In December 2001, a new antenna became ready for integration into the Plateau de Bure Interferometer. The array has started with 3 elements in 1990. At that time it was already decided to add a fourth antenna. Serious discussions to go to six antennas, i.e. 15 simultaneous baselines, started in about 1993 but it was only after Antenna 5 had been finished in 1996 that a 'green light' was given to build the 6th antenna.
The first industrial contracts for Antenna 6 were signed in 1998, and in 1999 the assembly of the mount had started. This came to an abrupt stop after the two terrible accidents that hit the Plateau de Bure Observatory in the second half of 1999. The decision to restart work on Antenna 6 depended not only on the possibility to bring the necessary staff up to the Plateau de Bure but also on a solution for transporting the heaviest piece of the antenna, the central hub with a weight of 5.4 tons, to the mountain. A unique opportunity for this arose during the necessary testing of the future cable car system for the transport of materials under heavy load conditions.
The assembly work for the reflector of antenna 6 as well as the remaining work on the thermal insulation was sub-contracted to outside companies. Most other tasks, including electrical cabling, the outfit of the receiver cabin, and the installation of the control system and necessary software changes were executed by IRAM staff.
After initial tests inside the hangar, the antenna was rolled out, and first tests were made on the sky, including a holographic measurement of the surface. Details of this are reported elsewhere in this newsletter.
Having successfully completed the construction of this antenna before the end of 2001 despite the many times difficult circumstances is a remarkable achievement to which many people inside and outside IRAM have contributed. It is a pleasure to thank all of them!
The IRAM highlight of the year 2001 was the First Light on Antenna 6. As planned, the antenna was moved from the hangar to station N13 on December 11. After a few days of software debugging and troubleshooting, two days of adverse weather conditions, and three hours invested in a coarse alignment, André Rambaud and Yvan Mourier, operators at the Plateau de Bure observatory, started to scan the sky for Jupiter on a fine-meshed grid.
In the early morning of December 16th, a first single dish signal was detected on Jupiter, and a few hours later, the antenna and the five-element interferometer were ready for the crucial attempt to obtain first fringes. At 21:35 UT the interferometer captured fringes on all the baselines connected to Antenna 6 (Fig. 1). After a few days of testing during scheduled technical time, Antenna 6 became finally available early January for the execution of scientific projects. Since then all the antennas are up and running continuously, weather permitting.
Substantial efforts have also been made to stabilize the tracking of the antenna in the presence of wind. Albeit a fine tuning is still required to meet the tracking specifications and reduce further antenna down-time, Antenna 6 has not shown any other significant problem. Today, antenna testing is still ongoing whenever there is a gap in the regular observing schedule. After almost two years of delay, Antenna 6 has finally joined the five-element array in regular science operations, and provides now after just a few weeks of testing, an interferometer with an unprecedented sensitivity and a high quality mapping capability. More than a dozen galactic and extragalactic projects have already been observed with the interferometer since the beginning of January. Many excellent results have already been obtained, and we are confident that many more will follow in the months to come.
The surface of Antenna 6 looks different than the other telescopes of the PdBI. Except for a few optically reflecting panels, which cover the surfaces of 3 other antennas entirely, and different from the aluminium-matte panels on 2 more antennas, the surface of this antenna is actually white.
The panels of this reflector are carbonfiber panels produced for the first generation Bure antennas. Because of the development of numerous pinholes in the backside aluminized Hostaflon layers with which these panels were delivered, the original surface layers have been removed without, however, destroying the definition of the high precision surface of these panels which results from a thin epoxy layer on top of the last carbonfiber layer.
Extensive tests have been performed to identify a two-layer surface coating with a conductive silver emulsion paint and a white paint for thermal reasons and as a protection against adverse meteorological conditions and the high UV flux levels at 2550 m altitude. IRAM was supported in this study by the Centre National d'Evaluation de Photoprotection (CNEP) in Clermont-Ferrand. A key criterion was, of course, the high reflectivity at 1mm- and 3mm-wavelengths of the conductive paint and minimal absorption in the protective layer. We also insisted on the possibility to remove the coating if necessary with solvents that will not attack the precious epoxy layer. The actual coating was done by the company SITEL in St. Martin d'Hères.
In the years to come, we intend to refurbish the surfaces of the remaining antennas with Hostaflon-sealed surfaces. In the meantime, we will build up experience with this new solution. Considering the performance of the new antenna which is practically already at the same efficiency at 1mm and 3mm wavelengths as the other antennas of the interferometer, we expect that re-painting the panels will provide a valuable way of maintaining the quality of the PdBI on the long term.