The VertexRSI design

The VertexRSI antenna is shown in Fig. 7.21. The mount is traditional, made from steel, covered with thermal insulation on the outside. The base provides a three-point

Fig. 7.21. Prototype 12 m diameter antenna for ALMA, designed and built by VertexRSI. Note the curved quadripod to minimise the spherical wave aperture blocking. (N.Emerson, NRAO/AUI)

connection to the foundation, from which it can quickly be released for transportation of the antenna to another baseline station. The fork of the alidade runs on a roller bearing of 2.5 m diameter. The elevation structure rotates on two bearings in the top of the yoke, providing a large receiver room directly behind the BUS. A receiver box of 1 m3 can be accommodated in this room. The elevation structure is made of steel, thermally insulated on the outside. The connection from the receiver cabin to the BUS is realised in invar steel to absorb the large difference in thermal expansion between the steel of the receiver cabin and the CFRP of the BUS.

The BUS is a box structure made from composite plates consisting of CFRP skins bonded to aluminium honeycomb cores. There are 24 sectors providing support for the surface panels. Vertex has not used their proven space-frame solution, pioneered with the HHT, thus avoiding the expensive fabrication of the nodes in CFRP. Although this was not exercised for one prototype, the box structure lends itself quite well to the use of assembly templates in the series fabrication of the antennas. Consequently the choice of the box structure undoubtedly is the cheaper solution. These structures can be analysed well by modern finite element methods and the deviation from homology in this case is only 5 mm rms. As we have seen before, apart from the lower weight, the CFRP BUS exhibits a very good thermal behaviour.

As clearly visible in Fig. 7.21, the quadripod support of the secondary reflector is strongly curved. This tends to decrease the spherical-wave blocking (see Sec.4.5). It was first used in the satellite ground station at Raisting, Germany in 1964. In the ALMA design, the Vertex group has made the quadripod part of the load-bearing structure, which results in a higher stiffness of the BUS-quadripod combination. The geometrical blocking percentage lies just under 3 percent, as specified.

The reflector surface is composed of 8 rings of aluminium panels, for a total of 264, each about 0.5 m2in area. The surface is machined to an rms accuracy of 6 mm on an accurate milling machine. The relatively small size of the panels is caused by the limited area coverage of the milling machine. Because the observation of the Sun for sizeable periods of time is a requirement, the reflector surface must be specially treated to avoid concentration of solar heat on the subreflector and the secondary focus. Vertex tried several ways of scattering the solar visible radiation by the slight grooves of the cutting edge, but they proved unsatisfactory because of the "gratinglike" reflection patterns. The final solution is chemical etching of the surface, which gives it a slight roughness at the 1 mm level, which scatters the visible light sufficiently.

The drives are pinion and gear-rack systems powered by dual dc-motors in anti-backlash configuration. To meet the very high pointing requirements Vertex includes a metrology system in the mount to determine the pointing errors which are not sensed by the angle encoders. It consists of an independently supported CFRP structure in the forkarms which is connected through displacement sensors to the elevation bearings. Together with a set of temperature sensors in the steel part of the antenna these should help to correct for pointing changes due to wind and temperature variations.

The total weight of the VertexRSI prototype ALMA antenna is 108 tonnes, equally divided over the azimuth and elevation sections.

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