Scientists and engineers working on the world’s largest ground-based astronomical project, the Atacama Large Millimeter/submillimeter Array (ALMA), have achieved another milestone – the successful linking of two ALMA astronomical antennas, synchronized with a precision of one millionth of a millionth of a second – to observe the planet Mars. ALMA is under construction by an international partnership in the Chilean Andes.
On April 30, the team observed the first “interferometric fringes” of an astronomical source by linking two 39-foot (12-meter) diameter ALMA antennas together with the other critical parts of the system. Mars was chosen as a suitable target for the observations that demonstrate ALMA’s full hardware functionality and connectivity.
“We’re very proud and excited to have made this crucial observation as it proves that the various hardware components work smoothly together,” said Wolfgang Wild, the European ALMA project manager. “This brings us another step closer to full operations for ALMA as an astronomical observatory.”
The two antennas used in this test will be part of ALMA’s array of 66 giant 39-foot (12-meter) and 23-foot (7-meter) diameter antennas that will observe as a single giant telescope.
ALMA will operate as an interferometer, capturing millimeter and submillimeter wavelength signals from the sky with multiple antennas, and combine them to create extremely high resolution images, similar to those that would be obtained by one, giant antenna with a diameter equal to the distance between the antennas used.
“This can only be achieved with the perfect synchronization of the antennas and the electronic equipment,” said Richard Murowinski, ALMA project engineer. “The extreme environment where the ALMA observatory is located, with its strong winds, high altitude, and wide range of temperatures, just adds to the complexity of the observatory and to the fascinating engineering challenges we face.”
The astronomers measured the distinctive “fringes” – a regular pattern of alternating strong and weak signals – detected by the interferometer as Mars moved across the sky.
The hardware used in this successful first test included two 39-foot (12-meter) diameter ALMA antennas as well as a complex series of electronic processing components needed to combine the signals.
Such pairs of antennas are the building blocks of imaging systems that enable radio telescopes to deliver pictures that approach or even exceed the resolving power of visible light telescopes. Each antenna is combined electronically with every other antenna to form a multitude of antenna pairs. Each pair contributes unique information that is used to build a highly detailed image of the astronomical object under observation.
When completed early in the next decade, ALMA’s 66 antennas will provide more than 1,000 such antenna pairings, with distances between antennas up to 10 miles (16 kilometers). This will enable ALMA to see with a sharpness surpassing that of the best space telescopes and to complement ground-based optical interferometers such as the European Space Observatory’s Very Large Telescope Interferometer.
“We are on target to do the first interferometry tests at the 16,400-foot (5000-meter) high site by the end of this year,” said Thijs de Graauw, ALMA director. “And by the end of 2011, we plan to have at least 16 antennas working together as a single giant telescope.”
