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The Array for Microwave Background Anisotropy (AMiBA) is a radiointerferometer for research in cosmology, currently operating 7 0.6m diameterantennas co-mounted on a 6m diameter platform driven by a hexapod mount. AMiBAis currently the largest hexapod telescope. We briefly summarize the hexapodoperation with the current pointing error model. We then focus on the upcoming13-element expansion with its potential difficulties and solutions.Photogrammetry measurements of the platform reveal deformations at a levelwhich can affect the optical pointing and the receiver radio phase. In order toprepare for the 13-element upgrade, two optical telescopes are installed on theplatform to correlate optical pointing tests. Being mounted on differentlocations, the residuals of the two sets of pointing errors show acharacteristic phase and amplitude difference as a function of the platformdeformation pattern. These results depend on the telescope's azimuth, elevationand polarization position. An analytical model for the deformation is derivedin order to separate the local deformation induced error from the real hexapodpointing error. Similarly, we demonstrate that the deformation induced radiophase error can be reliably modeled and calibrated, which allows us to recoverthe ideal synthesized beam in amplitude and shape of up to 90% or more. Theresulting array efficiency and its limits are discussed based on the derivederrors.

Platform deformation refined pointing and phase correction for the AMiBA hexapod telescope