The Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mission is a Japanese astronomical infrared satellite project optimized for mid' to far-infrared observatories. It will be launched at ambient temperature and cooled down on orbit by mechanical coolers on board with an efficient radiative cooling system, which allow us to have a 3.5m cooled (4.5K) telescope in space. SPICA will answer a number of important problems in present-day astronomy, ranging from the star-formation history of the universe to the formation of planets, owing to its high spatial resolution and unprecedented sensitivity in the mid- to far-infrared. The large aperture mirror for cryogenically use in space, however, demand a challenging development for the telescope system. A single aperture design of the primary mirror will be adopted for the SPICA telescope rather than deployable mirror designs to avoid further complexity and ensure the feasibility. The number of actuators for the primary mirror, if needed, will be minimized. Silicon carbide and carbon-filter reinforced silicon carbide are extensively investigated at present as the prime candidate materials for the SPICA primary mirror. This presentation reports the current status of the SPICA telescope system development.
We are developing high-strength reaction-sintered silicon carbide (RS-SiC) mirror as one of the new promising candidates for large-diameter space-borne optics.
In order to observe earth surface or atmosphere with high spatial resolution from geostationary orbit, larger diameter primary mirrors of 1-2 m are required. One of the difficult problems to be solved to realize such optical system is to obtain as flat mirror surface as possible that ensures imaging performance in infrared - visible - ultraviolet wavelength region. This means that homogeneous nano-order surface flatness/roughness is required for the mirror.
The high-strength RS-SiC developed and manufactured by TOSHIBA is one of the most excellent and feasible candidates for such purpose. Small RS-SiC plane sample mirrors have been manufactured and basic physical parameters and optical performances of them have been measured. We show the current state of the art of the RS-SiC mirror and the feasibility of a large-diameter RS-SiC mirror for space-borne optics.
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.
Next generation space telescopes, which are currently being developed in the US and Europe, require large-scale lightweight
reflectors with high specific strength, high specific stiffness, low CTE, and high thermal conductivity. To meet
budget constraints, they also require materials that produce surfaces suitable for polishing without expensive overcoatings.
HB-Cesic® - a European and Japanese trademark of ECM - is a Hybrid Carbon-Fiber Reinforced SiC composite
developed jointly by ECM and MELCO to meet these challenges. The material's mechanical performance, such as
stiffness, bending strength, and fracture toughness are significantly improved compared to the classic ECM Cesic®
material (type MF). Thermal expansion and thermal conductivity of HB-Cesic® at cryogenic temperatures are now
partly established; and excellent performance for large future space mirrors and structures are expected.
This paper will present the whole manufacturing process of such a space mirror starting from the raw material
preparation until the polishing of the optic including cryo testing .
The letters "HB" in HB-Cesic® stand for "hybrid" to indicate that the C/C raw material is composed of a mixture of
different types of chopped, short carbon-fibers.
Next generation space telescopes, which are currently being developed in the US and Europe, require large-scale light-weight
reflectors with high specific strength, high specific stiffness, low CTE, and high thermal conductivity. To meet
budget constraints, they also require materials that produce surfaces suitable for polishing without expensive over-coatings.
HB-Cesic - a European and Japanese trademark of ECM - is a Hybrid Carbon-Fiber Reinforced SiC composite
developed jointly by ECM and MELCO to meet these challenges. The material's mechanical performance, such as
stiffness, bending strength, and fracture toughness are significantly improved compared to the classic ECM Cesic
material (type MF). Thermal expansion and thermal conductivity of HB-Cesic at cryogenic temperatures are now partly
established; and excellent performance for large future space mirrors and structures are expected.
This paper presents the design and manufacturing of an 800-mm mirror for space application, starting with the C/C raw
material preparation to the finishing of the components, including the polishing of the mirror.
The letters "HB" in HB-Cesic stand for "hybrid" to indicate that the C/C raw material is composed of a mixture of
different types of chopped, short carbon-fibers.
KEYWORDS: Mirrors, Space telescopes, Telescopes, Silicon carbide, Cryogenics, Infrared telescopes, James Webb Space Telescope, Space mirrors, Composites, Astronomy
SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japanese astronomical infrared satellite project
with a 3.5-m telescope. The target year for launch is 2017. The telescope is cooled down to 4.5 K in space by a
combination of newly-developed mechanical coolers with an efficient radiative cooling system at the L2 point. The
SPICA telescope has requirements for its total weight to be lighter than 700 kg and for the imaging performance to be
diffraction-limited at 5 μm at 4.5 K. Material for the SPICA telescope mirrors is silicon carbide (SiC). Among various
types of SiC, primary candidates comprise normally-sintered SiC, reaction-sintered SiC, and carbon-fiber-reinforced
SiC; the latter two have been being developed in Japan. This paper reports the current design and status of the SPICA
telescope along with our recent activities on the cryogenic optical testing of SiC and C/SiC composite mirrors, including
the development of an innovative support mechanism for cryogenic mirrors, which are based on lessons learned from a
SiC 70 cm telescope onboard the previous Japanese infrared astronomical mission AKARI.
One of the key technologies for next generation space telescope with a large-scale reflector is a material having high specific strength, high specific stiffness, low coefficient of thermal expansion and high coefficient of thermal conductivity. Several candidates such as fused silica, beryllium, silicon carbide and carbon fiber reinforced composites have been evaluated. Pitch-based carbon fiber reinforced SiC composites were developed for the SPICA space telescope mirror to comply with such requirements. Mechanical performance such as bending stiffness, bending strength and fracture toughness was significantly improved. Evaluation procedures of thermal expansion and thermal conductivity behavior at cryogenic temperatures (as low as 4.5K) were established and excellent performance for the SPICA mirror was demonstrated.
Newly developed high-strength reaction-sintered silicon carbide, called New-Technology Silicon Carbide (NT-SiC) is an attractive material for lightweight optical mirror with two times higher bending strength than other SiC materials. The material has advantages in its fabrication process. The sintering temperature is significantly lower than that of pure silicon carbide ceramics and its sintering shrinkage is smaller than one percent. These advantages will provide rapid progress to fabricate large structures. The characteristics of the material are also investigated. The polish of the test piece demonstrated that the polished surface has no pore and is suited to visible region as well as infrared without CVD SiC coating. It is concluded that NT-SiC has potential to provide large lightweight optical mirror.
The high-strength reaction-sintered silicon carbide (RS-SiC) developed and manufactured by Toshiba and NEC-Toshiba Space Systems, NT-SiC, is one of the most promising, excellent and feasible candidates for light-weighted large-diameter space-borne optics that are applied to geostationary earth observations and astronomical observations. Small NT-SiC sample mirrors were manufactured to study basic physical parameters and features, and optical performances of the material, such as the surface conditions of polished NT-SiC, the condition of inner crystal grains, the correlation between the surface roughness and polishing, scattering characteristics, absorbance of solar light and infrared emissivity, and adhesiveness of metal coating. The current state of the art of the development of the NT-SiC mirror and the feasibility of light-weighted large-diameter NT-SiC mirrors for space-borne optics are described. Although technical challenges to achieve the surface roughness that is applicable to ultraviolet mirrors still remain, the optical performance and the physical properties of the present NT-SiC show that it is one of the most excellent mirror material in optical-infrared wavelength region.
We report the surface structure and roughness of the mirrors made of carbon fiber reinforced silicon carbide (C/SiC) composite improved for the SPICA (Space Infrared telescope for Cosmology and Astrophysics) mission. The improved C/SiC is a candidate of material for the SPICA light weight mirrors because of its superior properties: high toughness, high stiffness, small thermal deformation, feasibility to make large single dish mirror, low cost, and short term for production. The surface of the bare C/SiC composite consists of carbon fiber, silicon carbide and silicon, each of which has different hardness, so it is difficult to polish this surface smoothly. Our improved polishing technique achieved the surface roughness of better than 20nm RMS for the C/SiC composite flat mirror, which satisfies the requirement of the SPICA mission. For curved bare surface of the C/SiC mirror, the roughness is larger than 30 nm and now under improving. The Change of Bidirectional reflectance distribution function (BRDF) of the bare C/SiC composite at cryogenic temperature was measured with 632.8nm lasar. No significant difference was found between the BRDFs at 95K and that at room temperature. In order to improve surface roughness further, we are planning to apply the SiSiC slurry coating on the surface of the improved C/SiC composite. This combination can realize the surface roughness well enough to be applied even for optical telescopes.
The SPICA (Space Infrared Telescope for Cosmology and Astrophysics), which is a Japanese astronomical infrared satellite project with a 3.5-m telescope, is scheduled for launch in early 2010s. The telescope is cooled down to 4.5 K in space by a combination of mechanical coolers with an efficient radiative cooling system. The SPICA telescope has requirements for its total weight to be lighter than 700 kg and for the imaging performance to be diffraction-limited at 5 µm at 4.5 K. Two candidate materials, silicon carbide (SiC) and carbon-fiber-reinforced SiC (C/SiC composite), are currently under investigation for the primary mirror. A monolithic mirror design will be adopted in both cases because of the technical feasibility and reliability. This paper reports the current design and status of the SPICA telescope together with some of our recent results on laboratory cryogenic tests for the SiC and C/SiC composite mirrors.
One of the key technologies for next generation space telescope with a large-scale reflector is a material having high specific strength, high specific stiffness, low coefficient of thermal expansion and high coefficient of thermal conductivity. Several candidates such as fused silica, beryllium, silicon carbide and carbon fiber reinforced composites have been evaluated. Pitch-based carbon fiber reinforced SiC composites were developed for the SPICA space telescope mirror to comply with such requirements. Mechanical performance such as bending stiffness, bending strength and fracture toughness was significantly improved. Evaluation procedures of thermal expansion and thermal conductivity behavior at cryogenic temperatures (as low as 4.5K) were established and excellent performance for the SPICA mirror was demonstrated.
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