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Largest optical telescope in the world driven by 2,000+ PI actuators

Three PI actuators per panel control the motion of 798 quarter-ton mirror segments on the E-ELT, the world's largest telescope.

 

 

High atop the Cerro Armazones mountain in the harsh but extremely dry environment of the Chilean Atacama Desert, more than 2,000 custom hybrid actuators will adjust 798 quarter-ton segments in the primary mirror of the European Extremely Large Telescope (E-ELT). As a leader in precision motion control solutions, PI (Physik Instrumente) designed a new type of actuator that would meet the extreme requirements and win the European South Observatory (ESO) contract.

Hybrid actuator concept solves the nanometer-precision, high-load/low-heat conundrum
PI's ultra-precision actuator was developed to meet requirements such as tracing performance better than 2-nm RMS, force variation 0N to 900N, 15-mm travel, and minimized heat dissipation. Several concepts were simulated and tested before a winner was picked. The best results were achieved with a hybrid design based on the combination of a piezo actuator, brushless motor, linear encoder, and a hybrid controller to drive both the brushless motor and piezo actuator simultaneously based on the position feedback of an optical position sensor providing sub-nanometer resolution. Three actuators are required per segment.

Click on the image above to see the full labelled design schematic for the E-ELT.

 

 

Why is this precision required?
In order to perform similar to one 39-m mirror (it is not feasible to make a single mirror of that size), all segments have to be aligned to each other to within nanometers all the time. The exact position of the mirror segments is influenced by several factors such as wind load, distortions, and vibration from the telescope structure; temperature fluctuations; and varying gravity effects during tracking motions of the telescope. The smallest alignment errors between the mirror segments will degrade the optical performance of the telescope.

How large is extremely large?
Touted as the largest telescope in the world, the 39-m-diameter E-ELT's individual hexagonal mirrors measure 1.4 m across and weigh about 500 lb each. The large aperture directly determines the light-collecting area, which in turn defines the exposure time. More area means that, for a given amount of exposure time, the telescope can detect more distant (fainter) stars and look deeper into the universe.

Artist's impression of the European Extremely Large Telescope. [Credit: ESO/L. Calcada]

 

 

With its total surface area of close to 1,000 m², the E-ELT is supposed to collect around 100 million times more light than the human eye and boasts about 250 times the light-gathering area of the Hubble Space Telescope.

The E-ELT is scheduled to see its first light in 2024.

Want more details on this unique and impressive actuator system? Read the in-depth PI blog, "Ultra-High Precision Positioning Actuators to Align Mirror Segments in the E-ELT Telescope."

Standard and custom
PI has over four decades of experience providing in-house engineered precision motion control solutions, and can quickly modify existing product designs or provide a fully customized OEM part to fit the exact requirements of the customer's application.

Source: PI (Physik Instrumente)

Published September 2017

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