ALMA Observatory increases the number of antennas in operation

The radio telescope in Chile receives the last of its 66 high-precision radio antennas. The antennas are expected to operate in synchronized fashion as if they were a single telescope 16 kilometers in diameter.

By Elton Alisson

Agência FAPESP – The Chajnantor Plateau, located 5,000 meters above sea level in the Atacama Desert in Chile, is the site of 66 high-precision radio antennas that will operate at millimeter and submillimeter wavelengths of the electromagnetic spectrum, between infrared radiation and radio waves.

When they are all in operation, these parabolic antennas, each 12 meters in diameter and weighing approximately 100 metric tons, will be able to operate together in synchronized fashion, as if they were a single radio telescope 16 kilometers in diameter. This array will be one of the most powerful in the world for observing the cold distant Universe, which is characterized by molecular gas, dust and residual radiation from the Big Bang.

The last of the antennas belonging to the ALMA (Atacama Large Millimeter/Submillimeter Array) telescope, which is considered to be one of the largest astronomical projects currently underway, was delivered in late 2013 and is in the process of being readied for operational start-up.

“It would be technically impossible, as well as cost prohibitive, to build a 16-kilometer diameter telescope like the ALMA,” said Gianni Marconi, Italian astronomer and member of the scientific committee of the ALMA Observatory.

“ALMA is a project in progress, and when it is ready, it will represent the largest window through which to observe the cold distant Universe,” Marconi told Agência FAPESP during a recent visit by Brazilian journalists to the observatory.

According to Marconi, 16 antennas have already begun operating in the observatory, inaugurated in March 2013 after 15 years of planning and construction at a cost of US$1.4 billion. By the end of 2014, 18 more antennas are expected to begin operating, making a total of 34. The goal, however, is that all of the antennas will be up and running by 2015.

“The idea is that the scientific community would get access to ALMA in stages. But so far, 16 antennas are already in operation, and ALMA is the most powerful submillimeter observatory in the world,” Marconi said.

The antennas have an estimated cost of nearly US$100 million each and were built by consortia of companies from Europe, North America and East Asia, whose countries are financing the astronomy project.

In Europe, the project is being financed by the European Southern Observatory (ESO). Financing from East Asia is coming from the National Institute of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) of Taiwan, whereas in North America, financing is being provided by a prominent US science research-sponsoring agency, the National Science Foundation (NSF), in cooperation with the National Research Council (NRC) of Canada.

“The number of antennas built by companies in North America, Japan and Europe more or less reflects the contribution these countries made to the project, which stands at 37.5% from North America, 25% from Europe and 25% from East Asia,” said Marconi.

North America supplied 25 antennas, each 12 meters in diameter. East Asia contributed 16 antennas, four measuring 12 meters in diameter and 12 with a diameter of 7 meters – the latter considered by project astronomers to be the “Ferraris” of antennas because of their faster, quieter and, consequently, more expensive engines. In turn, the European consortium built 25 antennas, each 12 meters in diameter.

The principal ALMA network will consist of 50 antennas, each 12 meters in diameter, that will work together with an interferometer – an instrument that measures angles and distances based on the interference generated by the electromagnetic waves as they interact.

Data integration

The astronomical signals captured by the antennas are converted to digital format and transmitted by fiber optic cables to a supercomputer known as a correlator, which is located in a central building also situated at the top of the Chajnantor Plateau.

Considered the “brain” of the ALMA and specially designed for the project, the supercomputer gathers the signals from the various antennas and combines them to generate data that can be later analyzed by astronomers. In addition, the correlator multiplies the signals from the antennas and saves the data in files that contain information for forming high-resolution images of the regions observed, similar to those that could be obtained with a single telescope 16 kilometers in diameter.

“The correlator is equivalent to three million computers operating at the same time,” explained Marconi. “It aligns the space signals captured by the antennas so that they are all received at the same time, and it amplifies their intensity.”

To increase the number of antennas at the Observatory, the computing power would need to be increased to handle the volume of data collected, Marconi explained. “This is why we have 66 antennas here,” he said.

The antennas can be moved along the desert plateau and separated by distances that range from 150 meters to 16 kilometers. Repositioning the antennas requires the use of two 130-ton transporters designed specifically for this purpose.

The transporters, named Otto and Lore, are remotely controlled by radio and can travel at top speeds of 20 km/hour.

Game

A free game just released on the Internet simulates the operation of the transporters that move the antennas and thus allow the ALMA’s focus to be changed and improved.

“ALMA’s wavelength allows observations of all that is ‘cold’ in the Universe, such as cold clouds of gas and dust that form new stars and galaxies like the Milky Way,” explained the astronomer. “ALMA can therefore be used to study the birth of planetary and galactic systems that are related to the emergence of the Universe,” he said.

According to Marconi, these vast, cold interstellar clouds, whose temperatures reach no more than a few degrees above absolute zero, form “blackout curtains” that darken and obscure these regions of the Universe, preventing their observation at the visible-light frequencies captured by optical telescopes.

It is possible to traverse these cold clouds of gas and dust to see what is behind them, however, by means of the millimeter and submillimeter radiation captured by the ALMA. Therefore, the two types of astronomy – optical and millimeter and submillimeter radiation – are complementary, he said.

“Optical telescopes are better able to observe very powerful phenomena such as the explosion and death of a star, while millimeter and submillimeter astronomy allows us to study the important phenomena related to the origin of the Universe,” he explained. “The two types of astronomy can provide a very complete picture of an astronomical object from birth to death."

The problem with using millimeter and submillimeter radiation to conduct astronomical observations, however, is that what little radiation arrives from space in these wavelengths is absorbed by the water vapor in the air and lost.

Therefore, the telescopes used in this type of astronomy have to be constructed in high and dry locations, such as the Chajnantor Plateau.

As one of the highest observatories in the world – higher than the Mauna Kea observatories in Hawaii, located at an altitude of 4,250 meters – the ALMA telescope is situated in one of the driest regions on Earth. The humidity at the Observatory site is, on average, 0.2 millimeters of water vapor.

“These climate characteristics are very good for the type of astronomy we do here,” said Marconi. “The less water vapor, the cleaner the radiation from the object we are trying to observe,” he stated.

However, for the astronomers and technicians who work at the Observatory, the thin air makes their work much more difficult.

“It’s very hard to concentrate on work with the low volume of oxygen here at the top of the plateau,” said Marconi. “It’s as if the brain is only working with 10% of its capacity, and, therefore, there are likely to be basic safety errors committed, such as leaving a door open that should have been kept closed,” he said.