Study of the Dynamics of Large Reflector Antennas with Accelerometers

The Atacama Large Millimeter Array (ALMA) will consist of up to 64 state-of-the-art sub-mm telescopes, subject to stringent performance specifications that will push the boundaries of the technology, and makes testing of antenna performance a likewise challenging task. Two antenna prototypes were evaluated at the ALMA Test Facility at the Very Large Array site in New Mexico, USA. The dynamic behavior of the antennas under operational conditions was investigated with the help of an accelerometer system capable of measuring rigid-body motion of the elevation structure of the antenna, as well as a few low- order deformation modes. This resulted in dynamic performance numbers for pointing stability, reflector-surface stability, path-length stability, and structure flexure. Special emphasis was given to wind effects, one of the major factors affecting performance on timescales of seconds to tens of minutes. Although the accelerometers could not directly measure antenna performance on timescales longer than 10 seconds, it was possible to use them to derive antenna properties that allowed extrapolation of the wind-affected performance to timescales of 15 minutes and longer. This paper describes the accelerometer system, its capabilities and limitations, and presents the dynamic performance results of the two prototype antennas investigated. In addition to verification of the performance requirements, we investigated the vibration environment on the antennas, relevant for vibration-sensitive equipment for use on the ALMA antennas; the lowest eigenfrequencies for the antennas; and the sensitivity to vibration generated by similar antennas operating nearby. This work showed that seismic accelerometers can successfully be used for characterization of antenna dynamics, in particular when complemented with simultaneous wind measurements and measurements for static performance. The versatility of the accelerometers also makes them a valuable tool for troubleshooting of unforeseen antenna features.