Additive manufacturing technology is expected to be the third processing method following cutting and machining. In recent years, there has been an increasing demand for higher precision in the fields for optical equipment. However, there are still few reports of additive manufacturing technology using zero thermal expansion material. The authors report the results for the purpose of developing additive manufacturing technology for zero thermal expansion metal.
Low expansion glass materials have been widely applied to space optics due to their extremely low thermal expansion properties and outstanding mirror polishing characteristics. Meanwhile, the authors have focused on cordierite ceramics as promising candidate materials for space optics because they have higher specific stiffness than the conventional low expansion glass materials, having extremly low coefficient of thermal expansion. In the previous study, we evaluated the material properties of cordierite ceramics and established the key technology for manufacturing lightweight structures and polishing precise mirror surfaces. However, a reliable strength evaluation on a large number of specimens with various surface conditions was insufficient. Because measurement of breakage stress is often susceptible to the method of evaluation, strength characteristics of the brittle material need to be evaluated using many specimens of different surface conditions and the same test standard and methodology. In this research, ring-on-ring test, an ISO standard (ISO EN 1288- 5), was selected as the strength test standard for brittle materials. The Weibull distribution was used as the extreme value distribution for the breakage analysis. The strength distribution of two cordierite ceramics (NEXCERA™ CD107 and CO720) with various surface conditions generated in grinding, polishing, and sintering processes were evaluated and compared with a low thermal expansion glass-ceramic (ZERODUR®). These results showed the cordierite ceramics tend to have higher strength and Weibull parameter than the glass-ceramic under some surface conditions. This paper describes the effect of the surface condition of cordierite ceramics on bending strength.
A feasibility study was conducted for an optical imager system assumed to be mounted on a geostationary orbit satellite for Earth observation. The targeted spatial resolution was less than 10 meters for panchromatic mode at nadir observation conditions, and the observation area was assumed to 100 × 100 square kilometers. The optical system was designed based on a Korsch three mirror anastigmat; the primary mirror was 3.5 meters in diameter, and the focal length was approximately 45 meters. The worst wavefront error was estimated at less than 0.017 λrms in the field of view. As the next step, the primary mirror was segmented, and a trade-off study was conducted on two types of segmented mirror configurations. The optical performance of each configuration was compared in terms of PSF and MTF. Moreover, the deterioration of optical performance due to the misalignment and distortion of the segmented mirror was discussed and numerically estimated by using the Monte Carlo method. The sensitivity of the wavefront error was consequently estimated for the segmented mirror assembly.
Optical mirrors for space telescopes, which require high precision and high thermal stability, have commonly
been made of glass materials such as ultra low expansion glass (e.g. ULE®) or extremely low expansion glassceramic
(e.g. ZERODUR® or CLEARCERAM®). These materials have been well-known for their reliability
due to their long history of achievements in many space applications.