ALMA – the birth of a giant telescope

Aerial view of ALMA
Chajnantor Plateau, located at an altitude of 5000 meters in the Chilean Andes, where the array of ALMA antennas is located. Credit: Clem & Adri Bacri-Normier (wingsforscience.com)/ESO

From idea to reality

This year a new chapter of the book of astronomy started with ALMA. On the 13th of March 2013, the observatory was officially inaugurated with more than 50 antennas pointing at the centre of our galaxy. Standing there, looking at how the antennas seamlessly moved their dishes together in different directions and pointed at the sky like a synchronised dancers team, it was hard to imagine that from the first idea to actually building ALMA, it had taken these men and women some 20 years.

Wolfgang Wild – project manager at the European project office of ALMA, and who has been with ALMA since its early days, shares his memories of how the idea started. He says: “Already back in the 1980’s astronomers in Europe, the US and Japan were thinking about a large millimetre wave interferometer. These three projects were independent, and ideas about the location were also independent.

“In fact, Japanese scientists were the first to explore a site in the Atacama Desert with a site testing campaign in November 1992.” he adds. “In the 1990’s these three activities then realised that joining forces and discussing one large observatory would benefit everybody. This was the start of looking into a large interferometer.”

The collaboration between Europe and the US started in 1999. In 2001, all three groups signed a resolution affirming a “mutual intent to construct and operate a giant radio telescope”. And in 2004, all three partners formally signed an agreement for the joint construction the ALMA project.

The real design work of a telescope starts when a scientific idea and technical concept start taking shape. At this point scientists and engineers discuss goals and feasibility. Once a concept is agreed to and funding is available, technical details can be worked out.

“While ALMA was taking a shape, the biggest discussions were on technical specifications, for example how many antennas would be needed and what the dishes’ diameter should be,” remembers Wild. “There were questions about the antennas’ receiver bands (their “instruments”), their sensitiveness and how to calibrate the array.”

Questions regarding ALMA’s technical specifications, such as the diameter, precision and number of antennas, characteristics of the detectors, requirements for the site, characteristics of electronics and dozens of other things were answered many years before the first antenna arrived on the Chajnantor Plateau in September 2009.

The location was also one of the crucial questions during the telescope building process. The ALMA site was indeed chosen based on a set of stringent requirements, such as low water vapour content in the atmosphere, high altitude to have less oxygen, and enough space to put 66 antennas on an almost flat area, with a distance of up to 16km between them. It also had to be in the southern hemisphere around 30 degrees latitude.

It sounds challenging enough to find such a place, but once it was found, extensive testing had to be carried out to ensure the site was adequate. Many studies were done about the weather patterns, rainfall, cloud coverage, water vapour in the atmosphere, and other characteristics of the site.

“Sometimes these measurements are carried out during a long time, like years, if a project can afford this. Usually there are only a few people involved, setting up automatic measurement equipment at the site to be tested, and later analysing the data,” points out Wild.

It became clear that such a massive and expensive project could only become reality after extensive studies and simulations on the telescope’s characteristics had been conducted. Indeed, how to know in the beginning that the chosen concept was the right one and the telescope would work? For that purpose astronomers build a prototype – called a precursor – of an important part of a telescope to test the technology, learn about its performance and improve the final technical setup.

Wild explains that the first prototype of an ALMA antenna was tested at a dedicated place in the United States. He says: “We needed to build prototypes of different antennas’ parts – also receivers which were tested to make sure they did what they were supposed to be doing. Quite often, testing simulates the operating conditions to see what problems can arise. And finally, good systems engineering is needed to make sure that all the individual parts work together well and create a complete system.”

And once extensive testing had been carried out, the design of the array had been confirmed and the site approved, construction of the observatory finally began in November 2003, while the construction of the European antennas began in 2006. Page 3 >>


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