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World’s largest mirror telescope relies on nanometer accuracy inductive sensors

03 July 2017

The European Southern Observatory based in Garching, Germany, is relying on high precision displacement measurement systems from Micro-Epsilon for a project to build the world’s largest mirror telescope in Chile, which also involves French cooperation partner Fogale.

The European Extremely Large Telescope (E-ELT) is a project of the European Southern Observatory (ESO) (Image courtesy of ESO)

The inductive sensor systems used for displacement measurement are the most precise ever used in a telescope and determine the position of each single mirror segment in three axes to nanometer accuracy.

The construction of the largest, optical and near-infrared telescope in the world is progressing, with completion planned in 2024. The European Extremely Large Telescope (E-ELT) is a project of the European Southern Observatory (ESO) on the Cerro Armazones mountain in Chile. A major contract for this project involving several thousand high precision sensors has now been awarded to Micro-Epsilon in Germany.

The sensors play a crucial role in this research project. The telescope uses a unique, optical system with five mirrors and requires optical and mechanical elements that push modern technology to its limits. The diameter of the main mirror alone will be 39m – half the length of a football pitch. With a total surface area of 978m2, the mirror consists of 798 single segments, each 1.4m in size but only 5cm thick. These honeycomb segments must be aligned precisely to each other in order to form a perfect optical system. Their relative position can change due to outside interference from wind loads, fluctuating temperatures and, last but not least, gravity, which has different effects depending on the respective alignment of the E-ELT. The sensors provided by the FAMES consortium (Fogale and Micro-Epsilon), ensure exact positioning to nanometer accuracies. Measuring the relative position to nanometer accuracies therefore represents the fundamental core of this complex system. Predestined for outdoor applications, the sensors are ideal for long-term temperature stability, as well as their high resistance to external influences.

The major challenge in this large-scale project involving more than 5,000 inductive displacement measurement systems is to achieve the required measurement precision to nanometer accuracy under such difficult ambient conditions. Micro-Epsilon’s managing director Martin Sellen is satisfied with the ESO cooperation. He comments that after developing these high precision sensors for such an important project, the company can finally deliver the essential component for the world’s largest mirror telescope. The whole construction project pushes the boundaries of technical feasibility and so requires a longer planning period. 

Micro-Epsilon inductive sensors on world’s largest telescope

The sensors used are based on the principle of inductive coupling and Micro-Epsilon’s knowledge of eddy current measurement technology. The sensors measure on a wear-free, non-contact basis providing the highest precision and resolution. A special advantage of the sensor is its immunity to external influences such as dirt, pressure and humidity. The sensor consists of a transmitter coil and an opposite located arrangement of several receiver coils on the adjacent mirror segment. The transmitter coil is supplied with alternating current. The voltage induced by inductive coupling in the receiver coils depends on the position of the transmitter coil. The patented evaluation of the partial signals enables the determination of the position of the segments relative to each other in three axes. The special coil is designed in accordance with Micro-Epsilon’s Embedded Coil Technology (ECT) eddy current sensors, which differ from the wound coils found in conventional sensors. The coil itself is embedded in an inorganic carrier material. This design provides the sensors with an extremely high temperature rating and long-term stability, as well as good repeatability.

Building such an enormous telescope fulfils an important role in ground-based astronomy and should expand astrophysical knowledge significantly. The next generation E-ELT telescope enables the exploration of red-shifted galaxies, star formation, exoplanets and protoplanetary disks. With this project, ESO wants the E-ELT telescope to revolutionise the exploration of the universe with its gigantic main mirror and adaptive optics (AO) technology – just as Galileo did 400 years ago, when he was the first to turn a telescope towards the sky. The telescope may help to answer some major scientific challenges of our time. 

The telescopic system has a total weight of 2,800 tonnes and can rotate through 360°. Compared to today’s high-end telescopes, the E-ELT telescope will be four to five times larger and receive 15 times more light. It will capture 100 million times more light than the human eye, 8 million times more than Galileo’s telescope, and in total, more light than all existing 8-10m telescopes on Earth.

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