Closing the loop for ALMA

Jan 04, 2010
Three ALMA antennas linked together as an interferometer for the first time, on the 5,000 m altitude plateau of Chajnantor. Having three antennas observing in unison makes it possible to correct errors that arise when only two antennas are used, thus paving the way for precise images of the cool Universe at unprecedented resolution. Credit: ALMA (ESO/NAOJ/NRAO)

On 20 November 2009 the third antenna for the ALMA observatory was successfully installed at the Array Operations Site, the observatory's "high site" on the Chajnantor plateau, at an altitude of 5000 metres in the Chilean Andes. Later, after a series of technical tests, astronomers and engineers observed the first signals from an astronomical source making use of all three 12-metre diameter antennas linked together, and are now working around the clock to establish the stability and readiness of the system.

"The first signal using just two ALMA antennas, observed in October, can be compared to a baby's first babblings," says Leonardo Testi, the European Project Scientist for ALMA at ESO. "Observing with a third antenna 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 wavelengths."

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 capabilities of the observatory. When complete, ALMA will have at least 66 high-tech antennas operating together as an "interferometer", 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 unprecedented 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 is a leap of vital importance into the future of the . This major milestone 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 atmospheric turbulence. By comparing the signals received simultaneously 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 emission 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 will steadily grow to reach its full scientific potential, with at least 66 antennas.

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

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ESO is the European partner in ALMA. ALMA, the largest astronomical project in existence, is a revolutionary telescope, comprising an array of 66 giant 12-metre and 7-metre diameter antennas observing at millimetre and submillimetre wavelengths. will start scientific observations in 2011.

Explore further: Hubble sees 'ghost light' from dead galaxies

More information: More info: ESO ALMA web pages: www.eso.org/alma
More info: ALMA Observatory web pages: www.almaobservatory.org/

Related Stories

ALMA observatory equipped with its first antenna

Dec 19, 2008

High in the Atacama region in northern Chile, one of the world's most advanced telescopes has just passed a major milestone. The first of many state-of-the-art antennas has just been handed over to the Atacama ...

Major milestone: First two ALMA telescope antennas linked

May 06, 2009

On 30 April, the team observed the first "interferometric fringes" of an astronomical source by linking two 12-metre diameter ALMA antennas, together with the other critical parts of the system. Mars was chosen ...

The antenna bride and bridegroom

Mar 07, 2007

The Atacama Large Millimeter/submillimeter Array (ALMA), an international telescope project, reached a major milestone on 2 March, when two 12-m ALMA prototype antennas were first linked together as an integrated system to ...

ALMA telescope reaches new heights

Sep 23, 2009

The ALMA (Atacama Large Millimeter/submillimeter Array) astronomical observatory took another step forward and upward, as one of its state-of-the-art antennas was carried for the first time to Chile's 16,500-foot-high ...

Birth of a colossus on wheels

Jul 30, 2007

The first of two spectacular vehicles for the ALMA (Atacama Large Millimeter/submillimeter Array) Observatory rolled out of its hangar and passed successfully a series of tests. This vehicle, the ALMA antenna ...

Worldwide effort bringing ALMA telescope into reality

Feb 15, 2008

In the thin, dry air of northern Chile's Atacama Desert, at an altitude of 16,500 feet, an amazing new telescope system is taking shape, on schedule to provide the world's astronomers with unprecedented views of the origins ...

Recommended for you

Hubble sees 'ghost light' from dead galaxies

11 hours ago

(Phys.org) —NASA's Hubble Space Telescope has picked up the faint, ghostly glow of stars ejected from ancient galaxies that were gravitationally ripped apart several billion years ago. The mayhem happened ...

When did galaxies settle down?

18 hours ago

Astronomers have long sought to understand exactly how the universe evolved from its earliest history to the cosmos we see around us in the present day. In particular, the way that galaxies form and develop ...

Image: Hubble views the whirling disk of NGC 4526

19 hours ago

This neat little galaxy is known as NGC 4526. Its dark lanes of dust and bright diffuse glow make the galaxy appear to hang like a halo in the emptiness of space in this image from the NASA/ESA Hubble Space ...

Planet-forming lifeline discovered in a binary star system

Oct 29, 2014

Scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) have detected a streamer of dust and gas flowing from a massive outer disk toward the inner reaches of a binary star system. This never-before-seen ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.