Description
Keywords: noema-intro

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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 2560m 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 15m diameter,

- baselines ranging up to 760m

- a high-performance wide-band correlator named PolyFiX

- dual polarization receivers covering the 3mm, 2mm and 1.3mm 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 368m long, and the east-west oriented arm extends 216m west and 544m 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 8m 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 15m diameter Cassegrain telescope constructed largely of carbon fiber. The primary mirrors have a surface accuracy below 50m 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 3mm, 2mm, and 1.3mm atmospheric windows, respectively. The 3mm receiver band covers sky frequencies between 71 and 119GHz, the 2mm receiver band covers sky frequencies between 127 to 182GHz, and the 1.3mm receiver band covers sky frequencies between 197 and 276GHz. Typical receiver noise temperatures range from 25K to 45K, at 3mm, from 35K to 55K at 2mm, and from 40K to 70K at 1.3mm. 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 7.744GHz 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 30m 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.

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