Description

The Plateau de Bure interferometer is located in the South of the French Alps, near St Etienne en Dévoluy in the Départment Hautes Alpes. The interferometer's altitude is 2560 m at the intersection of the Azimuth and Elevation axes of the telescopes, and its longitude and latitude are 05:54:28.5 E and 44:38:02.0 N at the array phase center. The interferometer comprises

• Six antennas of 15m diameter,
• one north-south track 368m long,
• one east-west track 760m long,
• a wide-band correlator WideX covering a bandwidth of 3.6GHz in dual polarization with a channel spacing of about 1.95MHz
• a flexible spectral correlator, made of 8 independent units, with resolutions ranging from 0.039MHz to 2.5MHz, and with 128 to 512 associated channels per unit and baseline,
• dual polarization receivers: a 3mm receiver band tunable between 80 and 116GHz, a 2mm band (129 to 174GHz), a 1.3mm receiver band tunable between 201 and 267GHz, and a 0.8mm receiver band covering the 277 to 371GHz frequency range.

The six antennas of the interferometer can be positioned on 32 stations layed out along a ``T'' shaped track (see Figure ).

The north-south arm is 368m long, and the east-west oriented arm extends 216m west and 544m east of the intersection. The angle between the arms is $75\deg$. The station names are taken from the arm orientation and a two digit code indicating the distance from the track intersection (station W00) in 8m units.

Each antenna is a 15m diameter Cassegrain telescope constructed largely of carbon fiber. The primary mirrors have a surface accuracy below 60$\mu$m rms. The antenna mounts incorporate self propelled transporters for moving the antennas along the tracks between stations. The antennas are equipped with four receiver bands, observing in dual polarization in the 3mm, 2mm, 1.3mm, and 0.8mm atmospheric windows, respectively. The 3mm SIS mixers have typical SSB receiver noise temperature between 40K and 55K, the 2mm mixers have typical noise temperature between 30K and 60K, the 1.3mm SIS mixers have typical receiver noise temperature between 40K and 60K, and the noise temperatures at 0.8mm are typically 30-50K. More details about the receivers are given in Sect. , a summary can be found in Table .

The two IF-channels (one per polarization), each 3.6GHz wide, are transmitted by optical fibers to the central building. Details on the IF transport and processing are given in Sect. .

The wide-band correlator WideX with fixed spectral resolution, capable of processing 28 baselines (8 antennas), is available in parallel with a narrow-band correlator, the latter comprising eight correlator units. Each of the eight units is a fully independent, flexible entity, capable of processing 15 baselines (6 antennas). Full 2-bit sampling scheme is used to give 88% efficiency. For details of the unit capabilities, see Section .

A 64-bit Linux computer and several embedded processors control the interferometer and acquire the data. The user interface is a variant of OBS, familiar to most users of the IRAM 30-m telescope. Additional Linux PCs are available on site to the operator and on-duty astronomer for offline data reduction with the CLIC calibration package and for data archiving before transfer to Grenoble.

Each interferometer ``configuration'', i.e. the 6 antennas on given stations, simultaneously provides 15 baselines (or uv-tracks). The interferometer is run with several projects progressing in parallel, allowing for a flexible scheduling of the observations well adapted to the actual meteorological conditions. Depending on the weather and on the season (antenna maintenance period in the summer semester, i.e. the array is observing in its 5 antenna D configuration), configurations are changed every two to six months, so a project that requires two configurations will on average take about four months to be completed.

Figure: Example of uv-coverage (left) and interferometer station layout (right), as produced by the ASTRO UV_TRACK command. The shaded circle at the center of the uv-plane shows the antenna shadowing region