The ALMA telescope array consists of 66 dishes that can work separately or together. // Courtesy: ALMA
By Rich Lohman
As an amateur astronomer with my own small observatory, I know some of the conditions that make for good astronomical viewing. But until I traveled to Chile with the National Science Foundation-sponsored Astronomy in Chile Educator Ambassador Program in June of 2017, I didn’t appreciate the full extent of the planning that goes into locating and designing a high-quality observatory.
Ingredients for a world-class observatory
The Atacama Large Millimeter/ submillimeter Array (ALMA), located in northern Chile, is especially notable. We don’t usually think of an observatory made up of antennas, but ALMA is a radio observatory consisting of 66 dish antennas linked together with sophisticated electronics and computers.
For any optical observatory (one that uses large mirrors) to capture the highest quality images night after night, it must have the following conditions: low or near-zero light pollution from surrounding towns and cities; very dry and steady air; high elevation above sea level. For a radio observatory such as ALMA, there are additional factors, which I will mention later.
Since the principal objective of a telescope is to capture the light from distant astronomical objects (stars, galaxies, nebulae), any additional light from nearby sources will simply degrade or even wash out the desired image. Consequently, locating an observatory far from light-polluting sources is essential.
The issue of very dry, steady air relates, primarily, to the transparency and lack of movement in the atmosphere above the observatory. Water vapor in the atmosphere absorbs some of the light coming from distant sources. But it also provides a kind of moving current, which can shift the incoming light in random directions, causing the astronomical images to become very blurry. There are new technologies that help eliminate some of these effects, but dry, steady air still makes for the highest quality images.
Finally, an observatory located at a high elevation puts the telescope above a certain fraction of Earth’s atmosphere. This means that the natural attenuation of the distant light by the water vapor and other atmospheric gases is reduced. The higher the elevation, the clearer and brighter the astronomical image.
ALMA was built in the Atacama Desert of Chile because of its superb conditions for observing. // Rich Lohman
ALMA’s amazing location
If we look at the conditions that exist for the ALMA radio observatory, we find a remarkable location. ALMA is located in the far north of Chile in the Atacama Desert. The specific location is about 30 miles from the nearest large desert town, so light pollution is not an issue. The Atacama Desert is an extremely arid location. In the days of the early moonshots, NASA astronauts trained with their vehicles there, because the very dry conditions approximated those on the Moon.
The ALMA antenna array is located at an elevation of 5,000 meters above sea level (approximately 16,400 feet). This puts the “telescope” above nearly half of Earth’s atmosphere. It means that the electromagnetic radiation coming from the cosmos can pass through to the antennas with considerably less attenuation than if they were located at sea level. ALMA has even been compared favorably to the Hubble Space Telescope, which is outside our Earth’s atmosphere. I’ll return to this issue of high altitude a bit later.
One other factor was very important to the siting of the ALMA observatory. Since it was to consist of an array of 66 antennas working together, the area for locating all the antennas needed to be reasonably flat and level. Had all the antennas been located at various elevations, their synchronization would have been extremely difficult.
But ALMA sits on an ideal site. Picture a relatively flat plane, high in the Andes mountains with 66 antenna dishes arranged over a 16-km (10 miles) radius. That is a huge plane. Its name is the Chajnantor Plateau. The name comes from the indigenous peoples who inhabited the area centuries ago. There are descendants of these people still living in the area today.
A few of ALMA's many antennas. // Rich Lohman
Searching for cosmic radio waves
Now I’ll turn to what these 66 antennas are detecting and measuring. Radio waves come in from the cosmos. They are reflected off the parabolic dish and focused onto a small, secondary mirror, set in front of the dish. The focused beam is then sent into the back end of the antenna, where there is a receiver. Much like an FM or AM radio receiver, these receivers are tuned to specific frequencies.
At ALMA, the receiver module will eventually contain 10 separate receiver units tuned to different frequency bands. The astronomer running the experiment can select the frequency band(s) she wishes to measure. The photo below shows the receiver for one single frequency channel.
The frequency bands available in the ALMA receivers correspond to wavelengths in the range from 0.3 to 7.5 mm (0.01 to 0.3 inches) of the electromagnetic spectrum. Hence the name “Millimeter/ submillimeter Array.” This range is between the infrared and the microwave. What is notable about this range is that most of this energy is absorbed by our atmosphere as it comes towards Earth. However, ALMA’s high-altitude location allows a significant portion of it to be detected by the ALMA antennas.
One of the bands installed within the ALMA antenna receivers. // Rich Lohman
The other notable aspect of this wavelength range is that it corresponds to what is termed the “cold universe.” Processes out in the cosmos that are in the temperature range of a few to 80 degrees Kelvin emit this type of radiation. These are the low-energy physical processes typical of the early universe. This is emission from the dust and gas in molecular clouds, from which stars are born in stellar nurseries. ALMA has the ability to determine the molecular content of this dust and gas by measuring its spectrum. ALMA can view circumstellar disks of dust and gas and examine the early stages of planet formation. No other observatory in the world is capable of detecting these processes to the extent that ALMA can.
A final, key component of ALMA gives this observatory its incomparable ability. The antenna dishes can be moved to many locations over the plateau to give astronomers selection over the detail they wish to obtain in the images they are taking. To a layperson this is much like the zoom feature we have on our cellphone cameras. The antennas are also linked together electronically in a system called an interferometer. The details of interferometry are well beyond the scope of this article, but the following is a simplified explanation: The data from each receiver from each antenna is fed into a super computer called a correlator. The computer, knowing the location of each antenna, has the programming that combines all of the data into a single image. The final image is equivalent to one taken by a single antenna that is up to 16 km in diameter! To me, that is the most amazing aspect of the ALMA observatory.
Here are a few images that illustrate ALMA’s capability (click for a larger view):
Left: A composite image of the dusty debris ring around the young star Fomalhaut. // ALMA/ESO/NAOJ/NRAO/M. MacGregor/NASA/ESA/Hubble/P. Kalas/B. Saxton/AUI/NSF; Right: The protoplanetary disk around HL Tauri. // ALMA (ESO/NAOJ/NRAO), NSF
The author stands among the antennas of ALMA high above the Atacama Desert. // Rich Lohman
A human process
The above description of ALMA and the beauty of these few images convey what ALMA is and what it can do. However, the experience of actually standing up at 16,000 feet among these giants of antennas cannot be adequately communicated in words. There I was, on the Chajnantor Plateau, where the ancient Atacameños must have spent night after night observing the heavens and creating myths to express their understanding of what’s “up there” and “out there.” Now, here I was centuries later, learning about how we modern-day humans are doing much the same thing with our antennas seeking light from great distances. Our new stories may go deeper into what we call the truth, but the process is a natural human one. We want to know more about what our senses are telling us. We want to know more about who we are, where we have come from and where we are going. Looking up and out helps us answer those questions.