ESO Shouts Out Progress On Earth’s Most Powerful Telescope –“Unlock the Secrets of Alien Planets” (VIDEO)



The construction of the 39-meter Extremely Large Telescope (ELT), the largest optical/near-infrared telescope in the world on target for first light in 2024, is moving forward. The giant telescope employs a complex five-mirror optical system that has never been used before and requires optical and mechanical elements that stretch modern technology to its limits.

Contracts were signed for major components of the ELT for the casting of the telescope’s giant secondary and tertiary mirrors; the supply of mirror cells to support these two mirrors; and the supply of the edge sensors that form a vital part of the ELT’s huge segmented primary mirror control system. The secondary mirror will be largest ever employed on a telescope and the largest convex mirror ever produced.



The first two contracts cover the casting of the ELT’s largest single mirrors — the 4.2-meter secondary and 3.8-metre tertiary mirror — from low-expansion ceramic material Zerodur, originally developed for astronomical telescopes in the late 1960s. It has almost no thermal expansion, which means that even in the case of large temperature fluctuations, the material does not expand.

Chemically, the material is very resistant and can be polished to a high standard of finish. The actual reflective layer, made of aluminum or silver, is usually vaporized onto the extremely smooth surface shortly before the telescope is put into operation. Many well-known telescopes with Zerodur mirrors have been operating reliably for decades, including  ESO’s Very Large Telescope in Chile.




Hanging upside-down at the top of the telescope structure, high above the 39-metre primary mirror, the secondary mirror will be largest ever employed on a telescope and the largest convex mirror ever produced. As it is a highly convex, aspherical mirror, fabrication of the secondary is a considerable challenge and the result will be a truly remarkable example of precision optical engineering. As with many elements of the ELT it will be a genuine first in this area of technology. The total weight of the secondary mirror and its support system is 12 tons— and since it hangs over the primary great care must be taken to prevent the mirror from falling!

The concave tertiary mirror is also an unusual feature of the telescope. Most current large telescopes, including the VLT and the NASA/ESA Hubble Space Telescope, use just two curved mirrors to form an image. In these cases a tertiary mirror is sometimes introduced to divert the light to a convenient focus — that mirror is typically small and flat. However, in the ELT the tertiary also has a curved surface, the use of three mirrors delivering a better final image quality over a larger field of view than would be possible with a two-mirror design.

The ELT secondary and tertiary mirrors will rival in size the primary mirrors of many modern-day research telescopes and weigh 3.5 and 3.2 tons respectively. The secondary mirror is to be delivered by the end of 2018 and the tertiary by July 2019.

The third contract covers the provision of the sophisticated support cells for the ELT secondary and tertiary mirrors and the associated complex active optics systems that will ensure these massive, but flexible, mirrors retain their correct shapes and are correctly positioned within the telescope. Great precision is needed if the telescope is to deliver optimum image quality.

The M2 and M3 cells are complex mechanisms more than 6.5 meters wide and weighing close to 12 tons including the mirrors themselves. They provide alignment and tracking capabilities with a high precision hexapod with an absolute accuracy of tens of micrometers. The cells also compensate for mirror surface deformations in the order of tens of nanometres by means of an innovative solution using warping harnesses and lateral supports.

The fourth contract covers the fabrication of a total of 4608 edge sensors for the 798 hexagonal segments of the ELT’s primary mirror.

These sensors are the most accurate ever used in a telescope and can measure relative positions to an accuracy of a few nanometres. They form a fundamental part of the very complex system that will continuously sense the locations of the ELT primary mirror segments relative to their neighbours and allow the segments to work together to form a perfect imaging system. It is a huge challenge not only to make sensors with the required precision, but also to produce them quickly enough for thousands to be delivered to the necessarily short timescales.

The Daily Galaxy via ESO

Image credit:


"The Galaxy" in Your Inbox, Free, Daily