Drive Technology for the World’s Largest Telescope by Physik Instrumente

Drive Technology for the World's Largest Telescope by Physik Instrumente

The European – Extremely Large Telescope, in short ELT, of the European Southern Observatory (ESO) has a segmented main reflector with a diameter of 39 m, a light collecting area of almost 1,000 m², and is the largest telescope for scientific evaluation of electromagnetic radiation in the visible and near infrared wavelength range. It will be put into operation atop the 3,046-meter high Cerro Amazones in the Chilean Atacama desert during 2024. One of the most important tasks of the telescope is to help us to find out more about exoplanets, i.e., planets that exist beyond the solar system.

Project Requirements

The main reflector will be made up of 798 individual mirror elements each with a diameter of 1.40 meters. Altogether, the segments will collect several tens-of-millions more light than the human eye is capable of and transfer this light into scientific instruments via further optics. To compensate for deviation from the optimum beam path and therefore avoid imaging errors, the mirror elements must be aligned exactly to each other. Deviation from the optimum beam path can result for example, from distortion of the telescope tube because of gravitation, thermal effects or wind pressure.

The main reflector (M1) is made up of 798 mirror segments.( ESO/L. Calçada/ACe Consortium)

Each mirror segment is positioned by three drives. The requirements push the technology to its limits: Relatively long travel ranges of up to 10 mm with a position and path accuracy of better than 2 nm are the challenges that need to be overcome when developing the actuators. Tracking an object during observation typically requires velocities of between several nanometers per second and +/- 0.45 µm/s. The average position deviation may not be more than 2 nm. If the telescope is to be aimed at a different object, velocities of up to +/- 100 µm/s are required. This involves moving considerable weights: A mirror segment weights 250 kg.

Due to the different alignments of the telescope, an individual drive has to move or hold loads between 463 N pulling force and 1050 N pushing force. Winds or an earthquake can cause these loads to be exceeded considerably, but the drives would then not be in an active operational state. Furthermore, the very high demands on the lifetime of the systems represents a considerable technical risk, which needs to be considered when realizing the project and the same applies to the maximum permissible exhaust heat for actuators and controllers. PI “customized” a hybrid drive for this very demanding task.

Signing the contract with PI at the ESO headquarters in Garching. After a longstanding cooperation, the ESO placed the order on June 19, 2017 for production of the actuators for aligning the mirror segments. (Source: ESO / M. Zamani)

For more information on ESO and on the signing of the contract click >> here