Platform Deformation Refined Pointing and Phase Correction for the AMiBA Hexapod Telescope

TL;DR

This paper presents a refined method for correcting platform deformation effects on pointing and phase errors in the AMiBA hexapod telescope, enhancing its accuracy for cosmological observations.

Contribution

It introduces an analytical deformation model and calibration techniques to improve pointing and phase accuracy for the upgraded 13-element AMiBA array.

Findings

Deformation affects optical pointing and radio phase, measurable via optical telescopes.

Calibration can recover up to 90% of the ideal beam shape.

Modeling reduces array efficiency loss due to deformation.

Abstract

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.