Ralf Jedamzik

Optical glass and glass ceramic historical aspects and recent developments: a Schott view

Since the time of Galilei 400 years ago the progress of optical systems was restricted due to the lack of optical glass types with different dispersion properties and due to poor material quality. With the work of Otto Schott, which started 125 years ago, glass became a tailorable, highly reproducible and homogeneous material, thus enabling systematic design of optical systems. The demand for new glass types is still going on as well as the requirement for ever tighter tolerances and their proofs. New measurement methods provide deeper insight in the material properties. Developments in processing allow new optical elements to be designed, further advancing technology. This also holds for zero-expansion glass ceramics, another key enabling material for optical systems. This publication highlights some milestones in the history of optical glass and glass ceramics, comments on present day glass development as well as new optical elements and measurement methods and provides some new information on the materials’ properties.

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.

Homogeneity of the linear thermal expansion coefficient of ZERODUR measured with improved accuracy

For future extremely large telescope projects like OWL or TMT with at least several hundreds of mirror blanks the homogeneity of the coefficient of linear thermal expansion (CTE) within a single blank is an important issue. The telescope designers are not only interested in the global CTE homogeneity but also in measuring the axial CTE gradient to the highest precision. It has been proven in the past in many projects like GTC and Keck that ZERODUR(r) itself is a material of highest homogeneity even in large dimensions and huge quantities. About 95.5% of all 2m class mirror segments of all projects exhibit a peak to valley homogeneity of better than 0.015*10-6K-1. The actual homogeneity of the material is even better because the results so far are largely influenced by the restrictions of the CTE measurement repeatability in the past. This paper introduces an advanced method for the measurement of the CTE of ZERODUR(r) exhibiting a significantly improved reproducibility. The dilatometer setup was especially optimized to cope with the demand of highly accurate homogeneity measurements of 2 m class ZERODUR(r) segments for giant astronomical telescopes. Detailed measurement results out of a single 1.5 m class ZERODUR(r) segment based on the current state of production will be shown. The results show CTE distributions in radial, angular and axial direction. SCHOTT has already improved the production capacity for ZERODUR(r) immensely, thereby the results represent the current status of quality of the available mass production facilities at SCHOTT.

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.

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.

Homogeneity of the coefficient of linear thermal expansion of ZEDRODUR

The low thermal expansion glass ceramic ZERODUR is the material of choice for many big astronomical telescope projects like VLT, Keck I + II, HET, LAMOST and GRANTECAN (GTC). For future giant telescope projects like OWL or TMT with at least several hundreds of mirror blanks the CTE homogeneity within a single blank and from blank to blank is an crucial issue. The ZERODUR production process is based on established and proven methods used in the production of high homogeneity optical glasses. Therefore ZERODUR itself is a material of highest homogeneity even in large dimensions and huge quantities. This paper presents an evaluation of the homogeneity of the thermal expansion coefficient within more than 250 mirror blanks. The observed homogeneity range is only slightly larger than the repeatability of the standard dilatometer measurement of ±0.005*10-6 K-1. To improve the accuracy of measurement and to get a deeper understanding of the thermal expansion behaviour of ZERODUR a new dilatometer was built exhibiting a repeatability of ±0.001*10-6 K-1. Detailed evaluations of the thermal expansion coefficient homogeneity of a 100 mm x 100 mm ZERODUR test block showed no variation within the repeatability of measurement of the improved dilatometer.

Status of ZERODUR Mirror Blank Production at SCHOTT

SCHOTT has a history of more than 35 years with the production of the zero expansion glass ceramic material ZERODUR. More than 250 ZERODUR mirror blanks were already delivered to the large segmented telescopes KECK I, KECK II, HET, GTC, and LAMOST. The increasing worldwide demand on large ZERODUR components for LCD display lithography machines is similar to the expected demand for an Extremely Large Telescope. Last year SCHOTT has ramped up its ZERODUR production capacity. These recent investments in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared also for a future production of mirror blanks for Extremely Large Telescopes. The present status of the production capacity and the mass production of ZERODUR mirror blanks for industrial applications are discussed.

Modeling of the thermal expansion behaviour of ZERODUR at arbitrary temperature profiles

Modeling of the thermal expansion behavior of ZERODUR® for the site conditions of the upcoming Extremely Large Telescopes (ELTs) allows an optimized material selection to yield the best performing ZERODUR® for the mirror substrates. The thermal expansion of glass ceramics is a function of temperature and a function of time, due to the structural relaxation behavior of the materials. The application temperature range of the upcoming ELT projects varies depending on the possible construction site between -13°C and +27°C. Typical temperature change rates during the night are in the range between 0.1°C/h and 0.3°C/h. Such temperature change rates are much smaller than the typical economic laboratory measurement rate, therefore the material behavior under these conditions can not be measured directly. SCHOTT developed a model approach to describe the structural relaxation behavior of ZERODUR®. With this model it is possible to precisely predict the thermal expansion behavior of the individual ZERODUR® material batches at any application temperature profile T(t). This paper presents results of the modeling and shows ZERODUR® material behavior at typical temperature profiles of different applications.

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.

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.