Platform Deformation Refined Pointing and Phase Correction for the AMiBA
  Hexapod Telescope
  The Array for Microwave Background Anisotropy (AMiBA) is a radio
interferometer for research in cosmology, currently operating 7 0.6m diameter
antennas co-mounted on a 6m diameter platform driven by a hexapod mount. AMiBA
is currently the largest hexapod telescope. We briefly summarize the hexapod
operation with the current pointing error model. We then focus on the upcoming
13-element expansion with its potential difficulties and solutions.
Photogrammetry measurements of the platform reveal deformations at a level
which can affect the optical pointing and the receiver radio phase. In order to
prepare for the 13-element upgrade, two optical telescopes are installed on the
platform to correlate optical pointing tests. Being mounted on different
locations, the residuals of the two sets of pointing errors show a
characteristic phase and amplitude difference as a function of the platform
deformation pattern. These results depend on the telescope's azimuth, elevation
and polarization position. An analytical model for the deformation is derived
in order to separate the local deformation induced error from the real hexapod
pointing error. Similarly, we demonstrate that the deformation induced radio
phase error can be reliably modeled and calibrated, which allows us to recover
the ideal synthesized beam in amplitude and shape of up to 90% or more. The
resulting array efficiency and its limits are discussed based on the derived
errors.
