Armin Thomas

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.

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.

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.

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.

Manufacturing of the ZERODUR 1.5-m primary mirror for the solar telescope GREGOR as preparation of light weighting of blanks up to 4-m diameter

The 1.5 m primary ZERODUR® mirror of the solar telescope GREGOR incorporates 420 pockets at the backside for active cooling to avoid the thermal load impact of the sun deteriorating the observation. This design is also under consideration for the 2 m Indian Solar Telescope and for the 4.2 m European Solar Telescope (EST). The tip and tilt M5 mirror of the European Extremely Large Telescope (E-ELT) requires an even more demanding approach in light weighting. The approximately 3 m × 2.4 m elliptical flat mirror is specified to a weight of less than 500 kg. During the successful manufacturing of the GREGOR light weighted mirror, SCHOTT developed a systematic approach for processing such complex and long lead items which are capable for being up-scaled to a dimension of 4 m. In parallel SCHOTT has tested the machining of challenging aspect ratios of rib thickness and pocket height to prove the machinability of the E-ELT M5 design suggestions. The improved data on the bending strengths of ZERODUR® enable aggressive designs for light weighted 4 m class mirrors.

ZERODUR mandrels for the next generation of x-ray telescopes

Following the actual X-ray satellites XMM-NEWTON and CHANDRA future missions are in discussion. ESA is planning the XEUS-satellite and NASA the CONSTELLATION-X mission. The increasing effective areas of the telescopes require nested thin-walled mirrors of large diameters. For the mass production of segmented shells the techniques of nickel electroforming and of epoxy replication are in evaluation. In both cases ZERODUR glass ceramic was chosen for the replication mandrels due to its high thermal stability and its proven ability to be polished to excellent surface qualities. SCHOTT GLAS has produced pre-shaped prototypes of a-spherical replication mandrels. The final polishing is done at CARL ZEISS, who is also the prime contractor for the finished mandrels. A demonstration mandrel for XEUS has been finished in 2000; the first prototype mandrel for CONSTELLATION-X will be delivered this year. It has been demonstrated that high precision mandrels can be produced with the required accuracy. Thereby ZERODUR is developing from a mirror substrate material (ROSAT, CHANDRA) to the preferred material of mandrels for the replication of X-ray mirrors. This demonstrates the broad variety of applications for this zero expansion glass ceramics.

Making sense of large data sets without annotations: Analyzing age-related correlations from lung CT scans

The analysis of large data sets can help to gain knowledge about specific organs or on specific diseases, just as big data analysis does in many non-medical areas. This article aims to gain information from 3D volumes, so the visual content of lung CT scans of a large number of patients. In the case of the described data set, only little annotation is available on the patients that were all part of an ongoing screening program and besides age and gender no information on the patient and the findings was available for this work. This is a scenario that can happen regularly as image data sets are produced and become available in increasingly large quantities but manual annotations are often not available and also clinical data such as text reports are often harder to share. We extracted a set of visual features from 12,414 CT scans of 9,348 patients that had CT scans of the lung taken in the context of a national lung screening program in Belarus. Lung fields were segmented by two segmentation algorithms and only cases where both algorithms were able to find left and right lung and had a Dice coefficient above 0.95 were analyzed. This assures that only segmentations of good quality were used to extract features of the lung. Patients ranged in age from 0 to 106 years. Data analysis shows that age can be predicted with a fairly high accuracy for persons under 15 years. Relatively good results were also obtained between 30 and 65 years where a steady trend is seen. For young adults and older people the results are not as good as variability is very high in these groups. Several visualizations of the data show the evolution patters of the lung texture, size and density with age. The experiments allow learning the evolution of the lung and the gained results show that even with limited metadata we can extract interesting information from large-scale visual data. These age-related changes (for example of the lung volume, the density histogram of the tissue) can also be taken into account for the interpretation of new cases. The database used includes patients that had suspicions on a chest X-ray, so it is not a group of healthy people, and only tendencies and not a model of a healthy lung at a specific age can be derived.

Game-changing approaches to affordable advanced lightweight mirrors: Extreme Zerodur lightweighting and relief from the classical polishing parameter constraint

Recent game-changing technology greatly extends the design possibilities and range of applications for aggressively lightweighted open-back Zerodur® mirrors. We have compared several lightweighting design approaches under this new technology. Analytic comparisons are for 1.2m mirrors, all constrained to have a free-free first Eigenfrequency of 200 Hz. Figures of merit include resulting mass, thickness and relative cost. Much more aggressive masses are now available in open-back mirrors, competitive with the more expensive closed-back sandwich mirrors. These breakthroughs are relevant to spaceborne implementation of lightweight mirrors ranging from a few tenths of a meter in diameter to up to 4 meters in diameter.

ZERODUR mirror blanks for ELTs: technology and production capacity at SCHOTT

SCHOTT has a history of 100 years in delivering mirror blanks for astronomy. Since more than 30 years the zero expansion glass ceramic material ZERODUR is well recognized in the astronomical community. More than 250 ZERODUR mirror blanks for large segmented telescopes have been successfully produced at SCHOTT and were already delivered to KECK I, KECK II, HET, GTC, and LAMOST. For the increasing world wide demand on large ZERODUR components for industrial applications SCHOTT is presently ramping up its production capacity. The investment in additional melting and ceramisation capabilities are accompanied by improvements of quality assurance and processing technology. SCHOTT is now prepared for a future production of ZERODUR mirror blanks for next generation of Extremely Large Telescopes with diameters of 30 m to 50 m. For other large optical elements needed SCHOTT can supply the requested materials like optical glasses, filter glasses, fused silica and calcium fluoride.

Zerodur® — A Low Thermal Expansion Glass Ceramic for Optical Precision Applications

When in 1957 Corning Glass Works announced the invention of a new class of materials called glass ceramics, Schott started research in this field to acquire competence for future product developments. Schott mainly concentrated its research on glass ceramics in the Li2O-Al2O3-SiO2-composition field and contributed to a broadened understanding of the nucleation and crystallization phenomena; for example, Sack and Scheidler [4.1] were the first to propose the use of a combination of the nucleating agents TiO2 and ZrO2- In these early stages of research and development, product ideas mainly centred around cookware. In discussions during international meetings with scientists from other laboratories, Schott scientists learned that the low-expansion Li2O-Al2O3-SiO2 (LAS) glass ceramics were considered as substrate materials for telescope mirror blanks, and perhaps they dreamed of developing such a material. But Schott was well aware that the development of a telescope mirror substrate material and its realization in the form of large castings depended not only on the corresponding demand but equally on the willingness of an institution to supply sufficient funding.