On 20 November 2009 the third antenna天线，触角 for the ALMA observatory² was successfully installed at the Array Operations Site, the observatory's "high site" on the Chajnantor plateau高原, at an altitude of 5000 metres in the Chilean Andes. Later, after a series of technical tests, astronomers³ and engineers observed the first signals from an astronomical⁴ source making use of all three 12-metre diameter antennas⁵ linked together, and are now working around the clock to establish the stability and readiness准备就绪，敏捷 of the system. "The first signal using just two ALMA antennas, observed in October, can be compared to a baby's first babblings婴儿发出的咿呀声," says Leonardo Testi, the European Project Scientist for ALMA at ESO. "Observing with a third antenna¹ represents the moment when the baby says its very first, meaningful word — not yet a full sentence, but overwhelmingly exciting! The linking of three antennas is indeed the first actual step towards our goal of achieving precise and sharp images at submillimetre亚毫米的 wavelengths⁶."

The successful linking of the antenna trio was a key test of the full electronic and software system now being installed at ALMA, and its success anticipates the future capabilities⁷ of the observatory. When complete, ALMA will have at least 66 high-tech⁸ antennas operating together as an "interferometer干涉仪", working as a single, huge telescope probing the sky in the millimetre and submillimetre wavelengths of light. The combination of the signals received at the individual antennas is crucial to achieve images of astronomical sources of unprecedented⁹ quality at its designed observing wavelengths.

The three-antenna linkup is a critical step towards the observatory's operations as an interferometer. Although the first, successful measurements employing just two antennas were obtained at the ALMA high site from October 2009 (see ESO Announcement) and demonstrated the excellent performance of the instruments, the addition of the third antenna is a leap of vital importance into the future of the observatory. This major milestone¹⁰ for the project is known as "phase closure" and provides an important independent check on the quality of the interferometry.

"The use of a network of three (or more) antennas in an interferometer dramatically enhances its performance over a simple pair of antennas," explains Wolfgang Wild, the European ALMA Project Manager. "This gives astronomers control over possible features which degrade the quality of the image, arising due to the instrument or to atmospheric¹¹ turbulence¹²大气湍流. By comparing the signals received simultaneously¹³ by the three individual antennas, these unwanted effects can be cancelled out — this is completely impossible using only two antennas."

To achieve this crucial goal, astronomers observed the light coming from a distant extragalactic source, the quasar QSO B1921-293, well known to astronomers for its bright emission¹⁴ at very long wavelengths, including the millimetre/submillimetre range probed by ALMA. The stability of the signal measured from this object shows that the antennas are working impressively well.

Several additional antennas will be installed on the Chajnantor plateau over the next year and beyond, allowing astronomers to start producing early scientific results with the ALMA system around 2011. After this, the interferometer will steadily¹⁵ grow to reach its full scientific potential, with at least 66 antennas.

ALMA, an international astronomy facility, is a partnership¹⁶ of Europe, North America and East Asia in cooperation with the Republic of Chile.