Description

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Description

The NOrthern Extended Millimeter Array (NOEMA) 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 currently comprises

- ten antennas of 15 m diameter,

- baselines ranging up to 760 m

- a high-performance wide-band correlator named PolyFiX

- dual polarization receivers covering the 3 mm, 2 mm and 1.3 mm wavelength bands

The ten antennas of the interferometer can be positioned on 32 stations layed out along a “T” shaped track (see Figure ). The north-south arm is 368 m long, and the east-west oriented arm extends 216 m west and 544 m east of the intersection. The angle between the arms is 75. The station names are taken from the arm orientation and a two digit code indicating the distance from the track intersection (station W00) in 8 m units.

Until 2021, two more antennas will join NOEMA and the baselines will be extended up to almost 1700 m on the East-West arms, bringing NOEMA close to its full capabilities.

Each antenna is a 15 m diameter Cassegrain telescope constructed largely of carbon fiber. The primary mirrors have a surface accuracy below 50 m rms. The antenna mounts incorporate self propelled transporters for moving the antennas along the tracks between stations.

The antennas are equipped with three receiver bands, observing in dual polarization and two sidebands in the 3 mm, 2 mm, and 1.3 mm atmospheric windows, respectively. The 3 mm receiver band covers sky frequencies between 71 and 119 GHz, the 2 mm receiver band covers sky frequencies between 127 to 182 GHz, and the 1.3 mm receiver band covers sky frequencies between 197 and 276 GHz. Typical receiver noise temperatures range from 25 K to 45 K, at 3 mm, from 35 K to 55 K at 2 mm, and from 40 K to 70 K at 1.3 mm. More details about the receivers are given in Sect. , a comprehensive summary can be found in Table .

The dual polar, 2SB receivers deliver four IF outputs (one per polarization and one per sideband), each 7.744 GHz wide. These are transmitted by optical fibers to the central building. An IF processor further splits each of the four IF outputs into two 3.872 GHz wide basebands per sideband and polarisation which are called outer and inner basebands. These eight basebands are then fed into the correlator (see also Figure ).

The wide-band correlator PolyFiX, in its low spectral resolution mode with a 2 MHz channel spacing, can process a total instantaneous bandwidth of 31GHz for up to twelve antennas (see Sect. for more details). The 31 GHz are thereby split up over the two sidebands and polarisations. In addition, a high spectral resolution mode with a channel spacing of 62.5 kHz is also available for a large number of spectral windows that can be placed in both sidebands and polarisations. Further modes will become available in the future.

A 64-bit Linux computer and several embedded processors control the interferometer and acquire the data. The user interface, OBS , is a variant of PaKo, familiar to most users of the IRAM 30 m telescope.

Each interferometer configuration, i.e. the placement of the 10 antennas on predefined stations within the array, provides 45 baselines simultaneously. For each configuration, a list of projects is provided to the on-site operator and the astronomer on duty, allowing for a flexible scheduling of the observations well adapted to the actual meteorological conditions. Depending on the weather and on the season, 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) for the 10D configuration, as produced by the ASTRO UV_TRACK command. The shaded circle at the center of the uv-plane shows the short spacing region that cannot be observed with the array.