ALMA "first fringes" at Chajnantor
16 November 2009
A team of astronomers and engineers at the Atacama Large Millimeter/submillimeter Array (ALMA) have made the first interferometric measurements of radio signals — so-called “fringes” — of an astronomical source from the observatory’s 5000-metre “high site” of Chajnantor. This is an important technical step for ALMA, as it used a full suite of the production equipment, including two of the 12-metre diameter antennas, and sophisticated electronic systems for receiving and correlating the signals. This is the first time that all these complex items, almost all of which are at the leading edge of technology, have been used together as a complete system.
The antennas were moved into position on the Chajnantor plateau on 16 and 17 October. Over the next few weeks the system was connected together and tested. Finally, “fringes” were measured, the effect seen when signals received by a pair of antennas are combined with exactly the right timing. To achieve this, they used a variety of well-known astronomical objects, including the Orion molecular cloud and the quasar 3C84. The team performed experiments to accurately determine the distance between the antennas (the “baseline”) which was measured as 159.8331 metres with a precision of half a millimetre, and the difference in the time the signals take to travel to the central building (the “delay”) which was 1959.642 nanoseconds with a precision of two hundredths of a nanosecond. At the same time the equipment and connections needed to enable operation at shorter wavelengths was tested. The measurement of fringes from the quasar 3C454.3 marks the first time that ALMA equipment has detected fringes at a wavelength of less than 1mm, so ALMA is now truly a “submillimetre” as well as millimetre-wave telescope.
In order to generate the fringes, which are the first step towards making detailed images of the astronomical objects being observed, the signals are acquired and processed by the antenna system detectors and electronics, and then transmitted along optical fibres to the ALMA Operations Site Technical Building, where they are correlated and stored in the archive.
The team are now looking forward to the next step in the process — the addition of a third antenna which will allow them to verify more of the software and hardware, by obtaining “phase closure” — an important capability which requires at least three antennas, to cancel out errors in the phase of the signals caused by the instruments and atmosphere.
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, Garching, Germany
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