Frank-Thomas Lentes

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.

Heritage of ZERODUR® glass ceramic for space applications

The zero-expansion glass ceramic ZERODUR® from SCHOTT is widely used for ground-based astronomical mirrors and in industrial applications. This paper points out that it is also well suited for satellite applications, especially with respect to the space radiation environment. Recent developments show that highly lightweighted components can be manufactured and that such structures are strong enough to survive launch vibrations. A series of thirty reference applications, where ZERODUR® has been or is currently used (including METEOSAT, SPOT, ROSAT, CHANDRA, and HST), demonstrate the high and long lasting performance of ZERODUR® components in orbit. The ongoing successful missions and upcoming new satellites continue to enlarge the space heritage of this unique material.

Blue glass lens elements used as IR cut filter in a camera design and the impact of inner quality onto lens performance

Every digital camera has an IR cut filter to adapt the spectral sensivity of the sensor to that of the human eye. The trend to backside illuminated CMOS chips enable higher “light gathering”, especially under higher angles of incidence (field of view). Due to the virtually angle-independent transmittance characteristics of absorptive filter glasses (e.g. IR blocking so-called “blue glass”), blue glass is more and more used. Pure interference filters have a high angle dependency and in addition reflect the IR light which can cause ghost images. A typical design of a smart phone camera will be used to design and analyze the quality of a blue glass absorption filter. Blue glass as plano-plano filter plate in front of a CMOS chip will be examined. Furthermore, a lens made out of blue glass (substituting the plano-plano filter) will be designed and analyzed. It turns out that the blue glass lens can be used as a crown glass in an achromatic lens. The required transmittance (filter) curve will be elaborated. Such a blue glass lens can shrink down the size of the digital camera significantly. The blue glass lens needs to have a certain inner glass quality, e.g. striae, and thus the effects of wavefront distortion due to inner glass quality will be investigated. As a result striae of blue glass used lens or as plano-plano filter plate needs to be at a certain level. The blue glass lens has tighter restrictions on striae. For both cases a recommendation of inner glass quality level in terms of wavefront distortion will be given.