In connection with the long-term plans to extend the capabilities of the Plateau de Bure Interferometer, work is currently progressing in parallel on the eastern and northern track extensions. The cargo lifting system (``blondin'') for the transport of heavy equipment and materials to the Plateau de Bure site is in operation since the beginning of May. Work on excavating and backfilling the trenches with steel reinforced concrete, work on the foundation of the new stations at the far end of the track extensions and on the electrical cabling are progressing close to schedule. Current plans foresee to have the track extensions completed in time for the upcoming winter 2005/2006 scheduling period.
With two new stations at the far ends of the eastern (E68) and northern track (N46), a new set of configurations had to be defined that reflects an optimized trade-off between desired uv-sample distributions and the difficulties of moving antennas on the plateau in snowy winter conditions. The current total number of 4 configurations is mainly imposed by current operational constraints. With E68 and N46, the six-element array will observe on baselines up to 760m east-west (+86%) and 368m north-south (+59%) with only 4 primary configurations. Despite this small number, the four new configurations and their combinations result in synthesized beams with low sidelobe levels and a wide range of spatial resolutions (see Tables for details), and provide a very significant improvement (factor 2.5) in the surface resolving power of the six-element array.
The four entirely new configurations range from the most compact (D) to the most extended (A). The design of the configurations has been a product of careful considerations, which are outlined together with the properties of the new beams in a forthcoming publication. The primary aim has been to find configurations which result in close to circular beams with low sidelobes and can be combined in pairs to better sample the uv-plane and achieve a variety of spatial resolutions. The process that was followed consisted of the optimisation of the most extreme (A and D) configurations first. A slightly less extended configuration (B) was adapted to match the uv-coverage of A and the last configuration (C) was designed to complement both B and D.
All in all, the winter conditions have been very good at the Plateau de Bure with almost no snow (contrary to other regions in Europe) and with long periods of excellent phase stability and low atmospheric opacity. The interferometer recorded a 50-60% observing efficiency from January to April. To optimize the observing efficiency with respect to the sun avoidance constraints of A-rated projects, the configuration schedule of the interferometer was slightly adjusted. The array was moved to the most extended configuration (A) at the end of December, moved to the B configuration at the beginning of February and to the C configuration at the middle of March. Because of the weather conditions, the array was rearranged to the most compact configuration (D) at the beginning of April. Global VLBI observations, which include the array in the 3mm phased-array mode, were carried out from April 15 to 20, 2005.
Almost every A-rated project was completed before the end of the winter period. We have also invested observing time on a number of B projects, and even on a few targets of opportunity. Since last December, a total of 48 different projects has successfully been scheduled for observations. Concerning projects that have been started shortly before the end of the winter period, we plan to bring these to completion in the next few months. A few deep integration and low-resolution observations of sources in the Orion-Taurus region had to be suspended because of sun avoidance contraints and are now deferred to the end of the summer semester.
Finally, we would like to remind users of the Plateau de Bure Interferometer that B-rated summer proposals which were not started by the September 08, 2005 deadline, should be resubmitted. Investigators, who would like to check the status of their project, may consult the interferometer schedule on the Web at ./IRAMFR/PDB/ongoing.html.