.

One of the primary research projects on ALMA will be to study molecule formation in galaxies that existed when the universe was still young. It will also look into the heart of radio galaxies and quasars in the era when galaxies were just being formed for the first time. Much, much closer to home, it should be able to image comets and asteroids with amazing clarity and study objects in an orbit about the sun beyond the orbit of Neptune and Pluto, the so-called iGper Belt objects. It should even be able to detect radio emission form the surface of distant "normal" stars and observe planetary formation processes in detail.

Initial science measurements should begin in 2007 with the full array expected to be operational in 2012.

A prototype of the radio dishes that will make up ALMA has been brought into operation {Figure 15.5). Kbwn as APEJGhe Atacama Pathfinder Experiment, it began operation in July of 2005 and is operated independently of ALMA by several European scientific organizations including the German Max-Planck-Society, the Swedish Research Council and the European Southern Observatory. It uses a modified ALMA antenna and is located at the ALMA site. In order to work effectively and efficiently at the submillimeter wavelength range these 12-m dishes have to be smooth to better than 17 thousandths of a millimeter, or. in the words of

The APEX Telescope at Chajnantor

FIGURE 15.5. APEXelescope. a prototype reflector for ALMA, now in operation at the ALMA site in Chile. Clearly the desert terrain at an altitude of over 15,000 ft does not look inviting as a place where visiting radio astronomers can relax between observing runs. The telescope is therefore remotely controlled as will be the case for the larger ALMA at the same site. (Image credit: ESO.)

an APE^ress release, less than one fifth of the thickness of a human hair. That gives the dish its shiny appearance.

15.4.2. LOFAR—Low Frequency A rray

This radio astronomical tool represents a classical case of thinking outside the box. or in this case outside the dish. Instead of moving cumbersome masses of metal to point at various directions in the sky. a practical alternative at low frequencies is to use simple antennas that individually have no resolving power. All their signals will be collected in a central computer and by adding various time delays to the electrical paths from elements of this array it is possible to process the data so as to simulate a beam in the sky whose width will be determined by the extent of the array. The beam can also be pointed electronically. It is a very clever idea made even more innovative by having individual antennas consisting of little more than four supports in the shape of a pyramid planted in the ground holding up simple dipole antennas that make use of the radio reflecting properties of the ground to aid in the efficiency of the resulting array. The maximum height of each antenna support will be 2 m (about 6 ft). Initial funding will see the construction of 15,000 of these simple, cheap antennas in the Netherlands spread in cluster of spread over 100 km. The prototype with 100 clusters of antennas should be btiill in 2006-2008. It will then be expanded to across the German border to encompass a total distance of 350 km with 25,000 relatively unobtrusive structures. This enormous radio telescope will have no moving parts.

LOFAR has another unique feature, In addition for radio astronomy studies of the early universe at 10. 100, 150, and 200 MHz (wavelengths of 30,3,2, and 1.5 m, respectively), it will be used for studies of crop growth and seismic shifts. The cluster of antennas will be equipped with biosensors and weather stations related to studies of crop growth over an enormous area, given that the radio astronomers are already linking up all those antennas electronically. Why not add some additional, useful, information w hile you are at it? Similarly, by adding geophones and seismic sensors, data relevant to what's going on under the Dutch and German countryside, believed to be sinking with the removal of natural gas. will also be obtained.

A consortium of European observatories is involved in making this project a reality with the Astronomical Institute in Dwingeloo, the Netherlands, the driving force and with collaboration from groups in Germany, Sweden, France, and the United ifigdom. It is expected to be fully operational by 2015. Given that LOFAR is extending both sensitivity and resolution by 100 to 1.000 times in a largely unexplored frequency range we can expect surprises once it becomes fully operational The history of astronomy has shown over and over again that totally new discoveries follow on the heels of opening a new part of the radio band, after which the discovery rate drops and the study of details takes over.

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