Thorsten Doehring

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.

Forming mandrels for x-ray telescopes made of modified Zerodur

For the next generation of X-ray observatories (CONSTELLATION-X and XEUS) a mass production of glass mirror segments is considered. The mirror substrates (SCHOTT D263 and SCHOTT BOROFLOAT 33) will be pre-shaped in a high temperature slumping process by use of precision forming mandrels. SCHOTT GLAS developed the glass ceramic material ZERODUR K20 to meet the requirements of these mandrels. The new material is a modification of the well-known ZERODUR. A heat driven transformation thereby changes the crystalline phase from high-quartz to keatite structure. The resulting ZERODUR K20 exhibits an increased stability at high temperatures of up to 850°C and a low thermal expansion coefficient (CTE) of approximately 20•10-7 K-1 (20°-700°C). Numerical simulations of the slumping process based on experimental parameters of Zerodur K20 and the mirror substrate materials are presented.

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.

The technical challenge of large ELT filters

For the planned extremely large telescope projects not only the primary mirror diameters, but also the dimensions of the other optical components are increasing. For the involved manufacturers of astronomical filters technical issues like polishing, coating, measurement, handling and cementing are demanding. Not only the availability of monolithic glass substrates of the required dimensions is critical, but also the required glass quality regarding homogeneity, bubbles, inclusions, and striae. Additionally an individual production of such unique astronomical components is an economical risk, as it does not fit to the usual mass production of small filter components. It is the goal of this paper to call attention to this potential critical path for the future astronomical projects. The status of the filter glass production at SCHOTT and the development needs for these challenging components are discussed.

Optical glasses and glass ceramics for large optical systems

Schott has delivered blanks for large lenses and prisms since many decades. Glass and glass ceramics objects with dimensions above 300 mm diameter or edge lengths will remain challenges for a glass manufacturer. This holds especially when the quality specifications exceed the standard level significantly. Optical glass blocks of more than half a ton have been produced with outstanding internal quality. Although the manufacturing process is well controlled there are restrictions on the availability of such objects (glass types, long process times e.g.). Implications of the glass production process are presented as a guideline for designers in order to avoid unnecessary time losses. The similarity of the production process of the glass ceramic ZERODUR to that of optical glasses results in high homogeneity with regard to the coefficient of thermal expansion as well as to the optical properties. This qualifies ZERODUR for even higher demanding applications especially when reproducibility in series production is required.