Shin Utsunomiya

Design and Performance of a Prototype Polarization Modulator Rotational System for Use in Space Using a Superconducting Magnetic Bearing

We present the design and the mechanical and thermal performances of a prototype rotational mechanism using a superconducting magnetic bearing (SMB) for a space compatible polarization modulator. The rotational mechanism consists of an SMB with an optical encoder and a three-grip mechanism that holds a levitating rotor until a high-temperature superconducting array (YBCO) cools down below its critical temperature. After the successful operation of a grip mechanism, the rotor magnet levitates at 10-16 K, and we conduct spin-down measurements. We estimate the heat dissipation from the rotor rotation and an optical encoder. From the mechanical and thermal performances of the prototype rotation mechanism, we did not find the potential no-go results from this SMB technology for use in a future space mission. The development of this rotational mechanism is targeting for use of a polarization modulator for a space mission to probe the comic inflation by measuring the cosmic microwave background polarization.

Analysis of Thermal Deformation on a Honeycomb Sandwich CFRP Mirror

Thermal deformation analysis was performed on a carbon fiber reinforced plastic (CFRP) mirror with sandwich structure. To obtain unexpected asymmetry of the surface sheet, we investigated the deformation of a quasi-isotropic laminate under hot and humid conditions. Despite the symmetric lay-up, quasi-isotropic laminate deforms into twisted saddle shape with time, and this deformation could be simulated by assuming ply angle misalignment. Then, the elastic moduli of honeycomb cores were calculated theoretically. A honeycomb sandwich mirror model was constructed by adopting a sheet model and using honeycomb elements. The thermal deformation analysis was performed considering the ply angle misalignment. The test results clarified that the deformation of the surface sheet was a critical factor in the dimensional stability of the CFRP mirror.

Time-dependent deformation of ROHACELL-core CFRP sandwich panels

In this study, we experimentally and analytically investigated time-dependent deformation of carbon-fiber reinforced plastic sandwich panels with three kinds of polymer foam-core ROHACELL, with the deformation caused by moisture absorption. Tensile tests for the ROHACELLs before and after moisture absorption were performed using the digital image correlation method, and we obtained a stress–strain relationship for each condition. Surface deformations of sandwich panels were calculated using finite element analyses to examine the elasto-plastic deformation of the ROHACELLs based on the tensile test results. Experimentally measured deformation in the sandwich panels varied depending on the cell size of cores, and the largest measured deformation after the moisture absorption was approximately 100 μm. This was much larger than the analytical result. On the basis of this study, we suggest that the main reason for the deformation may be an irreversible shrinkage of the ROHACELLs caused the absorption of moisture during exposure to high-temperature and high-humidity environments.

Studies of the moisture absorption of thin carbon fiber reinforced plastic substrates for x-ray mirrors

Abstract. We study a lightweight x-ray mirror with a carbon fiber reinforced plastic (CFRP) substrate for next-generation x-ray satellites. For tightly nested x-ray mirrors, such as those on the Suzaku and ASTRO-H telescopes, CFRP is the suitable substrate material because it has a higher strength-to-weight ratio and forming flexibility than those of metals. In flat CFRP substrate fabrication, the surface waviness has a root mean square (RMS) of ∼1  μm in the best products. The RMS approximately reaches a value consistent with the RMS of the mold used for the forming. We study the effect of moisture absorption using accelerated aging tests in three environments. The diffusivity of the CFRP substrate at 60°C and at relative humidity of 100% is ∼9.7×10−4  mm2·h−1, and the acceleration rate to the laboratory environment was 180 times higher. We also develop co-curing functional sheets with low water-vapor transmissivity on the CFRP substrate. Co-curing the sheets successfully reduced the moisture absorption rate by 440 times compared to the un-co-cured substrate. Details of the CFRP substrate fabrication and moisture absorption tests are also reported.

Structure design of the telescope for Small-JASMINE program

Small-JASMINE program (Japan Astrometry Satellite Mission for INfrared Exploration) is one of applicants for JAXA (Japan Aerospace Exploration Agency) space science missions launched by Epsilon Launch Vehicles, and now being reviewed in the Science Committee of ISAS (Institute of Space and Astronautical Science), JAXA. Telescope of 300 mm aperture diameter will focus to the central region of the Milky Way Galactic. The target of Small-JASMINE is to obtain reliable measurements of extremely small stellar motions with the highest accuracy of 10 μ arcseconds and to provide precise distances and velocities of multitudes of stars up to 30,000 light years. Preliminary Structure design of Small- JASMINE has been done and indicates to satisfy all of requirements from the mission requirement, the system requirement, Epsilon Launch conditions and interfaces of the small science satellite standard bus. High margin of weight for the mission allows using all super invar structure that may reduce unforeseen thermal distortion risk especially caused by connection of different materials. Thermal stability of the telescope is a key issue and should be verified in a real model at early stage of the development.

Development of CFRP mirrors for space telescopes

CFRP (Caron fiber reinforced plastics) have superior properties of high specific elasticity and low thermal expansion for satellite telescope structures. However, difficulties to achieve required surface accuracy and to ensure stability in orbit have discouraged CFRP application as main mirrors. We have developed ultra-light weight and high precision CFRP mirrors of sandwich structures composed of CFRP skins and CFRP cores using a replica technique. Shape accuracy of the demonstrated mirrors of 150 mm in diameter was 0.8 μm RMS (Root Mean Square) and surface roughness was 5 nm RMS as fabricated. Further optimization of fabrication process conditions to improve surface accuracy was studied using flat sandwich panels. Then surface accuracy of the flat CFRP sandwich panels of 150 mm square was improved to flatness of 0.2 μm RMS with surface roughness of 6 nm RMS. The surface accuracy vs. size of trial models indicated high possibility of fabrication of over 1m size mirrors with surface accuracy of 1μm. Feasibility of CFRP mirrors for low temperature applications was examined for JASMINE project as an example. Stability of surface accuracy of CFRP mirrors against temperature and moisture was discussed.

Development of CFRP mirrors for low-temperature application of satellite telescopes

Ultra-lightweight and high-accuracy CFRP (carbon fiber reinforced plastics) mirrors for space telescopes were fabricated and their feasibility for low temperature applications was demonstrated. The CFRP mirrors were composed of sandwich panels with CFRP skins and CFRP honeycomb cores. Surface was deposited with epoxy thin layers by using a replica technique. The surface accuracy of the demonstrate mirrors of 150 mm in diameter was 0.8 μm RMS and the surface smoothness was improved to 5 nm RMS. Surface accuracy degradation was 0.6μm RMS (root mean square) from ambient temperature to liquid nitrogen. Surface asperity was classified with respect of their wave intervals and measurement areas. Surface accuracy and dimensional stability were strictly affected by raw materials and manufacturing conditions. Surface accuracy was measured at each process on the way of mirror forming. Manufacturing conditions to depress asperity were discussed.

Performance of lightweight large C/SiC mirror

Very lightweight mirror will be required in the near future for both astronomical and earth science/observation missions. Silicon carbide is becoming one of the major materials applied especially to large and/or light space-borne optics, such as Herschel, GAIA, and SPICA. On the other hand, the technology of highly accurate optical measurement of large telescopes, especially in visible wavelength or cryogenic circumstances is also indispensable to realize such space-borne telescopes and hence the successful missions. We have manufactured a very lightweight Φ=800mm mirror made of carbon reinforced silicon carbide composite that can be used to evaluate the homogeneity of the mirror substrate and to master and establish the ground testing method and techniques by assembling it as the primary mirror into an optical system. All other parts of the optics model are also made of the same material as the primary mirror. The composite material was assumed to be homogeneous from the mechanical tests of samples cut out from the various areas of the 800mm mirror green-body and the cryogenic optical measurement of the mirror surface deformation of a 160mm sample mirror that is also made from the same green-body as the 800mm mirror. The circumstance and condition of the optical testing facility has been confirmed to be capable for the highly precise optical measurements of large optical systems of horizontal light axis configuration. Stitching measurement method and the algorithm for analysis of the measurement is also under study.

Studies of print-through and reflectivity of x-ray mirrors using thin carbon-fiber-reinforced plastic

Abstract. We fabricated x-ray mirrors from carbon-fiber-reinforced plastic with a tightly nested design for x-ray satellites, using a replication method for the surfaces. We studied the effects of print-through on the mirror surface as a function of curing temperature. With room temperature curing, the root-mean-square value of the surface error was 0.8 nm. The reflectivity was measured using 8-keV x-rays, and the roughness was calculated as 0.5 nm by model fitting—comparable to that of the ASTRO-H/HXT mirror. We verified the long-term stability of the mirror surface over 6 months. We fabricated Wolter type-I quadrant-shell mirrors with a diameter of 200 mm and performed x-ray measurements at BL20B2 in the SPring-8 synchrotron radiation facility. We obtained reflection images of the mirrors using a 20-keV x-ray spot beam with a slit size of 10×1  mm in the radial and circumferential directions, respectively. The averaged half-power diameter (HPD) of the images in one mirror was 1.2 arc min in the circumferential center of the mirror and 3.0 arc min at the edge. In the spot images with a smaller slit size of 10×0.2  mm, we achieved an HPD of 0.38 arc min in the best case.

Studies of lightweight x-ray telescope with CFRP

We studies lightweight X-ray mirror with Carbon Fiber Reinforced Plastic (CFRP) substrate for next generation X-ray satellites. CFRP is suitable material as substrate for thin-foil highly nested X-ray mirrors like telescope of Suzaku, ASTRO-H since it has properties of higher strength-to-weight ratio and flexibility of forming than that of metals. In the current year we made flat panels for basic research and full/partial shell substrates by quasi-isotropic laminate with 8 ply prepregs, and performed reflector replication based on technique for the HXT mirror.