8.2 Description

The Plateau de Bure interferometer is located in the South of the French Alpes, near St-Etienne en Dévoluy in the Département of Hautes Alpes. The interferometer's altitude is 2552m (2560m at the intersection of the azimuth and elevation axes of the telescopes) and its longitude and latitude at the array phase center are 05:54:28.5E and 44:38:02.0N. After the cable-car accident, two means of transport to Plateau de Bure have been made available: transport by helicopter or on the ground using a four-wheel drive in summer, a ratrack in winter time, and a final foot-path to get atop the Plateau. A hangar in which the sixth antenna is currently under construction, is used for antenna maintenance, overhaul periods and, in general, for antenna repair work which needs to be carried out in between times but under safety conditions. The hangar houses a few workshops for cryogenics, mechanics, electronics, a power station which provides electric autonomy in case of interruptions in the external power supply and finally, the correlator room and the control room for remote array operation. Almost adjacent to the hall are the living quarters for the staff who supports the uninterrupted round the clock operation of the interferometer.

Figure 8.1: PdB interferometer station layout as of March 2001. The interferometer origin is defined as the center of the circle which goes through the stations N20, E24 and W12, the so-called IRAM phase center, and thus gives a unique vector definition to each station.

Currently, the interferometer consists of five antennas arranged in a T-shaped pattern extending over a maximum of 408 meters east-west and 232 meters north-south. A sixth antenna is expected to be ready for 2002, and the extension of the northern track is still under construction.

The antennas are conceptually identical: they all have a fully steerable alt-az mount which incorporates a self-propelled transporter for moving the antennas (130 tons) along the tracks between stations. Each antenna is a 15m diameter Cassegrain telescope with the backstructure and quadrupod legs largely made of carbon fiber for high thermal stability. The high precision of the reflecting antenna surface (40-60$\mu$m) guarantees best performance: all antennas have essentially the same sensitivity (22Jy.K$^{-1}$ at 3mm, 35Jy.K$^{-1}$ at 1mm - see Chapter 12 by A.Dutrey), and very similar pointing and focussing characteristics.

All the antennas are equipped with dual-frequency SIS receivers operating simultaneously in the 82GHz$-$115GHz and 205GHz$-$245GHz range. Typical double sideband receiver noise temperatures are between 25K and 50K at 3mm and between 40K and 60K at 1mm. The receivers upper and lower sidebands are separated by the correlators with a rejection better than 26dB. The lower to upper sideband gain ratio depends on the receiver and varies typically between 0.2 and 4.0 under standard operating conditions in the 3mm band, and yields essentially a double-side band tuning in the 1mm band. Pure single sideband tuning (with rejection 15 to 25 dB) is also possible in the 3mm band, with receiver temperatures around 60 to 80K.

Eight totally independent correlators units are available that provide an 87% correlation efficiency (for more details see Chapter 6by H.Wiesemeyer). Each correlator unit provides by default 7 choices of bandwidth/channel configurations down to a nominal velocity resolution of 50m.s$^{-1}$ at 230GHz. The correlators can independently be connected either to the 3mm or to the 1mm receiver (100-650 MHz) IF2.

A central control computer coordinates the entire interferometer (antennas, receivers and correlators and quite some other equipment) and makes the data acquisition. Raw data corresponding to the individual dumps of the correlator buffers will not be available as real-time jobs apply automatic calibrations (clipping correction, apodization, FFT, sideband separation, small delay correction, bandpass correction and other corrections) and make automatic data quality assessments (marking bad data, shadowing, phase lock, just to cite a few flags) before data is written to disk. A second workstation provides the software resources for offline data analysis and for data archiving before transfer to the Grenoble headquarters.