Millimeter-Wave Frequencies are sometimes associated with military missile guidance systems and commercial automotive radars. They are also essential to radio astronomy, including the world’s most advanced millimeter/submillimeter wavelength radio telescope, the Atacama Large Millimeter/submillimeter Array (ALMA). The giant array is under construction high in the Chilean Andes Mountains as part of an international project to design, build, and operate a multi-element interferometer operating in 10 different frequency bands between 31.3 and 950 GHz. ALMA will push the forefront of receiver and antenna technology to allow unprecedented views of the universe with resolution rivaling that of the Hubble Space Telescope.

Although other telescopes around the world are already operating in this spectral region, ALMA will provide a combination of frequency agility, resolution, and sensitivity that is substantially better than any current instrument. When completed, ALMA will be comprised of 66 individual antennas operating together as an interferometer (Fig. 1). The signals from the 66 dishes will be electrically combined by a state-of-the-art digital correlator, providing imaging capabilities that will allow the array to produce pictures of the millimeter-wave/submillimeter-wave radio sky with a resolution better than 10 milliarcseconds (1/360,000 of a degree), or about ten times better than the Hubble Space Telescope. This level of resolution is equivalent to resolving the individual letters of this article at a distance of about 35 km.

The basic characteristics of the ALMA telescope are:

• Fifty 12-m main antennas, plus a compact array of four 12-m and twelve 7-m antennas.
• Relocatable antennas providing maximum baseline coverage between 150 m and 14 km.
• Frequency coverage spread across 10 noncontiguous bands throughout the 31.3-to-950-GHz region.
• Angular resolution better than 10 milliarcseconds.
• The ability to map extended structures between several arcminutes to several degrees in extent (depending on frequency and array configuration).

A number of technological challenges are being met to make ALMA a reality. Atmospheric attenuation is a serious consideration at millimeter and submillimeter wavelengths, and the site location is critical. At such high frequencies, antenna surfaces must be very smooth to provide good efficiency, and the antennas themselves must have excellent pointing accuracy. The antennas must not only be accurate, but movable too, since the array will be reconfigured at regular intervals to provide variable-resolution or “zoom” capability. Receivers must be extremely sensitive, stable, and easy to reproduce for each of the 66 separate antennas. And the remote location of the telescope site dictates that the components be low maintenance.

The ALMA project is truly international. It is a partnership between Europe, Japan, and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Southern Observatory (ESO) and Spain, in Japan by the National Institutes of Natural Sciences (NINS) in cooperation with the Academia Sinica in Taiwan, and in North America by the United States National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC). ALMA construction and operations are led on behalf of Europe by ESO, on behalf of Japan by the National Astronomical Observatory of Japan (NAOJ), and on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI). The North American collaborators and ESO are the majority partners in the project, which is expected to cost approximately $1.2 billion when completed.

SEARCHING FOR A SITE
At millimeter and submillimeter wavelengths, the atmosphere becomes a serious impediment to signal transmission. Resonant absorption by oxygen and water molecules causes substantial frequency-dependent attenuation that can reach 100 dB/km or more at sea level sites at the highest frequencies used by ALMA. Between resonant frequencies, certain atmospheric “windows” allow better signal transmission, but attenuation is still considerably greater than in centimeter-wavelength bands.

Because the attenuation depends on both the density of the atmosphere and the amount of water vapor present, better propagation conditions can be obtained at high, dry sites—the higher and drier the better. For this reason, ALMA is being built in northern Chile, at a very high site in one of the driest places on Earth (Fig. 2). Specifically, the ALMA site is at 5059 m elevation on the Llano de Chajnantor in the District of San Pedro de Atacama, in the Andean Antiplano of northern Chile (south latitude 23°01’, west longitude 67°45’).

At the elevation of the ALMA site, the atmospheric density is about 50 percent of its sea-level value. The high desert atmosphere at the ALMA site contains typically approximately 1 to 2 mm of precipitable water vapor, which is about 20 to 30 times less than typical sea-level sites. The combination of lower atmospheric density and less water vapor provides significantly lower attenuation at millimeter and submillimeter wavelengths, allowing the observation of faint radio sources without being overwhelmed by thermal noise added by the Earth’s own atmosphere. The atmosphere above the ALMA site has also been shown to provide good phase stability, meaning that the blurring of radio images caused by atmospheric turbulence will be manageable.

The main interferometer array will be comprised of 50 individual parabolic dishes, each 12 m diameter (Fig. 3). Onehalf of the antennas are being procured by the European partners of ALMA, which awarded a 147 million Euro (approximately $190 million US) contract in December 2005 to a consortium led by Alcatel Alenia Space, including Italy’s European Industrial Engineering and Germany’s MT Aerospace. The North American partner awarded a $169 million contract to General Dynamics C4 Systems in July 2005 to build the other 25 antennas, which are being built under General Dynamics’ VertexRSI brand (Fig. 4).

Due to the high frequency of operation, the antenna requirements are quite stringent. Each antenna must maintain a root-mean-square (RMS) surface accuracy no worse than 25 µm, which is about one-twelfth the wavelength at ALMA’s highest operating frequency. The antennas must have an absolute pointing accuracy of 2 arcseconds (0.0006 deg) anywhere across the sky, and an offset pointing accuracy better than 0.6 arcsecond (0.0002 deg). The offset pointing accuracy is important since the antennas will be rapidly slewed on-source and off-source to help calibrate atmospheric effects. Pointing errors also cause reduced sensitivity to angularly compact sources.

Japan will be providing an additional four 12-m antennas and one dozen 7m antennas. The 16 additional antennas will be situated in a compact subarray that, when combined with the 50 main antennas, will provide better sensitivity of the entire ALMA telescope to angularly extended radio emissions. Mitsubishi Electric Co. (MELCO) will build the Japanese antennas.