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.
The receivers, which detect and amplify the extremely weak cosmic emissions picked up by the ALMA antennas, are key components in the array. The 31.3-to-950-GHz frequency range has been broken up into 10 separate noncontiguous bands that take advantage of the “windows” of lower atmospheric attenuation. These bands are (1) 31.3 to 45 GHz, (2) 67 to 90 GHz, (3) 84 to 116 GHz, (4) 125 to 163 GHz, (5) 163 to 211 GHz, (6) 211 to 275 GHz, (7) 275 to 373 GHz, (8) 385 to 500 GHz, (9) 602 to 720 GHz, and (10) 787 to 950 GHz. Each receiver will provide as much as 16 GHz total bandwidth distributed across two sidebands and two polarizations.
The receivers are being built as modular “cartridges” that will be simpler to maintain and replace. The receivers will use super-conducting tunnel junction mixers operating at 4 K to achieve the requisite low-noise performance. The end-to-end RMS flux density sensitivity goals of the telescope range from –304 dB(W/m2/Hz) at 110 GHz to –291 dB(W/m2/Hz) at 675 GHz, assuming that the noise fluctuations in the receiver are averaged for 60 seconds (most astronomical observations use time averaging to reduce noise fluctuations). To make rough comparisons with terrestrial receiver systems, these sensitivities translate into approximately –210 dBm into an isotropic receiving antenna assuming a receiver bandwidth of 8 GHz.
Requirements for radio astronomy receiver systems are stringent due to the extremely low signal levels involved. All ALMA frequency band receiver cartridges are being custom-designed and built by ALMA partners. Band 3 (84 to 116 GHz) receivers, under construction at the Herzberg Institute for Astrophysics in Canada, will be among the first installed in the ALMA antennas.
While there are relatively few terrestrial transmitting systems operating in the millimeter-wave/submillimeterwave bands, especially in the remote geographic region in which ALMA operates, forethought has been given to protecting the spectrum environment. Resolution 1055 of the Telecommunications Secretariat of Chile’s Ministry of Transport and Telecommunications was adopted in September 2004. The Resolution establishes a 30-km protection zone around the ALMA site in which no transmitting stations will be authorized to operate within any of the 10 receiver bands used by ALMA. Resolution 1055 further establishes a 120-km coordination zone surrounding the ALMA site in which any operator seeking a license to transmit within the ALMA bands must coordinate their use with ESO and AUI before authority to transmit is granted.
Unfortunately, the biggest threat to radio astronomy observatories is from airborne and satellite transmitters, which are not covered by Resolution 1055. Several satellite systems use frequencies in the millimeter-wave spectrum, and will remain an interference threat to ALMA operations.
The ultimate goal of the ALMA project is to produce new world-class science. The telescope will observe the very faint natural radio emissions from a large variety of sources. ALMA will probe this region of the electromagnetic spectrum with unprecedented sensitivity and resolution. Among ALMA’s targets will be the infrared emission from very distant galaxies. The original emission left these distant galaxies very long ago, near the earliest stages of the formation of the universe. These distant galaxies are receding from Earth at such great speed that their infrared emission has been red-shifted down to millimeter-wave and submillimeter-wave radio emission. The dust emission will tell us a great deal about the conditions within early galaxies leading to the formation of the earliest stars.
Observations of the Milky Way galaxy and other nearby galaxies will provide new information about the molecular conditions within regions of star formation, yielding new clues as to how stars are made. ALMA will be an excellent spectroscopic instrument that will help us understand the role of chemical and isotopic gradients in the formation of the familiar spiral structure that characterizes many galaxies.
Within our own Solar System, ALMA will provide detailed images of asteroids and the nuclei of comets. The telescope will image active regions of the sun and investigate the physics of particle acceleration on the sun’s surface. These same particles often have significant impact on ionospheric and atmospheric weather on Earth. Perhaps the most exciting science offerings from ALMA are the unforeseen discoveries that will be made using its unprecedented resolving power and sensitivity.
Construction of the production antennas and delivery to the ALMA site is expected to begin near the end of 2006. Development, testing, and construction of the receiver cartridges for various ALMA bands are ongoing. Early science operations from a partially completed array are expected in 2010. Full operation of the world’s most advanced millimeter-wave/submillimeter-wave telescope is expected in 2013. After construction is complete, science operations of the array will be headquartered at three regional science centers, one each in North America, Europe, and Japan.
Having passed all major technological hurdles over more than 20 years of design and development, astronomers are now keen to access the tremendous science potential of ALMA. For more information on the ALMA project, visit the dedicated website at www.alma.info