Micro-Epsilon’s displacement measurement sensors to be used in European telescope project

The European Southern Observatory based in Garching, Germany, is relying on 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 inductive sensor systems used for displacement measurement 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 and will be second to none in size. A major contract for this project involving several thousand high precision sensors has now been awarded to Micro-Epsilon in Germany. With more than 6,000 engineer-years of experience and 120 patents, Micro-Epsilon has long-term knowledge in high precision sensor technology. The globally operating sensor manufacturer employs around 1,000 people in the Group, with almost 400 at its Lower Bavarian headquarters.

The sensors for the giant telescope, supplied by Micro-Epsilon, 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 39 metres – half the length of a football pitch. With a total surface area of 978 square metres, the mirror consists of 798 single segments, each 1.4 metres in size but only 5 centimetres 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. Micro-Epsilon is responsible for manufacture of the sensors. Measuring the relative position to nanometer accuracies therefore represents the fundamental core of this complex system.

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.

The sensors used are based on the principle of inductive coupling and Micro-Epsilon’s
knowledge of eddy current measurement technology. 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. The coil itself is embedded in an inorganic carrier material.

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. Do other Earth-like planets exist that we could live and survive on? What are the characteristics of the first stars and galaxies, dark matter, dark energy and early Black Holes?

The telescopic system has a total weight of 2,800 tonnes and can rotate through 360 degrees. 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 8-10 metre telescopes on Earth.


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