The Netherlands builds support structure for the world’s largest telescope

TNO designs structure for the main mirror of the Extremely Large Telescope (ELT) • 19 Apr 2018

VDL ETG Projects, part of VDL Groep, will build the support structure for the main mirror of the Extremely Large Telescope (ELT) of ESO (European Southern Observatory) in northern Chile. The design was realised together with TNO, supported by NOVA (Netherlands Research School for Astronomy). The support structure consists of 798 supports for mirror segments, which together form the main mirror (with a diameter of more than 39 meters) of the telescope. This order, which will be delivered over a period of about five years, is worth tens of millions of euros. This represents a special achievement by Dutch industry.

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Space & Scientific Instrumentation

At the headquarters of the European Southern Observatory in Garching near Munich in Germany, ESO Director-General Xavier Barcons and CEO Willem van der Leegte of VDL Groep today signed the contract for the order.

Kees Buijsrogge, TNO Director of Space and Scientific Instrumentation: "TNO's ambition is to develop innovative space and scientific instruments and systems together with industry, users and other knowledge institutes. To this end we use our experience in the field of optical instrumentation and precision mechanics for the aerospace, astronomy and semiconductor industry for the design and testing of structures that have to be positioned on a nanometre scale (one millionth of a millimetre). We are therefore proud to have contributed to this very attractive assignment for Dutch industry."


The ELT is already being called the world's biggest eye on the sky. The size of the ELT is comparable to that of a football stadium. The 'eye' of the telescope is almost as large as half a football pitch and will absorb more light than all the large, professional optical telescopes of this moment do together. The ELT will make scientific discoveries possible in the area of planets, the composition of nearby galaxies and the deep universe. The hexagonal, mechatronic support structure keeps the main mirror of the telescope in the desired shape, with nanometre precision. VDL ETG Projects is responsible for building these structures while the design has been realised together with TNO, supported by NOVA (Netherlands Research School for Astronomy). The main mirror with its adaptive control system will therefore be the largest telescope for observations in visible light.

It is impossible to make the main mirror of 39 meters in one piece, so the mirror is made up of 798 hexagonal mirror segments of approximately 1.4 metres in size and 5 centimetres thick. Each mirror segment has its own supporting structure and is driven by several electric motors, which means that the mirror can be positioned very accurately. The continuous correction is done by 9 electric motors on each frame and 3 actuators under each frame, which contains a piezo motor that enables precision setting to the nanometre. Since mirrors must be replaced regularly to be given a new reflective layer, a total of almost 950 mirror segments and support structures are being built.

Over one billion euros has been made available for the realisation of the ELT. The construction of the foundation of the ELT has already started. The telescope should be operational in 2024.

Dutch cooperation

TNO is the world leader in optical and optomechatronic systems for space and astronomy. In 2010 TNO made the first prototypes for the support structure for the telescope. Four years later, in 2014, VDL and TNO won a contract with NOVA for the design of the final prototypes of the load-bearing structures and the construction of six frames.

VDL ETG, also with extensive experience in the semiconductor industry, analytics and Science & Technology segment, has particular expertise to optimise precision mechatronics system designs for cost control, reliability and maintenance. All this goes together to create the final design for economically feasible, mass production manufacture.

NOVA, the collaborative association of astronomers from Dutch universities, has specialist knowledge of the use of large telescopes and the optimisation of the interaction between the telescope and the scientific instrumentation behind the telescope. Together they ensure that ESO is disposed of optimum synergy between the key disciplines referred to above. Today's order, awarded to VDL ETG Projects, therefore includes the series production of the support structure.

The European Southern Observatory (ESO)

ESO is the most important intergovernmental astronomical organisation in Europe and by far the most productive observatory in the world. This European Southern Observatory is supported by fifteen Member States: Belgium, Denmark, Germany, Finland, France, Italy, the Netherlands, Austria, Poland, Portugal, Spain, the Czech Republic, the United Kingdom, Sweden and Switzerland, and by the host country Chile. ESO is implementing an ambitious programme aimed at designing, building and managing large observatories that enable astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in the field of astronomy. ESO manages three world-class observation locations in Chile: La Silla, Paranal and Chajnantor. At Paranal is ESO's Very Large Telescope (VLT), the most advanced optical observatory in the world, and two survey telescopes. VISTA works in infrared and is the largest survey telescope in the world and the VLT Survey Telescope is the largest telescope specifically designed to map the sky in visible light. ESO also plays an important role as partner at ALMA, the largest astronomical project of the moment. And at Cerro Armazones, near Paranal, ESO is building the 39-meter Extremely Large Telescope, the ELT, which will be 'the biggest eye on the sky' in the world. The headquarters of ESO is located in Garching near Munich in Germany


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