  The Atacama Large Millimeter Array (ALMA) will consist of up to 64
state-of-the-art sub-mm telescopes, subject to stringent performance
specifications which 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 behaviour 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, resulting 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. This paper describes the accelerometer system, its
capabilities and limitations, and presents the dynamic performance results of
the two prototype antennas investigated.

Astrophysics