Thorsten Döhring

Forty years of ZERODUR mirror substrates for astronomy: review and outlook

Initiated in 1968 by the first order of the Max-Planck-Institute in Heidelberg the successful history of ZERODUR® continues now since 40 years. ZERODUR® zero expansion glass ceramic from SCHOTT has been the material of choice in astronomy for decades, thanks to its special properties such as its extremely high thermal and mechanical stability. Today most of the major modern optical telescopes of the 4 m class and of the 8 m to 10 m class are equipped with ZERODUR®. For the future several Extremely Large Telescope (ELT) projects are in development, which are designed with even larger primary mirrors ranging from 30 m to 42 m. Also here ZERODUR® is under consideration. A historical review, the actual status of developments and an outlook to the future is given in this paper.

ZERODUR ® Glass Ceramics for High Stress Applications

Recently SCHOTT has shown in a series of investigations the suitability of the zero expansion glass ceramic material ZERODUR® for applications like mirrors and support structures of complicated design used at high mechanical loads. Examples are vibrations during rocket launches, bonded elements to support single mirrors or mirrors of a large array, or controlled deformations for optical image correction, i.e. adaptive mirrors. Additional measurements have been performed on the behavior of ZERODUR® with respect to the etching process, which is capable of increasing strength significantly. It has been determined, which minimum layer thickness has to be removed in order to achieve the strength increase reliably. New data for the strength of the material variant ZERODUR K20® prepared with a diamond grain tool D151 are available and compared with the data of ZERODUR® specimens prepared in the same way. Data for the stress corrosion coefficient n of ZERODUR® for dry and normal humid environment have been measured already in the 1980s. It has been remeasured with the alternative double cleavage drilled compression (DCDC) method.

ZERODUR® Glass Ceramics-Strength Data for the Design of Structures with High Mechanical Stresses

With an increasing number of applications mirrors and support structures made of the zero expansion glass ceramic material ZERODUR® has to endure high mechanical loads, e.g. rocket launches or controlled deformations for optical image correction. Like for other glassy materials, the strength of glass ceramics is dominated by its surface condition. Test specimens have been ground with fine grain tools (e.g. D64 diamond grains) and / or subsequently etched. The strength data basis for the design of highly stressed structures has been extended and new information has been derived for the extrapolation to low failure probabilities.

Manufacturing of lightweighted ZERODUR components at SCHOTT

There is a broad range of applications for lightweighted components made from ZERODUR(R) glass ceramic. The main markets are secondary and tertiary mirrors for astronomical telescopes, mirrors and structural components for satellites, and mechanical structures for industrial applications, mainly in microlithography. Prominent examples from astronomy are VLT-M3, GEMINI-M2, SOFIA-M1, MAGELLAN-M2, MMT-M2, and METEOSAT-SEVIRI. At SCHOTT components with blind or undercut semiclosed holes are manufactured, typically with circular, hexagonal, rectangular or triangular shapes. The classical grinding process results in weight reduction factors of about 70 %. By additional acid etching technologies even higher lightweighting factors and rib thicknesses below 1 mm have been achieved.

Optical materials for astronomy from SCHOTT: the quality of large components

The new generation of survey telescopes and future giant observatories such as E-ELT or TMT do not only require very fast or very large mirrors, but also high sophisticated instruments with the need of large optical materials in outstanding quality. The huge variety of modern optical materials from SCHOTT covers almost all areas of specification needs of optical designers. Even if many interesting optical materials are restricted in size and/or quality, there is a variety of optical materials that can be produced in large sizes, with excellent optical homogeneity, and a low level of stress birefringence. Some actual examples are high homogeneous N-BK7 blanks with a diameter of up to 1000 mm, CaF2 blanks as large as 300 mm which are useable for IR applications, Fused Silica (LITHOSIL®) with dimensions up to 700 mm which are used for visible applications, and other optical glasses like FK5, LLF1 and F2 in large formats. In this presentation the latest inspection results of large optical materials will be presented, showing the advances in production and measurement technology.

Strength Aspects for the Design of ZERODUR® Glass Ceramics Structures

In some applications mirrors and support structures from the zero expansion glass ceramic material ZERODUR(R) have to endure mechanical loads, e.g. rocket launches or controlled deformations for optical image correction. Like for other glassy materials the strength of glass ceramics is dominated by its surface condition. Similar to other glass ceramics ZERODUR(R) has higher strengths than glasses for comparable surface conditions. For the design of ZERODUR(R) parts well known rules of thumb for its strength are not sufficient in any case. So new information and data with enlarged sample sets and hence better statistics have been collected to improve the understanding of its behavior under mechanical loads. Finally an outlook is given on the application of ZERODUR(R) in ambitious current and future space projects.

Properties of Zerodur Mirror Blanks for Extremely Large Telescopes

SCHOTT produces the zero expansion glass ceramics material ZERODUR since 35 years. More than 250 ZERODUR mirror blanks were already delivered for the large segmented mirror telescopes KECK I, KECK II, HET, GTC, and LAMOST. Now several extremely large telescope (ELT) projects are in discussion, which are designed with even larger primary mirrors (TMT, OWL, EURO50, JELT, CFGT, GMT). These telescopes can be achieved also only by segmentation of the primary mirror. Based on the results of the recent production of segment blanks for the GTC project the general requirements of mirror blanks for future extremely large telescope projects have been evaluated. The specification regarding the material quality and blank geometry is discussed in detail. As the planned mass production of mirror blanks for ELTs will last for several years, economic factors are getting even more important for the success of the projects. SCHOTT is a global enterprise with a solid economical basis and therefore an ideal partner for the mirror blank delivery of extremely large telescopes.

Industrialization scenario for X-ray telescopes production based on glass slumping

Large X-ray segmented telescopes will be a key element for future missions aiming to solve still hidden mysteries of the hot and energetic Universe, such as the role of black holes in shaping their surroundings or how and why ordinary matter assembles into galaxies and clusters as it does. The major challenge of these systems is to guarantee a large effective area in combination with large field of view and good angular resolution, while maintaining the mass of the entire system within the geometrical and mass budget posed by space launchers. The slumping technology presents all the technical potentiality to be implemented for the realization of such demanding systems: it is based on the use of thin glass foils, shaped at high temperature in an oven over a suitable mould. Thousands of slumped segments are then aligned and assembled together into the optical payload. An exercise on the mass production approach has been conducted at Max Planck Institute for Extraterrestrial Physics (MPE) to show that the slumping technology can be a valuable approach for the realization of future X-ray telescopes also from a point of view of industrialization. For the analysis, a possible design for the ATHENA mission telescope was taken as reference.

CTE characterization of ZERODUR® for the ELT century

This review paper summarizes the extensive investigations that have been performed at SCHOTT to achieve a deeper understanding of the CTE homogeneity of ZERODUR® within single blanks and the casted formats (boules). Especially for the upcoming Extremely Large Telescope (ELT) projects like E-ELT or TMT with at least several hundreds of mirror segments the reproducibility of the mean CTE, CTE homogeneity and axial gradient is very important while keeping the CTE quality assurance process economic at the same time. Statistics of CTE homogeneity measurements on a ZERODUR® boule suitable for an economical production of ELT mirror substrates using the improved dilatometer will be presented. It will be shown, that it is possible to achieve tight CTE specifications by utilisation of processes existing at SCHOTT, while at the same time guaranteeing a long term reproducibility. The CTE measurement is optimized for a temperature interval from 0°C to 50°C. We developed a model to extrapolate the CTE behaviour to specific temperature conditions at the telescope site.

Mirrors for solar telescopes made from ZERODUR glass ceramic

The zero expansion glass ceramic material, ZERODUR®, is well known for night-time telescope mirror substrates. Also for solar telescopes ZERODUR® is often selected as mirror blank material. Examples are the Swedish 1 m Solar Telescope (SST), the balloon-born telescope SUNRISE, and the New Solar Telescope (NST) of the Big Bear Solar Observatory. The properties of ZERODUR® are discussed with respect to the special technical requirements of solar observatories, resulting in the conclusion that mirrors made of this glass ceramic material are an excellent choice for solar telescopes.