Both IRAM observatories participated in the Global session; Pico Veleta to 100% under good conditions. Plateau de Bure observed only to a small fraction due to bad meteo conditions with snow, freezing fog and winds up to 160 km/h (a large part of southeastern France suffered from heavy snowfalls, bad traffic conditions and power failures at the time). Directly after the main Global session, a weather improvement allowed Bure to participate in the observation of the Galactic center with a recording rate of 1 Gbit/s (the maximum data rate possible with the Mark5A system installed in 2004), although only with 3 antennas, as the other three had developed substantial icicles on the subreflector and quadrupods.
From the technical point of view, the PdBI phased array has operated with several improvements. The results of the February 2005 tests on Bure have been applied, and the Bure VLBI system has now a reduced phase noise and improved efficiency. For the April 2005 session, the constraints were purely meteorological.
A polarization VLBI test between Bure, Pico Veleta and Effelsberg followed. Bure joined late due to high wind, first with four antennas, then with six, as the daylight progressively de-iced the frozen antennas. The experiment showed that Bure can observe simultaneously with two sub-arrays, one in Left Circular Polarization (LCP) and one in Right Circular Polarization (RCP), but that some software modifications are still necessary before this observing mode can be offered in Global VLBI sessions.
During the session, Bure still operated with the EFOS-1 maser on loan from the geodetical station Wettzell, Germany. Directly after the VLBI session, the repaired CNRS maser was transported by cablecar to the PdBI and successfully installed in the correlator room, where EFOS-1 was disconnected from the system.
The Pico Veleta and Plateau de Bure teams were reinforced by visiting astronomers from the MPIfR Bonn. At both observatories, the good preparations by the technical staff were fundamental for the success of the experiment; on the Plateau de Bure, additional efforts were necessary to de-ice all antennas with a motorized platform at the beginning of the Global session (one removed icicle had reached a length of more than two meters!). Many thanks to all who have made the observations possible.
EFOS-1 left the Plateau de Bure on the 19. May 2005, after being switched to standby mode (stop of the maser emission, but the rest of the instrument remaining under power) and conditioned for transport two days earlier. Figure 1 gives a short summary of the return voyage. This time, the maser was brought down from the mountain via cablecar, using a spacious platform which had not been available last year. The operation was supervised on the PdB by the IRAM safety engineer D. Thievent, and from the lower cablecar station on to its final destination by the maser technician G. Blaser (Observatoire de Neuchâtel) and M. Bremer. The transport from St. Etienne en Devoluy (France) to Wettzell (Germany) was done by road with a rented minivan, and the maser ion pumps and heating were kept under power with four 180 Ah 12V-batteries. Numerous precautions were taken to avoid shocks and to reduce vibrations for this high precision frequency standard.
On arrival at the Fundamentalstation Wettzell
the maser was installed on its usual place in the basement and
successfully restarted. Many people from Wettzell, Neuchâtel and IRAM
were involved in the logistics and technical aspects of this
operation, and their contributions have been essential in this