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50 Hexapod 6-DOF Alignment Systems for the Sub-Reflectors of ALMA, the Largest Telescope Project in the World


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50 Hexapod 6-DOF Alignment Systems for the Sub-Reflectors of ALMA

The ALMA (Atacama Large Millimeter Array) observatory is the largest and most advanced astronomical telescope system in the world. ALMA is not a classical optical telescope but a combination of up to 66 radio-telescope dishes receiving electromagnetic radiation at millimeter and sub-millimeter wavelength.  The combination of many antennas, spread out over a large area, has a similar effect as a much larger single antenna, but at a lower cost. The individual antennae are spread between 150 m and a maximum of 12 km.

This video above explains the construction of the ALMA telescope.
The PI Hexapod is shown around 0:38.

The antenna array is located in the Atacama Desert of northern Chile at an altitude of 5,000 meters. These are favorable conditions for the best possible observations, since millimeter radiation is absorbed by water vapor in the atmosphere and humidity levels at this altitude are extremely low, minimizing the attenuation mm and sub-mm wavelength radiation. In its final state, there will be a combination total of 66, 39ft and 23ft antennas. A typical radio telescope antenna consists of a main reflector (the large dish) and a secondary, sub-reflector that needs to be aligned properly in regard to the main dish for maximum efficiency. This precision alignment task is carried out by special Hexapod 6-axis parallel positioning systems from PI. It gives insights into the birth of new galaxies, stars, and planets, but also into star systems of the early universe that are more than 13 billion light years away.

Technological demands here are high: low air pressure, temperature differences of up to 50°C, strong winds, dust and rain are additional challenges in 5,000 meters above sea level that have to be faced.

One of the challenges for designing the high precision systems are the adverse conditions with low pressure, high winds, and huge temperature swings between day and night, of up to 90°F  (50°C).

To keep the antenna performance at its optimum, the sub-reflectors even out external impacts on the mechanical system. Deviations can occur, for example, with antenna tracking that compensates for global rotation, with bending of telescope components due to gravitation, or through thermal effects or wind load.

The sub-reflectors are mounted on the hexapods which provide motion with 6 degrees of freedom. The PI Hexapod combines a load capacity of 200 kg with sub-micron linear resolution and microrad-level angular resolution. A digital motion controller provides advanced features, such as a user-programmable virtual pivot point, critical in complex alignment applications. Target positions in 6-space are commanded in Cartesian coordinates and reached by smooth vectorized motion. Hexapod parallel kinematics are lighter, more compact, and stiffer than conventional serially stacked multi-axis motion systems. The effect is a higher natural frequency with improved dynamics, faster step & settle, and higher stability.


PI (Physik Instrumente) L.P.
16 Albert Street
Auburn , MA 01501
United States

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