ALMA Antennas

The antennas are central to the ALMA project. Their quality and performance define the overall functionality of ALMA. The specifications of each antenna are 2 arcsec absolute pointing over the whole sky, 0.6 arcsec tracking, and a 25 micrometre r.m.s. surface accuracy. These are very tight specifications for radiotelescopes fully exposed to the harsh weather environment at 5000m altitude.

In view of the difficulties in fullfilling these requirements, prototype antennas were supplied by three companies: the AEC Consortium (procured by ESO), Vertex RSI (procured by NRAO for North America) and Mitsubishi Electrical Company (procured by NAOJ, Japan). All three prototypes were extensively tested at the ALMA Test Facility in Socorro, New Mexico. Several groups of international experts, both internal and external to ALMA, reviewed the performance of the prototype antennas and concluded that on the basis of the tested functioning their expected performance at the ALMA site conform to the technical requirements.

The North American partners of the ALMA project, through AUI, signed a contract to supply up to 25 antennas, with options to increase to 32 antennas, with Vertex RSI on July 11, 2005. On December 6, 2005, the ESO Director General signed a contract with the AEM (Alcatel Alenia Space France, Alcatel Alenia Space Italy, European Industrial Engineering S.r.L., MT Aerospace) Consortium for the supply of 25 ALMA antennas, with options to increase the number of antennas to 32. The four antennas of 12 metres diameter and the twelve antennas of 7 metres diameter to be provided by Japan, have been ordered from Mitsubishi Electrical Company.

The first antennas supplied by Vertex RSI have been delivered in Chile during the second half of 2007. The first antenna to be supplied by the AEM Consortium is expected by the third quarter of 2008. Despite the later delivery of the first AEM antenna, both suppliers are expected to deliver their 25th antenna by the end of 2011. This is due to a higher delivery frequency of AEM antennas. The four Japansese 12 metre antennas have also been delivered at the Operations Support Facility (OSF).

Manufactoring of the steel structure of the AEM antennaThe Vertex antenna at the Operations Support Facility

The ALMA Antenna Transporters

The ALMA antennas will be operating at an altitude of 5000 metres. The antenna array can be reconfigured in order to achieve the imaging requirements by relocating antennas on the stations at the Array Operations Site (AOS). There is a compact configuration in which all antennas operate within an area of 160 x 250 metres, and there is an expanded configuration for which the maximum separation between antennas reaches about 15 km. In order to move antennas, each weighing more than 100 tons, the ALMA project has designed a special, dedicated transport vehicle. Two units have been manufactured. These transporters are truly unique. They will first move antennas from their assembly area, the OSF (2900 metres) to dedicated positions at the AOS (5000 metres).

After the initial move from the OSF they will move antennas around on the AOS - to compact or expanded configurations - and position the antennas to an accuracy of a few millimetres. In addition, these transporters shall also move antennas for extended maintenance and repair from the AOS to the OSF.

The weight of the antennas, their high precision and the hostile, high altitude environment impose severe boundary conditions on the transport vehicles. They have a weight of about 150 tons, and their dimensions are about 10 m x 15 m x 6 m (width x length x height). The two transporters have been delivered to the OSF in early 2008.