SiC Ultra Light Mirror for Large Space Telescope and for Extremely Huge Ground Based Telescope II

Transcription

1 SiC Ultra Light Mirror for Large Space Telescope and for Extremely Huge Ground Based Telescope II Noboru Ebizuka *a,b, Hiroaki Eto a, Yutang Dai a, Toru Suzuki a,wimin Lin a, Hitoshi Omori a, Toshikazu Ebisuzaki a and Hideki Takami b a RIKEN (The Institute of Physical and Chemical Research), b National Astronomical Observatory of Japan Abstract SiC (silicon carbide) lightweight mirrors are used for a large number of space telescopes, and SiC is also candidate as hopeful material for segmented mirrors of the next generation ground based telescopes from 30 to 100 m in diameter. However, an SiC mirror is difficult to shape because the material is very hard and brittle. We are developing an SiC mirror by means of the ELID (ELectrolytic In-process Dressing) grinding method, a grinding machine with rotary table of 800 mm in diameter and precision of 10 nm in control resolution, and computational simulations. The ELID grinding method is versatile for fabrications of very hard materials. In this study, we introduce test fabrications of SiC mirrors with 360 mm in diameter and equilateral triangle rib structures in the rear face. We developed a support tool with air actuator and oil pressure clamp for suppression of the mirror deformation for manufacturing of the thin mirror. Key words: Diamond grinding, Cast iron binder, On-machine measurement 1. Introduction The primary mirror of a space telescope requires extremely lightweight, various kinds of new materials, structures and techniques are applied for the mirror. Lightweight mirrors of SiC (silicon carbide) are currently used for a large number of space telescopes because the material has advantages with high thermal stability (thermal conductivity to CTE ratio), high stiffness (Young s modulus to density ratio) and so on. For example, Astro-F (Japanese infrared astronomical satellite, scheduled for launch in 2005, Fig. 1 left) and Herschel Space Observatory (HSO, ESA, scheduled for launch in 2007) employ SiC primary mirror with 710 mm and 3.5 m in diameter respectively. The SiC mirror for Astro-F is porous SiC substrate sandwiched with CVD (chemical vapor deposition)-sic [1]. The primary mirror of HSO employs a sintered SiC. SiC is a candidate material for a primary mirror of SPICA (Japanese infrared space observatory, Fig. 1 right) [2] and JTPF (Japanese terrestrial planet finder) missions. SiC is not only material for space applications but also available for a huge ground based telescope of the next generation from 30 to 100 m in diameter. Fig. 1 Imaginary picture of Astro-F (left) and SPICA (right). Astro-F has the primary mirror with 710mm in diameter cooled down to 6K, and will observe in the wavelength range from 1.8 to 200µm. SPICA has the primary mirror with 3.5m in diameter cooled down to 4.5K, and will observe in the wavelength range from 5 to 200µm. (JAXA home page) * RIKEN, Wako, Saitama, , Japan; ebizuka@riken.jp; Home page: (Japanese)

2 Fig. 2 ELID (Electrolytic in-process dressing) grinding method with a diamond wheel of cast iron binder. However, SiC is a very hard material next to diamond, it is difficult to obtain a precise and smooth mirror surface by using ordinary grinding method. In order to solve the problem, we study the manufacturing process for an SiC lightweight mirror by combination of the ELID grinding method, a nano-precision rotary grinder and computational simulations. 2. ELID grinding method A diamond grindstone with cast iron binder is used for the ELID (ELectrolytic In-process Dressing) grinding method. Diamond is easy to vanish within pure iron at high temperature because pure iron has strong reaction of chemical diffusion for carbon. On the other hand, the diffusion of cast iron for diamond is little since the cast iron saturates with carbon. Moreover, holding force of cast iron for diamond grains is increased by tiny diffusion. However, the grindstone had not been practical because it was difficult to true and dress owing to its strong holding force, and cast iron binder is hard to abrade even if diamond grains are wore out, consequently, it is easy to clog. Therefore the ELID grinding method, diamond grains are adequately projected on surface of a grindstone by electrolysis of cast iron binder, was invented [3, 4]. Figure 2 shows the schematic representation of the ELID grinding method. The ELID system consists of a metal bonded grindstone, electrolytic power source, and electrolytic coolant (Fig.2, right). The grindstone is connected to the positive terminal of a power source with a smooth brush contact, and a fixed electrode is made negative. The clearance between the grindstone and fixed electrode keeps approximately 0.5 mm. As shown in figure 2 light, initial electrolytic process removes cast iron binder of the grindstone, and protrudes abrasives of diamond grains, and then a ferrous compound layer forms onto the grindstone surface. According to the compound layer becomes thicker, the electrolytic process decreases because the layer has resistance. As the protruded abrasives wear off during grinding process, and the compound layer becomes thinner, electrolytic process increases. By the ELID process, the thickness of the compound layer is automatically maintained to give stable abrasive protrusion for the grindstone. The compound layer works not only a current controller but also protector of diamond abrasives and polish. Fig. 3 silicon wafer with 8 inches and germanium immersion grating (30 x 30 x 72 [mm]) [5-7] fabricated by ELID grinding method.

3 Fig. 4 Rotary grinding machine "RG-800" with turntable of 800nm in diameter and 10 nm in control resolution. Fig. 5 Mirror blank of sintered SiC with 360nm in diameter. The ELID grinding method is versatile for fabrications of very hard materials, such as crystalline silicon (Fig. 3 left) and sapphire, ceramics, glasses, hard metals and so on. By using a 3D profiler with nano precision and the ELID grinding method, we had successfully fabricated germanium immersion gratings (Fig. 3 right) for the mid-infrared high dispersion spectrograph of the 8.2m Subaru telescope [5-7]. 3. Trial fabrications of SiC mirrors We are fabricating a spherical mirror of sintered SiC with equilateral triangle rib structures in the rear face by using a rotary grinding machine RG-800 with rotary table of 800mm in diameter and precision of 10nm in control resolution. The spherical mirror is 360mm in diameter and 2380mm in radius of curvature, an optical face and rib structures are 5mm in thickness, as shown in figure 5. Although SiC ceramics are very hard and brittle, mirror deformation by grinding force could not disregard. Before the fabrication of the mirror with 360 mm in diameter, we performed test fabrications of a flat mirror with triangle ribstructure of rear face and a spherical mirror without rib-structure to obtain fundamental data of grinding force and profile deformation for FEM simulations. The flat mirror is 80 mm in diameter and 13mm in total thickness, an optical face and rib structures are 3mm in thickness (Fig. 6) [8]. The spherical mirror is 200 mm in diameter and 2380 mm in radius of curvature. We developed a support tool for suppression of the mirror deformation as shown in figure 7. The mirror is fixed by central hale with taper, and supported by three fixed points and nine actuators of axial direction. Three actuators of radial direction have functions for prevention of vibration and stopper of rotation. The actuators of axial and radial direction Fig. 6 Test fabrication of a flat mirror with 80 mm in diameter and triangle rib structure of rear face. Maximum deformation is approximately 330nm measured by interferometer and profiler of contact type.

4 Fig. 7 Support tool of air actuator and oil pressure clamp for thin mirror with 360mm in diameter. are lifted by air pressure and contact to the mirror with tiny pressure, and then the actuators are clamped at the position by oil pressure. Therefore, the deformation of mirror is reduced against the grinding force, and vibration of mirror by grinding is suppressed. The support is designed, if the mirror face and rib structures become thinner, thirty-six actuators could add for directly supporting an optical face. Figure 8 shows simulation the mirror deformation within and without support. The simulation was performed by application software named ANSYS [9]. Figure 9 shows a picture of the SiC mirror with 360 mm in diameter finished by a grinding wheel with #4,000. The mirror blank of sintered SiC had corrected perpendicularity of basement to outline and center hole, axial center of outline and center hole, and radius and center of the mirror curvature by preparatory grinds. The blank was ground by grinding wheels of #1,200 and #4,000 abrasive with 305 mm in diameter. The rotation speed of the table and wheels were set 12 and 1,000 rpm respectively. The depth of grind was 1 to 1.5 µm for #1,200 and 0.6 µm for #4,000. Figure 10 shows a deviation from curvature of the mirror measured by on-machine profiler of contact type. The deviation with large value between 150 and 180 mm in radial position is caused by an over correction with #1,200 grind because the profiler could not use at that time. The deviation between 60 and 140 mm in radial position is 0.25 µm in P-V and µm in rms, and surface roughness is 0.80 µm in P-V and 0.14 µm in rms. The deviation value is acceptable to an infrared telescope for observations from 2µm to longer wavelength. On the other hand, a telescope for visible and ultraviolet observations demands deviation value blow 0.05 µm in P-V and 0.02 µm in rms. To achieve the precise shape of a mirror for visible and ultraviolet, on-machine measurement and feedback fabrications are necessary. We are developing an interferometer for on-machine measurement and software for feedback fabrications. Fig. 8 Results of profile deformation by simulation. Maximum deformations for grinding force of 4Kg are 0.515mm without support (left) and 0.08mm with support.

5 Fig. 9 SiC mirror with 360 mm in diameter finished by a Fig. 10 Deviation from curvature of the SiC mirror grinding wheel with #4,000. of Figure 9. Conclusions We demonstrate, combination of ELID grinding method and rotary grinding machine with 10nm in control resolution is suitable for manufacturing of an SiC mirror, through our experiments. And a result of FEM calculation for the deformation of thin mirror caused by grinding force is consistent with experiments. The technologies are promising method for manufacturing of a large-scale optical element such as an astronomical telescope, X-ray mirror, optical antenna of free space optical communications and so on. The authors acknowledge staff members of Japan Aerospace Exploration Agency (JAXA) and National Astronomical Observatory of Japan (NAOJ) for critical comments and continuous encouragement to support this research. We wish to thank colleagues of RIKEN for advice on fabrications and their help. This work is partly funded by a grant-in-aid of NAOJ. References 1. T. Onaka, Y. Sugiyama and S. Miura, "Telescope system of the infrared imaging surveyor (IRIS)", Infrared Astronomical Instrumentation, Proc. SPIE 3354, , H. Kaneda et al, "Optical performance of the ASTRO-F telescope at cryogenic temperatures", IR Space Telescopes and Instruments, Proc. SPIE 4850, , H. Ohmori, "Electrolytic in-process dressing (ELID) grinding technique for ultraprecision mirror surface machining", International Journal of Jpn. Soc. Prec. Eng. 26, , H. Ohmori and T. Nakagawa, "Mirror surface grinding of silicon wafer with electrolytic in-process dressing", Annals of CIRP 39, , N. Ebizuka, K. Oka, A. Yamada, M. Ishikawa, M. Kashiwagi, K. Kodate,Y. Hirahara, S. Sato, K.S. Kawabata, M. Wakaki, S. Morita, T. Shimizu, S. Yin, H. Omori and M. Iye, Grism and Immersion Grating for Space Telescope, The 5th International Conference on Space Optics, Proc. ICSO2004, CD-ROM, ebizuka.pdf, N. Ebizuka, K. Oka, A. Yamada, M. Ishikawa, M. Kashiwagi, K. Kodate,Y. Hirahara, S. Sato, W. Fuji, M. Wakaki, K.S. Kawabata, M. Watanabe, H. Suto,M. Tamura and M. Iye, Grism and Immersion Grating for the 8.2m Subaru Telescope, Diffractive Optics 2003, CD-ROM, pp , N. Ebizuka, S. Morita, T. Shimizu, Y. Yamagata, H. Omori, M. Wakaki, H. Kobayashi, H. Tokoro, Y. Hirahara, "Development of Immersion Grating for Mid-Infrared High Dispersion Spectrograph for the 8.2m Subaru Telescope", Proc. SPIE 4842, pp , N. Ebizuka, Y. Dai, H. Eto, W. Lin, T. Ebisuzaki, H. Omori, T. Handa, H. Takami and Y. Takahash, "Development of SiC Ultra Light Mirror for Large Space Telescope and for Extremely Huge Ground Based Telescope", Proc. SPIE, 4842, , H. Eto, Y. Dai, N. Ebizuka, Y. Saito, T. Suzuki, W. Lin, H. Ohmori, T. Ebisuzaki and H. Takami, ELID Grinding of Ultra Lightweight Mirror, The 5th International Conference on Space Optics, Proc. ICSO2004, CD- ROM, eto.pdf, 2004.