Visible light from distant astronomical objects is shifted to longer wavelengths, so the search for such objects at ever greater distances pushes observations towards the IR range (>5μm). Planets are bright in the IR, and direct imaging at those wavelengths, instead of in the visible, is advantageous because of the greater contrast of the planets to the central star. As a result, IR observations are a promising technique for the study of extra solar planets. Furthermore, the evolution of material in the universe can be studied most efficiently in the IR.1 Ground-based astronomical IR observations, however, are severely hampered by the absorption of the terrestrial atmosphere and by the thermal emission of the telescopes and the atmosphere.
Space-borne telescopes that are cooled to cryogenic temperatures (<10K) are ideal facilities for making sensitive observations in the IR. The first of such telescopes (the Infrared Astronomical Satellite) made an all-sky survey in 1983 and clearly demonstrated the capabilities of this telescope class. However, space missions are strictly constrained by the mass of the payload and the ability of the instruments to withstand harsh launch conditions. Materials used in the telescope designs must be lightweight and have high-stiffness. In addition, cryogenic telescopes require high thermal stability to achieve good imaging performances. The selection of a suitable mirror material is crucial to the success of IR space telescope missions.
We selected silicon-carbide (SiC), which has relatively good specific stiffness and thermal stability (see Figure 1)2, as the mirror material for the first Japanese IR space telescope mission, AKARI.3 For this purpose, we developed a special SiC material that is composed of a porous lightweight core and a dense chemical vapor deposition (CVD) coat.4 Although beryllium (Be)—with a high specific stiffness and mild thermal stability—was previously used as the mirror material in several IR missions, we chose a SiC material because of its superior thermal stability.
Figure 1. Thermal stability versus specific stiffness for different mirror materials. Thermal stability (in units of Wμm−1) is the ratio of thermal conductivity (λ) to the coefficient of thermal expansion, CTE (α). Specific stiffness (in units of kNmg−1) is the ratio of Young's modulus (E) to specific weight (ρ). Red circles: Metallic materials. Blue squares: Low-CTE glasses. Orange diamond: Carbon-fiber reinforced plastic (CFRP) with low CTE. Black triangles: Silicon-carbide (SiC) materials, including carbon-fiber-reinforced SiC (C/SiC).
We conducted cryogenic tests on several small (160mm in diameter) test mirrors and were able to significantly increase the thermal stability of the AKARI SiC test mirrors by improving the CVD coating process, so that there is almost no thermal distortion (mirror No. 3 in Figure 2). This fabrication process was used for the flight model of the AKARI telescope.
Figure 2. Change in the surface figure error with changing temperature for different test mirrors (160mm diameter). Purple and blue lines: Mirrors made from the AKARI SiC material. Black line: Mirror made from carbon-silicon-carbide (C/SiC). Red line: Mirror made from a carbon-fiber-reinforced SiC composite, HB-Cesic. Photographs are the front (left) and rear (right) sides of a HB-Cesic test mirror. RMS: Root mean square the surface figure error for a wavelength (λ) of 632.8nm.
We found that the cause of AKARI flight telescope wavefront errors was dominated by cryogenic deformation of the mirror support structure.5 We then investigated materials that would facilitate improved support structures. We developed several new SiC-based materials that can be used in the mirrors of the next generation of IR missions, as well as for other space applications.
Our new materials include reaction-sintered SiC, carbon-fiber-reinforced plastic, and a carbon-fiber-reinforced SiC composite (C-SiC).6 C-SiC, in particular, has great potential to improve the fragility of SiC (that arises due to its ceramic nature) and could be used in more sophisticated mirror support structures.7 C-SiC needs to be polished carefully, and its homogeneity controlled so that it can be suitable for cryogenic use. HB-Cesic—a carbon-fiber-reinforced SiC material—has short, chopped, and randomly-oriented carbon fibers that reduce the anisotropy of the composite and improve its thermal stability.8 After several trials, we found that the cryogenic performance of HB-Cesic and the AKARI SiC are comparable.
Our development activities, together with the successful European Space Agency (ESA) Herschel Space Observatory mission that used a 3.5m sintered-SiC mirror,9 have resulted in several new promising materials for future cryogenic telescopes such as the Space Infrared Telescope for Cosmology and Astrophysics (SPICA). SPICA is the next-generation Japanese IR space mission and is a collaborative project with ESA.10 It will carry a 3.2m cryogenic telescope, cooled by onboard mechanical coolers, and will be able to achieve unprecedented sensitivity in the mid- to far-IR spectral region (5–210μm). The telescope will be used to advance our understanding of origin and evolution of our universe, as well as the process of planetary formation. Our future work will continue to follow on improving these materials.
The authors acknowledge all the members of the AKARI and the mirror development projects for their continuous support. This work was supported in part by KAKENHI grants (#10559005 and 13349001) from the Japan Society of Promotion of Science, as well as a Special Coordination Fund for Promoting Science and Technology from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
University of Tokyo
Takashi Onaka received his PhD in astronomy from the University of Tokyo and has been a professor since 2001.
Graduate School of Science
Hidehiro Kaneda received his PhD in physics from the University of Tokyo and has been a professor since 2012.
Mitsunobu Kawada, Keigo Enya, Takao Nakagawa
Laboratory of Infrared Astrophysics
Institute of Space and Astronautical Science
Mitsunobu Kawada received his PhD in physics from Nagoya University and has been an associate professor since 2010.
Keigo Enya received his PhD in astronomy from the Graduate University for Advanced Studies and has been an assistant professor since 2007.
Takao Nakagawa received his PhD in astronomy from the University of Tokyo and has been a professor since 1999.
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