Six antennas gazing at the cosmos
The other observatory of IRAM, the Plateau de Bure interferometer, is the most advanced facility existing today for millimeter radio astronomy.
Located in the Hautes-Alpes on the wide and isolated Plateau de Bure at an elevation of 2550 meters, the interferometer consists of six antennas, each 15 meters in diameter. Each antenna is equipped with state-of-the-art high-sensitivity receivers. Two rails, extending on a north-south and east-west axis, enable the antennas to be moved up to a maximum separation of 760 meters.
During observations, the six antennas function as a single telescope, a technique called interferometry. With the antennas pointing towards the same cosmic source, the signals received by each of them are subsequently combined. The angular resolution achieved during observation is that of a single telescope, whose diameter corresponds to the maximum distance between the individual antennas. In the case of the IRAM interferometer, this is equivalent (for the longest baselines) to a telescope with a diameter of 760 meters, which can distinguish two one-cent coins placed next to each other at a distance of 5000 meters.
Due to the complexity of such an advanced antenna array system, only IRAM operators perform the observations.
To obtain a complete image of a cosmic object, interferometry uses the earth’s rotation, which slowly turns the antennas with respect to the source, thus enabling step by step to scan the source’s structure. Astronomers are able, after several hours of observation, to reconstruct a high angular resolution image of a cosmic object and study its detailed morphology.
The two IRAM facilities can also be combined with other radio telescopes into one giant interferometer with intercontinental baselines (Very Large Baseline Interferometry or VLBI). This global technique is particularly well adapted to the exploration of ultra luminous cosmic phenomena, such as brightly shining matter around black holes (quasars) or shells ejected by dying stars. The angular resolution that is achieved with this technique is such that it would be possible to detect a golf ball on the moon! VLBI is also used to measure the movements of the tectonic plates and to pinpoint the position of a spacecraft.
Thousands of tons of materials and machinery were transported up to the Plateau de Bure to build the interferometer and in particular the east-west and north-south tracks. These tracks enable the antennas to move to their observation configurations where their positions can be measured to within a fraction of a millimeter.
Since the beginning of construction in the early 1990s, the Plateau de Bure interferometer has been constantly upgraded. With only three antennas at the beginning, the number of antennas was doubled in the next ten years, and the baselines have been extended to about three times their original length. Recently, a new generation of receivers was installed that has significantly increased the sensitivity and therefore the efficiency of the Plateau de Bure interferometer.