About the IRAM Interferometer
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The Plateau de Bure Interferometer (PdBI) is one of the World's most sensitive millimeter interferometers. Situated on the Plateau de Bure at 2550m altitude in the French Alps, its precise geographical position is
Longitude: 05:54:28.5 East, Latitude: 44:38:02.0
During its history, the PdBI underwent several track extensions, received additional antennas (all of 15m diameter and similar construction as the first ones) and technical upgrades. From a three antenna interferometer with a maximum baseline of 288 meters in 1988, it has evolved to a six-antenna array with baselines up to 760 meters in 2005. A new generation of powerful dual-polarisation receivers for the 3mm and 1mm observing bands was installed in 2006, and extended to the 2mm observing band in late 2007.
The construction of the array was started in 1985, and the first interferometric fringes were obtained in 1988. Since 1990, the interferometer is open to the world-wide scientific community, and issues twice a year a call for observing proposals.
Radio interferometry is a powerful but complex technique. To make its use attactive to a wider community, observations at the PdBI are not performed by the astronomers who propose them, but by the telescope operators and IRAM staff astronomers, who also provide assistance at various stages of data reduction. In this way, radio interferometry can be applied as a powerful tool also by the non-specialists from other branches of astronomy, such as optical, infrared or X-ray astronomers.
The imaging properties of an interferometer differ from those of a single antenna. The signals from several antennas combine to provide the resolving power (image detail) of a single antenna whose diameter would be equal to the maximum separation between the individual antennas. Each possible combination between antenna pairs contributes to the image:
2 antennas: 1 baseline
3 antennas: 3 baselines
4 antennas: 6 baselines
5 antennas: 10 baselines
6 antennas: 15 baselines
By passing from three to six antennas, the PdBI has been able to boost its imaging speed by a factor of five. The antennas of the IRAM interferometer can move on rail tracks up to a maximum separation of 760 m in the E-W direction and 368 m in the N-S direction, corresponding to a resolution of 0.5 arcsecs at an observing wavelength of 1.3 mm (230 GHz). This is the apparent size of an apple at a distance of 60 km. The field of view of the interferometer is determined by the ratio of observing wavelength to diameter for each antenna, and is 20 arcsecs at 230 GHz.