>120 ---

A cross-section drawing of the antenna is shown in Fig. 7.15. The mount is traditional and made of steel. The structural optimisation included the steel section of the elevation structure, the CFRP BUS and the CFRP quadripod simultaneously. The cone-section of the elevation structure terminate in a flat disc and the CFRP BUS is supported by 24 steel blades ("knife-edges") placed along the outer circumference of the disc. The new aspect of this design is the optimisation of the structure for several loading conditions simultaneously. The design team succeeded beautifully and the specifications were significantly surpassed for most conditions.

The fabrication of the reflector panels also involved some new aspects. MAN Technologie had obtained considerable experience in the replication of CFRP composite panels from steel molds for the antennas of the IRAM interferometer. Our specification of 7 mm manufacturing accuracy was however more than twice as severe. The panels are composed of an aluminium honeycomb core to which CFRP plates are bonded on both sides. The parabolic form is obtained by lay-up of the CFRP prepreg on the curved mold and the panel on the mold is cured at high temperature to obtain its final shape. Two new elements enabled us to achieve the specified goal. For the molds we used pyrex glass, ground to 3 mm rms accuracy at the Optical Sciences Center of the University of Arizona. The design of the composite panel aimed at a thermal expansion coefficient of 3 • 10-6 K-1, equal to that of the mold material. This would minimize the introduction of stresses during the heating and cooling procedures of the fabrication. However, a slight warping of the panel after release from the mold could be expected. These were corrected in the final surface measurement by the existence of 5 or 6 adjusters per panel. Small scale errors within the panel surface were removed by a second lay-up on the mold in which the reflecting aluminium foil of 40 mm thickness was bonded to the panel in a cold-curing process. A consequence of this procedure is that the physical temperature of the panels must stay below 50 C. This is not a problem at the site which is at 3200 m elevation, but it required caution during storage over a Tucson summer where air temperatures of more than 40 C are not uncommon.

The overall performance of the antenna, along with the major specifications, is summarised in Table 7.4. All specifications have been surpassed and the final overall rms surface error is about 12 mm, the lowest value achieved to date with a 10 m diameter antenna. The surface measurement was done with holography, using the LES 8 satellite at a frequency of 37 GHz (Baars et al., 1999).

Table 7.4. HHT specifications and achieved performance Error component Specification Achieved

Homology imperfection, assembly

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