Atacama Large Millimeter Array
March 27, 2013
The deserts of western South America contain some unusual archaeological artifacts. These are huge geoglyphs, which are drawings either scratched into the desert sands (a negative geoglyph), or built up from rocks (a positive geoglyph).
These became a topic of popular culture in the late 1960s with publication of the book, "Chariots of the Gods?," by Erich von Däniken (Putnam, 1968). The theme of this book is that there are historical and religious records of visits by extraterrestrials. The book attracted significant public interest, since it was published at the start of the Space Age, and more than ten million copies were sold.
Von Däniken argued that since geoglyphs, such as the Nazca lines in Nazca, Peru, can only be appreciated from a high altitude, they were built to be viewed by the extraterrestrial "gods" in their spacecraft. It makes a nice story, but scientists don't see much merit in this conjecture.
Just south of the Nazca geoglyphs are their cousins in the Atacama Desert of
Chile, including the Atacama Giant, as shown below. This geoglyph, which is about 120 meters in longest dimension, is thought to have been an astronomical calendar based on the setting Moon. Astronomy is continuing at the Atacama Desert with the recent commissioning of the US $1.3-1.4 billion Atacama Large Millimeter Array (ALMA), a new radio telescope.[1-6]
There are many radio telescopes in the world, so why build another, and why place it in the desert? The clues are in its name. First, there the millimeter part, which refers to the operating wavelength. The first radio telescope, built by Karl Jansky, operated at a frequency of 20.5 MHz, corresponding to a wavelength of about 14.6 meters. Using the familiar conversion formula of wavelength λ into frequency ν,
ν = c/λ,
in which c is the speed of light, shows that a wavelength of one millimeter corresponds to a frequency of about 300 GHz. That's more than 10,000 times higher an operating frequency than Jansky's telescope, and far above the common frequency bands, as shown in the table.
The highest frequency devices most people will encounter are Wi-Fi equipped computers that now operate as high as 5 GHz, and potentially at 60 GHz. Direct-broadcast satellite equipment receives signals slightly above 12 GHz (Ku-band). The highest frequency with which I've had practical experience is X-band, the operating frequency of an electron paramagnetic resonance spectrometer I helped construct as an undergraduate. Microwave ovens for the home operate at 2.45 GHz.
Why have a radio telescope operate at such small wavelengths? Smaller wavelength gets you better angular resolution for the same size dish antenna; provided, however, that the dish surface is smooth to that wavelength. Instead of a single, large dish antenna, ALMA is designed to have a sixty-six, fifty-seven of which were operational on opening day, March 13, 2013. These antennas range in diameter from 7-12 meters and have a surface accuracy of better that 25 micrometers. When the antenna signals are combined, ALMA acts as if it were a 16 kilometer (10 mile) wide telescope.
|Band Designation||F (GHz)||
||Band Designation||F (GHz)|
Most people are familiar with the iconic Very Large Array (VLA) radio telescope, near Socorro, New_Mexico. It was the setting for the opening scene of the film, 2010, and several other films. The angular resolution of ALMA, 10 milliarcseconds, is ten times better than that of the VLA, and it has twenty times the point source detection sensitivity of the VLA.
Now, the question of why site ALMA at a desert plain in the middle of nowhere. It's another consequence of the millimeter operating wavelength. Radio astronomers, like their optical astronomer colleagues, are sometimes defeated by Earth's atmosphere. The atmosphere is generally transparent to radio waves above 50 MHz, where the ionosphere can be ignored. Absorption from water vapor and atmospheric gases becomes important, however, at much higher frequencies. (see figure).
|The first antenna placed at ALMA.|
The 100 ton antennas are movable, so the antenna array can be reconfigured for different observations. They are moved by two 28-wheel transporters.
(ALMA/ESO/NAOJ/NRAO photo, Creative Commons Attribution 3.0 licensed, via Wikimedia Commons.)
Not only is the ALMA site in the Atacama Desert, which is naturally dry, it's also on a plateau at a 5,000 meter (16,400 foot) altitude that puts it above much of Earth's atmosphere.[1-3,6] The observatory building is apparently the second highest building on Earth, surpassed only by a train station in Tibet.[1-3] The air is so rarefied that the astronomers must pass a medical screening to work there, and they sometimes need supplemental oxygen.[1,3]
The digital combination of signals from so many separate antennas to give the equivalent resolution of a single, large antenna is no mean feat. It requires 17 quadrillion calculations per second, all done on a highly parallel supercomputer called the Alma correlator, as shown in the photograph. These calculations are only possible if the signals from the antennas are synchronized to a picosecond.
||Average atmospheric absorption of mm radio waves at sea level (20°C, 1013.24 millibar, water vapor density 7.5 g/m3.|
Fig. 5.1 of ref. 7, redrawn for clarity.
(click for larger image.)
- Vladimir Hernandez, "Alma telescope: Ribbon cut on astronomical giant," BBC News, March 13, 2013.
- Hayley Dixon, "£1 billion telescope can see the beginning of time," Telegraph (UK), March 13, 2013.
- Clara Moskowitz, "ALMA telescope, the world's largest, will go deeper and farther than ever before," Christian Science Monitor, March 13, 2013.
- World's largest ground-based telescope opens in Chile, The Engineer, March 14, 2013.
- Eric Hand, "Radio astronomy: The patchwork array," Nature, vol. 495, doi:10.1038/495156a, (March 14, 2013), pp. 156-159 .
- David Cornish, "Alma telescope inaugurated as 50th antenna goes live," Wired (UK), March 13, 2013.
- Section 5 of Kevin R. Petty and William P. Mahoney III, "Weather Applications and Products Enabled Through Vehicle Infrastructure Integration (VII)," United States Department of Transportation - Federal Highway Administration Report No. FHWA-HOP-07-084, January 2007. Full report (36.5 MB PDF file).
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