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Exposing the secrets of the Universe with a collection of mirrors...

25/10/2002

This telescope will use 36 hexagonal mirrors that will fit together in a honeycomb pattern, making it a challenge of exact mathematical calculation and precision fitting. So what will these very special mirrors be made of?

To make our mirror, not just any material will do. The invention of Zerodur in the 1970s by the German company SCHOTT gave science a material that combined lightness with a very low coefficient of thermal expansion: it does not expand and contract with fluctuations in temperature.

The need to deliver materials with an ever lower coefficient of thermal expansion has given rise to lines of research and technological developments that can also benefit society by improving the quality of life of ordinary people.

Laser instruments for the ARIANNE rocket programme, optical devices for the METEOSAT meteorological satellite and mirrors for telescopes like the European Southern Observatory’s (ESO’s) Very Large Telescope (VLT) are just some of the uses that Zerodur has been put to.

The secret is in the temperature

The properties of Zerodur did not go unnoticed by technical staff at the GTC. Although part of the production process is jealously guarded by SCHOTT, the secret of Zerodur lies in the unusual thermal conditions that it is subjected to in its process of transformation from ordinary glass to vitreous ceramic material.

Why was Zerodur chosen? It is a type of vitreous ceramic like the material used for cooker hobs and its main characteristic is its low coefficient of thermal expansion. This means that fluctuations in temperature will make the mirrors expand or contract very little, so that any deformation will be almost undetectable.

There are three steps in its manufacture. Casting, the first step, sees the raw materials - based on silicon oxide like any other glass - heated to 1,600º C. The resulting product is poured into moulds and left to cool rapidly.

Ceramification, the next step, causes internal changes in the material - the tiny crystals that give Zerodur its special property of zero thermal expansion are formed. SCHOTT has developed a delicate thermal process for this. The material is placed in an oven and subjected to a thermal cycle, which lasts three to four months in the case of the derivative used on the GTC, but up to a year for larger items like the 8 metre diameter mirrors of the VLT telescope.

Finally, to reduce internal tension in the Zerodur it is heated to lower temperatures than those used for ceramification. This is called annealing. Once cooled, the segments are cut to the specified dimensions and now, fully imbued with all the properties they need, they are ready to be polished.

Would you like to find out more about the company making the segments?

Visit the SCHOTT webpage at www.schott.com.


Text:
José Manuel Abad Liñán
Natalia R. Zelman

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