Relying on high precision displacement measurement systems to build the world’s largest mirror telescope

Written by: Eureka! | Published:

The construction of the largest, optical and near-infrared telescope in the world is progressing, with completion planned for 2024. The revolutionary 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 been awarded to sensor innovators Micro-Epsilon.

The sensors for the giant telescope play a crucial role in the research project with the telescope using a unique, optical system that incorporates five mirrors. It requires optical and mechanical elements that push modern technology to the limit.

The diameter of the main mirror alone will be 39m and 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 in order to form the perfect optical system. Their relative position can change due to outside interference from wind loads, fluctuating temperatures and even gravity, which has different effects depending on respective alignment.

The sensors provided by the FAMES consortium (Fogale and Micro-Epsilon), ensure exact positioning to nanometre accuracies.

Micro-Epsilon is responsible for the manufacture of the sensors, which are the most precise ever used in a telescope. Predestined for outdoor applications, the sensors stand out due to their long term temperature stability, as well as their high resistance to external influences.

The major challenge in the project involving more than 5,000 inductive displacement measurement systems is to achieve the required measurement precision under such difficult ambient conditions.

Micro-Epsilon’s managing director Martin Sellen, says: “The whole construction project pushes the boundaries of technical feasibility and so requires a longer planning period. In addition to economic importance, we can also bring our knowledge into international top-level research.”

Design Principles

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 several receiver coils placed opposite on the adjacent mirror segment. 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 significantly from the wound coils found in conventional sensors. The coil itself is embedded in an inorganic carrier material. Their innovative design provides the sensors with an extremely high temperature rating and long term stability, as well as excellent repeatability.

Project goals

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 of the big questions in astronomy such as if Earth has a life-capable sister planet elsewhere in the galaxy and what is dark matter?

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 8-10m telescopes on Earth.


Universal instruments

The European Extremely Large Telescope (E-ELT) will have a number of scientific instruments that will enable it to analyse different parts of the distant universe in different ways. Among them are:

The High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI): A light instrument installed on

the Extremely Large Telescope (ELT) for spectroscopy in the wavelength range 0.47–2.45µm. The versatile instrument will offer a set of spatial scales to optimise observations for a wide range observing conditions.

The Multi-conjugate Adaptive Optics RelaY (MAORY):An adaptive optics module based on Multi-Conjugate Adaptive Optics (MCAO) — designed to help compensate for distortions caused by turbulence in the Earth’s atmosphere. MAORY is designed to work with the imaging camera MICADO, which needs stable and sharp images across a large field of view in the near-infrared (wavelengths from 0.8–2.4µm) in order to make very precise measurements of the positions, brightness, and motions of stars.

The Mid-infrared ELT Imager and Spectrograph (METIS): A powerful spectrograph that will make full use of the giant 39m main mirror and focus exoplanets, proto-planetary disks, Solar System bodies, active galactic nuclei, and high-redshift infrared galaxies.

It will also study the Martian atmosphere and the centre of the Milky Way.

Multi-Adaptive Optics Imaging Camera for Deep Observations (MICADO): A diffraction limited imaging instrument capturingnear-infrared wavelengths. The design was driven by a desire for high sensitivity and resolution, astrometric accuracy, and wide wavelength coverage spectroscopy. MICADO will also be a uniquely powerful tool for exploring environments where gravitational forces are extremely strong, such as close to the supermassive black hole at the centre of the Milky Way.



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