1 of 11 The Current and Future State-of-the-art Glass Optics for Space-based Astronomical Observatories Abstract Recent technology advancements show significant promise in the ability to reduce the cost, schedule and risk associated with producing segmented Primary Mirrors (PMs) as well as monolithic optics larger than Hubble Space Telescope (HST) scale to the surface figure and smoothness required of current and future astronomical systems. This paper describes the present state-of-the art technology for glass mirrors at ITT and a path to next generation technology for use in a wide range of applications. In-process development activities will be discussed as well as the areas in which future investments can further enhance glass PM technologies. Active, passive, monolithic, and segmented mirror technologies will be discussed along with some basic descriptions of the different ways by which light-weighted glass mirror blanks are fabricated. There will be an emphasis on Corning’s Ultra Low Expansion (ULE®) and borosilicate optics, with some discussion of glass ceramics and other material substrates. The paper closes with a table that summarizes potential areas of investment that will continue to advance the state of the art for the use of glass and other materials in optical systems. Robert Egerman ITT Corporation 585-269-6148 [email protected]Co-Authors (from ITT) Gary MatthewsJeff Wynn*Charles Kirk Keith Havey *ITT Retiree Acknowledgments The authors would like to thank David Content from the NASA Goddard Space Flight Center for his continued support of the development of borosilicate corrugated optics and his efforts to further this technology through NASA’s Industrial Partnership Program.
1 of 11 1.0Introduction Glass and glass ceramics have always been the primary candidate optical materials for space based reflective telescope systems that study electromagnetic radiation in the cosmos in wavelengths ranging from x-rays to the near infrared. ULE®is perhaps the best known, since it was the material of choice for the Hubble Space Telescope Primary Mirror (HST PM). ULE®has many favorable characteristics; the most notable of which is its extremely low coefficient of thermal expansion (CTE) around room temperature (<30ppb/°K from 5-35°C). Schott Zerodur®, a glass ceramic, has material characteristics comparable to ULE®and was the material substrate of choice for the optics in the Chandra X-ray Observatory and the segmented Hobby-Eberly telescope mirrors finished by ITT. Borosilicate and fused silica are two other generic glasses that have been used for flight and ground based systems. For telescopes that require extremely stable optics, these two materials are preferable for optical systems operating at temperatures of ~40K and ~100K respectively. Since the completion of the fabrication of the HST PM in 1981, significant advancements have been made in glass optic fabrication technologies to reduce mirror areal density (see Figure 1), enabling increased PM aperture sizes, while reducing risk, cost and schedule.