In search of our cosmic origins

Seven of the 66 antennas that the ALMA observatory will have when it is finished, surrounded by bases awaiting the arrival of new antennas. The North American and European partners have committed to providing 25 antennas each, while Japan will install 16
Credit: ALMA (ESO/NAOJ /NRAO), W Garnier. Acknowledgement: General Dynamics C4 Systems
Date:14 March 2013 Tags:, ,

This week, in a remote part of the Chilean Andes, the Atacama Large Millimetre/submillimetre Array (ALMA), was inaugurated at an official ceremony that gripped the attention of astronomers across the world. The event marked the completion of all the major systems of the giant telescope and the formal transition from a construction project to a full-fledged observatory. The observatory was conceived as three separate projects in Europe, the US and Japan in the 1980s, merging into one in the 1990s. Total construction cost will be around R12,9 billion.

Able to observe the Universe by detecting light that is invisible to the human eye, ALMA will show us never-before-seen details about the birth of stars, infant galaxies in the early Universe, and planets coalescing around distant suns. It will also discover and measure the distribution of molecules – many of them essential for life – that form in the space between the stars.

The antennas of the ALMA array, fifty-four 12 m and twelve smaller 7 m dish antennas, work together as a single telescope , employing interferometry to perform their astronomical miracles. Each antenna collects radiation coming from space and focuses it on to a receiver. The signals from the antennas are then brought together and processed by a specialised supercomputer, the ALMA Correlator. The 66 ALMA antennas can be arranged in different configurations, where the maximum distance between antennas can vary from 150 m to 16 km.

In theory, the basic notion of interferometry is quite simple. A signal from the sky is captured by two or more antennas which are combined in order to analyse the signal and thus obtain information about its source (whether a star, a planet or a galaxy). By combining the radio waves collected by several antennas, it’s possible to construct images. Such images are comparable to those that would be obtained with a hypothetical giant telescope or antenna 14 000 m in diameter. Since constructing and operating an antenna that size is technically impossible (at least with current technologies), constructing several small antennas and using them combining their output is far more feasible.

Nevertheless, this is not so simple in practice. To operate properly, ALMA must have its 66 antennas and electronics working in perfect synchrony, with a precision of one millionth of a millionth of a second. In addition, the signals from the different antennas must be combined in a way that the path followed from each antenna until it is combined at the central computer (the correlator) must be known with an accuracy equal to the diameter of a human hair (that is, hundredths of a millimetre).

And as if that’s not challenging enough, there is the problem of reducing the possible attenuation and perturbation suffered by the signal from the time it touches each antenna until it is digitised and transmitted over several kilometres of optic fibre to the central computer. Even earlier, as soon as the signal penetrates the Earth’s atmosphere, it is partially absorbed, deviated and delayed by molecules of carbon dioxide, oxygen and water (even at 5 000 m altitude and in the dry conditions encountered in the Atacama Desert). Seven weather stations and specially built water vapour radiometers, which measure the amount of line-of-sight water vapour present in the atmosphere, will be used to correct for these atmospheric effects.

For the full story, cool images, videos and other interesting stuff, visit the ALMA observatory