Mobile astronomy

25 April 2008

Each ALMA transporter will be 10 m wide, 4.5 m high and 16 m long

Each ALMA transporter will be 10 m wide, 4.5 m high and 16 m long

In a joint effort between Europe, North America and Japan, in co-operation with Chile, a new astronomy facility will be built on the Llano de Chajnantor site in the Atacama Desert of Northern Chile. The completed Atacama Large Millimetre Array project (ALMA) will comprise 64 radio antennas, which can be positioned on 250 specially prepared foundations separated by baselines of up to 18 km over an area of about 5,600 square metres. Initially, 25 antennas have been ordered with an option for another seven. The new facility is expected to start operations by 2010-2011.

Before then, however, the antennas have to be moved to site. Also part of the project is the construction of a central assembly and service station, located at an altitude of 2,900 m. The disassembled antennas will arrive there from different manufacturing locations around the world. Initially the service station will also facilitate antenna assembly. After completion and testing the Scheuerle-built ALMA Antenna Transporters come in to play.

Transporter design

The basic transporters will be about 16 m long, 10 m wide and up to 4.5 m high. Each transporter will weigh about 150 tonnes. They will have four pendulum axles at the front and rear, two axles per line. Each axle will have two large tyres. Hydrostatically driven motors inside the wheel hubs provide the traction, powered by two 500 kW diesel engines mounted in power packs at the rear of the transporters. In this way the combined output will be 1,000 kW or the equivalent of 1,340 hp.

The power packs also run a set of arms capable of loading and discharging a fully assembled 12 m diameter antenna weighing about 110 tonnes. The hydraulically activated arms, mounted at both sides of a transporter, pick and lift an antenna over the front and position the base of the antenna on a lowbed between both sets of axles. In this way the centre of gravity can be kept as low as possible.

When loaded, the transporters have to travel from the service station, already at an altitude of 2,900 m, to the site on a plateau at 5,000 m. A specially prepared road through the mountains requires a journey of about 28 km, comprising gradients up to 7%.

The extreme altitude is also a factor. The air contains less oxygen by volume, reducing efficiency of the diesel engines by some 50%, and requiring people to take extra precautions. On site the antennas have to be discharged and accurately positioned on the docking paths in a pattern as predetermined by the astronomers.

When the initial 25 antennas are in place they will work as one giant telescope. The Scheuerle ALMA Antenna Transporters will also be used to move the antennas in to other configurations, allowing for different observing modes, comparable with a zoom lens on a camera, offering different degrees of resolution and sky coverage as needed by the astronomers.

The first transporter is scheduled to be delivered to San Pedro de Atacama in Chile in 2007 to match the delivery of the first antennas to Chajnantor.

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