|
The Atacama Large Millimeter/Submillimeter Array (ALMA) is a joint project between astronomical organizations in
Europe, North America, and East Asia, in collaboration with the Republic of Chile. ALMA will consist of at least 54
twelve-meter antennas operating in the millimeter and sub-millimeter wavelength range. It will be located at an altitude
above 5000m in the Chajnantor Plateau in northern Chile.
There are 192 antenna foundations under construction at ALMA's Array Operations Site (AOS). Interchangeability
between foundations will permit a variety of array configurations. Foundations provide the physical interface to the
bedrock, as well as to the underground signal and power cable conduits. To achieve ALMA's precision requirements, the
antenna pointing angular error budget is strict with anticipated non-repeatable error on the order of a few arc seconds.
This level of precision imposes rigorous requirements on antenna foundations.
The objective of this study is to demonstrate the methodology of precision tilt measurements combined with finite
element simulation predictions to portray the qualitative nature of the antenna foundation surface deformation.
Characteristics of foundation surface tilt have been examined in detail. Although the actual foundation has demonstrated
much less resistance to tilt than the finite element representation, the simulation has predicted some key characteristics
of the tilt pattern. The large deviations from the ideal have incited speculations into the compliance of materials,
ambiguities in the construction, thermal effects and several other aspects described herein. This research has served as
groundwork to characterize ALMA's foundation surface behavior on a micro-degree level and to identify subsequent
studies to pursue. This in turn has contributed to the diagnosis of antenna pointing anomalies.
|
