GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is one of the three candidates of ISAS/JAXA’s Strategic L-class mission for the 2030s. The 1.2 m aperture, 50 K cryogenic space telescope with the wide-field camera (WFC) will provide the 1,260 square arcmin field-of-view for five photometric bands between 2 and 8 μm. The high resolution spectrometer (HRS) will observe the 10–18 µm with a wavelength resolution of 30,000. The GREX-PLUS WFC field-of-view is 130 times larger than that of the James Webb Space Telescope and similar to those of Euclid and Roman Space Telescope. Since these two survey missions are limited to the wavelength less than around 2 µm, GREX-PLUS will extend the wavelength coverage beyond 2 μm, providing versatile legacy imaging survey significantly improved from previous Spitzer imaging survey in the same wavelength range. The spectral resolution of the GREX-PLUS HRS is 10 times higher than that of the James Webb Space Telescope, opening a new window of the mid-infrared high-resolution spectroscopy from space. The main scientific themes are the galaxy formation and evolution and the planetary system formation and evolution. The GREX-PLUS WFC aims to detect the first generation of “bright” galaxies at redshift z > 15. The GREX-PLUS HRS aims to resolve the Kepler motion of water vapor molecules and identify the location of the water “snowline” in ∼ 100 proto-planetary disks. Both instruments will provide unique data sets for a broad range of scientific topics including galaxy mass assembly, origin of super massive blackholes, infrared background radiation, molecular spectroscopy in the interstellar medium, transit spectroscopy for exoplanet atmosphere, planetary atmosphere in the Solar system, and so on. This paper presents the status of the concept design of GREX-PLUS, including telescope system, WFC, HRS, cooling system, and spacecraft bus system.
Theoretical calculations predict that high-resolution spectroscopy of H2O gas lines in the mid-infrared region is the most promising method to observationally identify the snow-line, which has been proposed as the critical factor separating gas giants from solid planets in the planetary formation process. This requires the spectroscopic observations from space with R = λ/Δλ ≥ 30, 000. For this purpose, we propose a mid-infrared (10-18 μm) high-resolution spectrometer to be onboard the GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) mission. We are developing "immersion grating” spectroscopy technology for high-resolution spectroscopy in space. We have chosen CdZnTe as a candidate for the optical material. We report the current status of the development of the CdZnTe immersion grating, including evaluation of its optical properties (absorption coefficient and refractive index) at cryogenic temperatures, development of an anti-reflection coating with a moth-eye structure for wide-wavelength coverage, and verification of machinability for grating production. We plan to make a prototype spectrometer to demonstrate the capability of the immersion grating with ground-based observations in the N-band (λ = 8–13 μm) and beyond.
We present the current status of our development of a new near-infrared spectrometer for the InfraRed Survey Facility (IRSF) 1.4-m telescope, located in South Africa. The spectrometer is designed to cover the wavelength range of 1.0–1.6 μm with the spectral resolution of 550 at 1.2 μm and 730 at 1.6 μm and have a small number of optical surfaces to achieve a high optical throughput of 55%. The spectrometer is also equipped with a near-infrared slit viewer with a 3′×4′ field of view to perform precise spectral monitoring and mapping. We have tentatively completed the development of the instrument by using a commercial InGaAs detector and confirmed its expected sensitivity and spectral resolution by test observations with the Kagoshima University 1-m telescope. We now plan to replace the current detector with a new InGaAs detector developed for astronomical observations. The new detector covers the same wavelength range as the current one but has a significantly lower dark current and a larger array format, which enables us to upgrade the spectrometer to have higher sensitivity and spectral resolution. We plan to mount the spectrometer on the IRSF telescope by early 2025 after the detector is replaced and the second test observation with the Kagoshima University telescope is done.
The balloon-borne Japan-United States Infrared Interferometry Experiment (JUStIInE) is a pathfinder for the first space-based far-IR interferometer. JUStIInE will mature the system-level technology readiness of spatio-spectral far-IR interferometry and demonstrate this technique with scientific observations. Operating at wavelengths from 30 to 90 µm, JUStIInE will provide unprecedented sub-arcsecond angular resolution and spectroscopic data. Our plan is to develop a cryogenic Michelson beam combiner and integrate it with an existing and tested telescope optical system and gondola from the Japanese Far-infrared Interferometric Telescope Experiment (FITE). With two JUStIInE balloon flights we plan to collect, calibrate, analyze, and publish scientific results based on the first far-IR spatio-spectral observations of young stellar objects, evolved stars, and the active galactic nucleus of NGC 1068. The NASA Astrophysics Roadmap envisages a future in which interferometry is applied across the electromagnetic spectrum, starting in the far-infrared. The Far-IR Probe recommended in the 2021 Decadal Survey presents an opportunity to take that important step. A Far-IR Probe mission based on this concept will enable us to understand terrestrial planet formation and spectroscopically study individual distant galaxies to understand the astrophysical processes that govern their evolution.
GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a new mission concept for ISAS/JAXA’s strategic L-class mission program in the 2030s. With a 1.2 m aperture, a 50 K cryogenic space telescope will have a < 1, 400 arcmin2 wide-field camera with 6 bands in the 2–10 μm wavelength range and a high-dispersion spectrometer with a wavelength resolution of < 30, 000 in the 10–18 μm band. The cryogenic infrared mission concept of GREX-PLUS is based on SPICA, exploiting the technical resources so far studied and developed, such as an active cooling system. The high-dispersion spectrometer of GREX-PLUS is based on the high-dispersion channel of the SPICA Mid-Infrared Instrument (SMI). The wide-field camera of GREX-PLUS is also based on previous concept studies for the ISAS/JAXA’s WISH mission concept. GREX-PLUS is a concept proposal for a Japan-led mission but international collaborations are also welcome.
We performed wave-optics-based numerical simulations at mid-infrared wavelengths to investigate how the presence or absence of entrance slits and optical aberrations affect the spectral resolving power R of a compact, high-spectral-resolving-power spectrometer containing an immersion-echelle grating. We tested three cases of telescope aberration (aberration-free, astigmatism, and spherical aberration), assuming the aberration budget of the Space Infrared Telescope for Cosmology and Astrophysics, which has a 20 μm wavelength diffraction limit. In cases with a slit, we found that the value of R at around 10 to 20 μm is approximately independent of the assumed aberrations, which is significantly different from the prediction of geometrical optics. Our results also indicate that diffraction from the slit improves R by enlarging the effective illuminated area on the grating window and that this improvement decreases at short wavelengths. For the slit-less cases, we found that the impact of aberrations on R can be roughly estimated using the Strehl ratio.
The mid/far infrared hosts a wealth of spectral information that allows direct determination of the physical state of matter in a large variety of astronomical objects, unhindered by foreground obscuration. Accessing this domain is essential for astronomers to much better grasp the fundamental physical processes underlying the evolution of many types of celestial objects, ranging from protoplanetary systems in our own milky way to 10-12 billion year old galaxies at the high noon of galaxy formation in our universe. The joint ESA/JAXA SPICA mission will give such access for the astronomical community at large, by providing an observatory with unprecedented mid- to far-infrared imaging, polarimetric and spectroscopic capabilities.
We present an overview of the cryogenic system of the next-generation infrared observatory mission SPICA. One of the most critical requirements for the SPICA mission is to cool the whole science equipment, including the 2.5 m telescope, to below 8 K to reduce the thermal background and enable unprecedented sensitivity in the mid- and far-infrared region. Another requirement is to cool focal plane instruments to achieve superior sensitivity. We adopt the combination of effective radiative cooling and mechanical cryocoolers to accomplish the thermal requirements for SPICA. The radiative cooling system, which consists of a series of radiative shields, is designed to accommodate the telescope in the vertical configuration. We present thermal model analysis results that comply with the requirements to cool the telescope and focal plane instruments.
SMI (SPICA Mid-infrared Instrument) is one of the three focal-plane science instruments for SPICA. SMI is the Japanese-led instrument proposed and managed by a university consortium. SMI covers the wavelength range from 10 to 36 μm with four separate channels: the low-resolution (R = 60 – 160) spectroscopy function for 17 – 36 μm, the broad-band (R = 5) imaging function at 34 μm, the mid-resolution (R = 1400 – 2600) spectroscopy function for 18 – 36 μm, and the high-resolution (R = 29000) spectroscopy function for 10 – 18 μm. In this presentation, we will show the latest design and specifications of SMI as a result of feasibility studies.
The ESA/JAXA SPICA mission is a candidate for the ESA Cosmic Vision Medium Class M5 opportunity. Since 2019 an Airbus Defence and Space team has been performing a trade-off study (on behalf of ESA) to establish a baseline telescope optical configuration and design, which can meet the mission scientific performance requirements. This paper describes the telescope baseline design selected, with first estimates of the expected optical performance. The optical design wavelength is 20 microns for an operating temperature of 8 K covering a total bandwidth of 12 to 420 microns over a 30 arc minutes field of view, with a total required collecting area of at least 4.0 m². The fundamental mission science driver is to achieve a sky background (astrophysical sources) limited performance. The telescope is designed to illuminate three instruments namely; SMI (JAXA - Japan), SAFARI (SRON - Netherlands) and B-BOP (CEA - France).
We measured the transmittance of low-resistivity (∼ 10^2 Ωcm) and high-resistivity (> 10^10 Ωcm) CdZnTe, which are candidates for Immersion grating (IG) in 10–18μm wavelength, at cryogenic temperature. IG is a compact diffraction grating and we are developing a cryogenically operated 10--18μm IG for SMI/HR (SPICA Mid-Infrared Instrument / High-Resolution spectrometer) of SPICA (SPace Infrared telescope for Cosmology and Astrophysics) We performed two experiments: transmittance measurement with a convergent light Fourier transform spectrometer, and with a collimated lamp beam system. Our result shows that the low-resistivity CdZnTe has large absorption (> 0.5 cm^{−1} ) and the high-resistivity CdZnTe has low absorption (< 0.1 cm^{-1} ) at 8.5K at 10--18 μm. The high-resistivity CdZnTe is promising as an IG material although higher precision measurement is needed to determine whether it meets the absorption coefficient requirement (< 0.01 cm^{-1}) at cryogenic temperature.
HiZ-GUNDAM is a future satellite mission which will lead the time-domain astronomy and the multi-messenger astronomy through observations of high-energy transient phenomena. A mission concept of HiZ-GUNDAM was approved by ISAS/JAXA, and it is one of the future satellite candidates of JAXA’s medium-class mission. We are in pre-phase A (before pre-project) and elaborating the mission concept, mission/system requirements for the launch in the late 2020s. The main themes of HiZ-GUNDAM mission are (1) exploration of the early universe with high-redshift gamma-ray bursts, and (2) contribution to the multi-messenger astronomy. HiZ-GUNDAM has two kinds of mission payload. The wide field X-ray monitors consist of Lobster Eye optics array and focal imaging sensor, and monitor ~1 steradian field of view in 0.5 – 4 keV energy range. The near infrared telescope has an aperture size 30 cm in diameter, and simultaneously observes four wavelength bands between 0.5 – 2.5 μm. In this paper, we introduce the mission overview of HiZ-GUNDAM.
We present an overview of the thermal and mechanical design of the Payload Module (PLM) of the next- generation infrared astronomy mission Space Infrared Telescope for Cosmology and Astrophysics (SPICA). The primary design goal of PLM is to cool the whole science assembly including a 2.5 m telescope and focal-plane instruments below 8 K. SPICA is thereby expected to have very low background conditions so that it can achieve unprecedented sensitivity in the mid- and far-infrared. PLM also provides the instruments with the 4.8 K and 1.8 K stages to cool their detectors. The SPICA cryogenic system combines passive, effective radiative cooling by multiple thermal shields and active cooling by a series of mechanical cryocoolers. The mechanical cryocoolers are required to provide 40 mW cooling power at 4.8 K and 10 mW at 1.8 K at End-of-Life (EoL). End-to-end performance of the SPICA cryocooler-chain from 300 K to 50 mK was demonstrated under the framework of the ESA CryoChain Core Technology Program (CC-CTP). In this paper, we focus on the recent progress of the thermal and mechanical design of SPICA PLM which is based on the SPICA mission proposal to ESA.
SMI (SPICA Mid-infrared Instrument) is one of the two focal-plane science instruments for SPICA. SMI is the Japanese led instrument proposed and managed by a nation-wide university consortium in Japan and planned to be developed in collaboration with Taiwan and the US. SMI covers the wavelength range from 12 to 36 μm with 4 separate channels: the low-resolution (R = 50-120) spectroscopy function for 17-36 μm, the broad-band (R = 5) imaging function at 34 μm, the mid-resolution (R = 1300-2300) spectroscopy function for 18-36 μm, and the high-resolution (R = 28000) spectroscopy function for 12-18 μm. In this paper, we show the results of our conceptual design and feasibility studies of SMI.
Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems.
SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultra-sensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions.
The SPICA instrument complement offers spectral resolving power ranging from ~50 through 11000 in the 17-230 µm domain as well as ~28.000 spectroscopy between 12 and 18 µm. Additionally, SPICA will be capable of efficient 30-37 µm broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 µm range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of ~5x10-20 W/m2 (5σ/1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.
The fairing of the launcher selected for the Space Infrared telescope for Cosmology and Astrophysics (SPICA) mission is not compatible with a primary mirror of 3.5m in diameter. Thus three alternative optical designs of the SPICA Telescope Assembly (STA) with a primary mirror of reduced size were defined and their theoretical optical performances assessed. The impact of the size reduction on the STA optical performances was then quantified. Based on the results of the study, we defined a STA optical design optimum in terms of optical performances and of accommodation of instruments in the STA focal surface.
Light-weight mirrors are developed for two Japanese infrared astronomical missions, ASTRO-F and SPICA. ASTRO-F is scheduled for launch in 2005, while the target year for launch of SPICA is 2010. The mirrors of the ASTRO-F telescope are made of a sandwich-type silicon carbide (SiC) material, comprising porous core and CVD coat of SiC on the surface. Cryogenic measurements of the ASTRO-F primary mirror and telescope assembly were performed extensively. As for the SPICA telescope, which has an aperture of 3.5-m diameter, carbon-fiber-reinforced SiC (C/SiC composite), as well as SiC, is one of the promising candidates for mirror material. C/SiC composite spherical test mirrors of 160-mm diameter has recently been manufactured and tested. This paper presents the experimental results of the cryogenic performance obtained for the sandwich-type SiC mirrors and the C/SiC composite mirrors.
We carried out various tests of 800-mm-diameter aperture, lightweight optics that consisted wholly of carbon fiber-reinforced SiC composite, called HB-Cesic. A cryogenic optical test was performed on the primary mirror to examine any CTE irregularity as a surface change, and only small deformations were observed. The primary mirror was assembled with a convex secondary mirror into an optical system and tested under vacuum at the 6-m-diameter radiometer space chamber at Tsukuba Space Center of JAXA, where we have prepared interferometric metrological facilities to establish techniques to test large optical systems in a horizontal light-axis configuration. The wavefront difference between under vacuum and under atmosphere was confirmed to be less than 0.1 λ at λ=633 nm, if we realigned the optical axis of the interferometer and flat mirror under vacuum. We also demonstrated a stitching interferometry using the Φ800-mm optics by rotating a mask wheel of subapertures in front of the optical reference flat. The wavefront stitched from eight individual measurements of Φ275-mm subapertures differs from the full-aperture measurement without the mask by about 0.1 λ nm RMS, which showed the technique could able to be applied to test large telescopes especially for infrared wavelength region.
A large-scale lightweight mirror that is made of silicon carbide-based material is required for the coming astronomical and earth observation missions. The influence of the inhomogeneity of the coefficient of thermal expansion (CTE) on specular surface accuracy was studied as an important technological issue for such a large optical component. At first, a systematic case study for the series of CTE’s main factors was conducted using the finite element method, and consequently a comprehensive equation to calculate the amount of surface deviation was derived. Based on that technology, finite element analysis to simulate the surface accuracy profile that a test mirror sample showed during cryogenic measurement was carried out using experimentally obtained CTE data from cutout test pieces, and the profile was successfully reproduced.
SMI (SPICA Mid-infrared Instrument) is one of the two focal-plane scientific instruments planned for new SPICA, and
the Japanese instrument proposed and managed by a university consortium in Japan. SMI covers the wavelength range of
12 to 36 μm, using the following three spectroscopic channels with unprecedentedly high sensitivities: low-resolution
spectroscopy (LRS; R = 50 - 120, 17 - 36 μm), mid-resolution spectroscopy (MRS; R = 1300 - 2300, 18 - 36 μm), and
high-resolution spectroscopy (HRS; R = 28000, 12 - 18 μm). The key functions of these channels are high-speed dustband
mapping with LRS, high-sensitivity multi-purpose spectral mapping with MRS, and high-resolution molecular-gas
spectroscopy with HRS. This paper describes the technical concept and scientific capabilities of SMI.
We present the latest results of the sensitivity estimate for spectrometers of the SPICA Mid-Infrared
Instrument (SMI). SMI has three spectroscopic channels; low resolution spectrometer (LRS), medium
resolution spectrometer (MRS) and high resolution spectrometer (HRS). Taking account of the results of
optical design of each spectrometer and the latest information of the expected performance of detector
arrays, the continuum sensitivity for a point source, the continuum sensitivity for an extended source,
the line sensitivity for a point source, the line sensitivity for an extended source, and the saturation limit
are calculated for LRS, MRS and HRS and are provided in this paper.
We present the design, fabrication and test results for a dichroic mirror, which was primarily developed for the SPICA Coronagraph Instrument (SCI), but is potentially useful for various types of astronomical instrument. The dichroic mirror is designed to reflect near- and mid-infrared but to transmit visible light. Two designs, one with 3 layers and one with 5 layers on BK7 glass substrates, are presented. The 3-layer design, consisting of Ag and ZnS, is simpler, and the 5-layer design, consisting of Ag and TiO2 is expected to have better performance. Tape tests, evaluation of the surface figure, and measurements of the reflectivity and transmittance were carried out at ambient temperature in air. The reflectivity obtained from measurements made on mirrors with 5 layers were < 80 % for wavelengths, λ, from 1.2 to 22 μm and < 90 % for λ from 1.8 to 20 μm. The transmittance obtained from measurements made on mirrors with 5 layers were < 70 % for λ between 0.4 and 0.8 μm. Optical ghosting is estimated to be smaller than 10-4 at λ < 1.5 μm. A protective coating for preventing corrosion was applied and its influence on the reflectivity and transmittance evaluated. A study examining the trade-offs imposed by various configurations for obtaining a telescope pointing correction signal was also undertaken.
We present the current status of SPICA (Space Infrared Telescope for Cosmology and Astrophysics), which is a mission optimized for mid- and far-infrared astronomy with a cryogenically cooled 3.2 m telescope. SPICA is expected to achieve high spatial resolution and unprecedented sensitivity in the mid- and far-infrared, which will enable us to address a number of key problems in present-day astronomy, ranging from the star-formation history of the universe to the formation of planets. We have carried out the “Risk Mitigation Phase” activity, in which key technologies essential to the realization of the mission have been extensively developed. Consequently, technical risks for the success of the mission have been significantly mitigated. Along with these technical activities, the international collaboration framework of SPICA had been revisited, which resulted in maintenance of SPICA as a JAXA-led mission as in the original plan but with larger contribution of ESA than that in the original plan. To enable the ESA participation, a SPICA proposal to ESA is under consideration as a medium-class mission under the framework of the ESA Cosmic Vision. The target launch year of SPICA under the new framework is FY2025.
In this report we describe our development of a prototype inverse-polished mirror for the passive correction of the static
and predictable wavefront errors (WFE) of space-based telescopes, in particular, especially for infrared coronagraphs.
An artificial WFE pattern with a root mean square (rms) value of 350 nm was numerically generated to facilitate the
design of the prototype mirror. The surface of the mirror is approximately flat, is 50.0 mm in diameter and 15.0 mm
thick at the edge. The designed WFE pattern was constructed on the mirror surface by micro-polishing. Both the figure
and roughness of the mirror surface were evaluated. The rms value of the measured surface figure was reduced to 135
nm after subtraction of the designed surface figure. The benefit of subtraction to mid-infrared coronagraph performance
was simulated, which showed the contrast was improved by a factor of ~100 close to the core (closer than 10 λ/D where
λ and D are the wavelength and telescope aperture diameter, respectively) of the coronagraphic image of a point source.
An analysis of the power spectrum density shows that the lower frequencies in the WFE are well reproduced on the
mirror, while the higher frequencies remain due to the limitations imposed on the controllable spatial resolution by the
fabrication process. In this study, inverse-polished mirrors combined with deformable mirrors and their application to
ground-based telescopes are also discussed. To fully explore the potential of the inverse-polished mirror, a systematic
allocation of the error budget is essential taking into account not only the fabrication accuracy of the mirror but also an
evaluation of the telescope and other factors with non-predictable uncertainties.
Focal Plane Arrays (FPA) are key items for modern astronomical observations in the near infrared wavelength, but it is very expensive and not easy to get them. Less expensive NIR FPAs with reasonable performance are very important to spread NIR observation extensively. FPA640×512 manufactured by Chunghwa Leading Photonics Tech is a 640×512 InGaAs detector covering the 0.9-1.7 μm wavelength. Since this array is significantly cheaper than the commonly used NIR FPAs in the astronomical observation, it is possible to be a good choice for particular projects which do not need many pixels, if FPA640×512 has acceptable performance for the purpose. We have evaluated one test grade array of FPA640×512 both in the room and low temperature environment. In order to evaluate the characteristics of this FPA in the low temperature environment, we cooled it down by the mechanical refrigerator and confirmed that it works at 100 K. We have found that the dark current reduces exponentially as the FPA temperature decreases, but it hits the bottom at~1000 e−/sec bellow 200 K with the default setting. We are trying to reduce the dark current by optimizing the bias voltage and the current to the MUX circuit. The latest experiments have shown the possibility that the dark current decreases to~200 e−/sec. This value is still higher than that of NIR FPAs used in the scientific observation, but it may be applicable for the particular purpose, for example, FPAs for slit viewer in spectrometers, wave front sensor, and so on.
Owing to its high specific stiffness and high thermal stability, silicon carbide is one of the materials most suitable for large space-borne optics. Technologies for accurate optical measurements of large optics in the vacuum or cryogenic conditions are also indispensable. Within the framework of the large SiC mirror study program led by JAXA, we manufactured an 800-mm-diameter lightweight telescope, all of which is made of HB-Cesic, a new type of carbon-fiber-reinforced silicon carbide (C/SiC) material developed jointly by ECM, Germany and MELCO, Japan. We first fabricated an 800-mm HB-Cesic primary mirror, and measured the cryogenic deformation of the mirror mounted on an HB-Cesic optical bench in a liquid-helium chamber. We observed the cryo-deformation of 110 nm RMS at 18 K with neither appreciable distortion associated with the mirror support nor significant residual deformation after cooling. We then integrated the primary mirror and a high-order aspheric secondary mirror into a telescope. To evaluate its optical performance, we established a measurement system, which consists of an interferometer in a pressure vessel mounted on a 5-axis adjustable stage, a 900-mm auto-collimating flat mirror, and a flat mirror stand with mechanisms of 2-axis tilt adjustment and rotation with respect to the telescope optical axis. We installed the telescope with the measurement system into the JAXA 6-m chamber and tested them at a vacuum pressure to verify that the system has a sufficiently high tolerance against vibrations in the chamber environment. Finally we conducted a preliminary study of sub-aperture stitching interferometry, which is needed for telescopes of our target missions in this study, by replacing the 900-mm flat mirror with a rotating 300-mm flat mirror.
Silicon carbide (SiC) has good thermal conductivity, high stiffness, and a relatively low specific density, all of which are advantageous to the application to telescopes operating at cryogenic temperatures. The first Japanese astronomical infrared space mission AKARI, which was launched in 2006 February and completed the second generation all-sky survey at 6 bands from mid- to far-infrared, employed a 700mm cryogenic telescope made of specially developed SiC. It was a sandwich-type of SiC composed of a lightweight porous core and a dense chemical vapor deposition (CVD) coat to decrease the specific density and facilitate machining for achieving the required surface figure accuracy. Measurements with an interferometer of 160-mm sample mirrors demonstrated that the AKARI mirror SiC had good thermal stability down to cryogenic temperatures (~6K), while the mirror support of the compact design became the primary source of the wave-front errors of the AKARI telescope. Taking the advantage of the heritage of the AKARI telescope development as well as ESA’s Herschel telescope, we are planning the next infrared space mission SPICA (Space Infrared Telescope for Cosmology and Astrophysics) of a 3.2m cooled telescope in participation of ESA using SiC-based materials. In this presentation, we summarize the development of AKARI SiC telescope and present the development activities of the SPICA telescope from the point of view of SiC being as the mirror material for cryogenic space infrared telescopes.
AKARI, the Japanese satellite mission dedicated to infrared astronomy was launched in 2006 February and exhausted its liquid helium in 2007 August. During the cold mission phase, the Infrared Camera (IRC) onboard carried out an all-sky survey at 9 and 18µm with better spatial resolution and higher sensitivity than IRAS. Both bands also have slightly shorter wavelength coverage than IRAS 12 and 25μm bands and thus provide different information on the infrared sky. All-sky image data of the IRC are now in the final processing and will be released to the public within a year. After the exhaustion of the cryogen, the telescope and focal plane instruments of AKARI had still been kept at sufficiently low temperatures owing to the onboard cryocooler. Near-infrared (NIR) imaging and spectroscopic observations with the IRC had continued until 2011 May, when the spacecraft had a serious problem in the power supply system that forced us to terminate the observation. The IRC carried out nearly 20000 pointing observations in total despite of its near-earth orbit. About a half of them were performed after the exhaustion of the cryogen in the spectroscopic modes, which provided high-sensitivity NIR spectra from 2 to 5µm without disturbance of the terrestrial atmosphere. During the warm mission phase, the temperature of the instrument gradually increased and changed the array operation conditions. We present a summary of AKARI/IRC observations, including the all-sky mid-infrared diffuse data as well as the data taken in the warm mission phase.
SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japan-led infrared astronomical satellite project
with a 3.2-m lightweight cryogenic telescope. The SPICA telescope has stringent requirements such as that for the
imaging performance to be diffraction-limited at the shortest core wavelength of 5 microns at the operating temperature
of 6 K. The design of the telescope system has been studied by the Europe-Japan telescope working group led by ESA
with the European industries, the results of which will be presented in other papers. We here present our recent optical
testing activities in Japan for the SPICA telescope, focusing on the experimental and numerical studies of stitching
interferometry. The full pupil of the SPICA telescope will be covered by a sub-pupil array consisting of small
autocollimating flat mirrors (ACFs), which are rotated with respect to the optical axis of the telescope. For preliminary
stitching experiments, we have fabricated an 800-mm lightweight telescope all made of the C/SiC called HBCesic, which
is a candidate mirror material for the SPICA telescope, and started optical testing with 900-mm and 300-mm ACFs at an
ambient temperature. ACFs can suffer significant surface deformation in testing a telescope at cryogenic temperatures,
which is difficult to be measured directly. We therefore investigate the effects of the surface figure errors of the ACFs on
stitching results by numerical simulation.
We present the current status of the development of the SPICA Coronagraph Instrument (SCI). SPICA is a next-generation
3-meter class infrared telescope, which will be launched in 2022. SCI is high-contrast imaging, spectroscopic
instrument mainly for direct detection and spectroscopy of extra-solar planets in the near-to-mid infrared wavelengths to
characterize their atmospheres, physical parameters and evolutionary scenarios. SCI is now under the international
review process. In this paper, we present a science case of SCI. The main targets of SCI, not only for direct imaging but
also for spectroscopy, are young to matured giant planets. We will also show that some of known exoplanets by ground-based
direct detection are good targets for SCI, and a number of direct detection planets that are suitable for SCI will be
significantly increased in the next decade. Second, a general design of SCI and a key technology including a new high-throughput
binary mask coronagraph, will be presented. Furthermore, we will show that SCI is potentially capable of
achieving 10-6 contrast by a PSF subtraction method, even with a telescope pointing error. This contrast enhancement
will be important to characterize low-mass and cool planets.
We present a test of optical metrology for 800-mm spaceborne optics in the 6-m radiometer thermal vacuum chamber at
JAXA's Tsukuba Space Center of JAXA. Under the framework of the JAXA's large-optics study program for astronomy
and Earth observations, we developed a test bench for interferometric metrology of large optics with an auto-collimation
method in the chamber. The optical system was aligned in a horizontal light-axis configuration within the facility limit to
handle a 3.5-m aperture telescope like SPICA. A high-speed interferometer was contained in an aluminum and titanmade
pressure vessel, which was mounted on the five-axis stage. We tested the 800-mm lightweight C/SiC optics using a
900-mm diameter flat mirror. Alignment changes in tilts of about ten arcseconds were observed as pressure went down
from 1 atm to vacuum. After we re-aligned the interferometer and flat mirror, the wavefronts through the optics under
vacuum were observed to increase in astigmatism aberration by 0.07λRMS at λ=633nm from under atmosphere, which
might be caused by a deformation in the test optics or flat mirror.
SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is a Japan-led infrared astronomical satellite project
with a 3-m-class telescope in collaboration with Europe. The telescope is cooled down to temperature below 6 K in space
by a combination of mechanical coolers with radiative cooling in space. The 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 6 K. The mirrors
will be made of silicon carbide (SiC) or its related material, which has large heritages of the AKARI and Herschel
telescopes. The design of the telescope system has been studied by the Europe-Japan telescope working group led by
ESA with European industries to meet the requirements. As for optical testing, responsibilities will be split between
Europe and Japan so that final optical verification at temperatures below 10 K will be executed in Japan. We present our
recent optical testing activities in Japan for the SPICA telescope, which include the numerical and experimental studies
of stitching interferometry as well as modifications of the 6-m-diameter radiometer space chamber facility at Tsukuba
Space Center in JAXA. We also show results of cryogenic optical testing of the 160-mm and 800-mm lightweight
mirrors made of a C/SiC material called HBCesic, which is a candidate mirror material for the SPICA telescope.
We present the current status of the development of a far-infrared monolithic Ge:Ga photoconductor array
proposed for the SAFARI instrument onboard SPICA, which is a future infrared space mission. SPICA has a
large (3-m class) cooled (<6 K) telescope, which enables us to make astronomical observations with high spatial
resolution and unprecedented sensitivity in the mid- and far-infrared wavelength. As a candidate detector to
cover the 45-110 μm band of a far-infrared focal plan instrument of SAFARI, we are developing a large format
monolithic Ge:Ga array. The monolithic Ge:Ga array is directly connected to cryogenic readout electronics
(CRE) using the Au-Indium bumping technology. Our goal is to develop a 64×64 Ge:Ga array, on the basis
of existing technologies and experience in making the 3×20 Ge:Ga monolithic arrays for the AKARI satellite.
In order to realize a larger format array with better sensitivity than that of the AKARI array, we have been
making some technical improvements; (1) development of the Au-In bumping technology to realize the large
format array, (2) optimization of the structure of the transparent electrode to achieve the better sensitivity, (3)
development of an anti-reflection coating to reduce interference fringe between the Ge substrate, and (4) Use
of the low-noise cryogenic readout electronics with low power consumption. We fabricated the prototype 5×5
Ge:Ga arrays to demonstrate and evaluate the properties of monolithic array. We demonstrate experimentally
the feasibility of these elemental technologies, and also show the results of performance measurements for the
prototype Ge:Ga arrays.
Space-borne large optics will be required in future missions for astronomy and earth observations. In order to realize
large-optics missions, JAXA has started the study of the ground measurement techniques of large optics. The 6m
diameter radiometer thermal vacuum chamber (6m chamber) at Tsukuba Space Center will be used for tests of JAXA's
future large-optics missions like Space Infrared Telescope for Cosmology and Astrophysics (SPICA). We measured the
vibration environment of the 6m chamber for the feasibility study of precise optical measurement. We placed a test
mirror inside the chamber and measured the surface figures of the mirror from outside the chamber with a high-speed
interferometer, while the chamber was being vacuum-pumped and cooled by liquid nitrogen; we also directly measured
the vibrational levels with accelerometers concurrently. The measurements were performed for each phase of the
chamber system operation including pumping and cooling processes. This paper presents the results about optical
measurement under the vibration environment on the 6m chamber. We confirm that the vibrations from pumps and
shroud have negligible effects on optical measurements owing to a vibration isolation system in the 6m chamber.
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.
Infrared Camera (IRC) onboard AKARI satellite has carried out more than 4000 pointed observations during the phases
1 and 2, a significant amount of which were performed in the spectroscopic mode. In this paper, we investigate the
properties of the spectroscopic data taken with MIR-S channel and propose a new data reduction procedure for slit-less
spectroscopy of sources embedded in complicated diffuse background structures. The relative strengths of the 0th to 1st
order light as well as the efficiency profiles of the 2nd order light are examined for various objects taken with MIR-S
dispersers. The boundary shapes of the aperture mask are determined by using the spectroscopic data of uniform zodiacal
emission. Based on these results, if the appropriate template spectra of zodiacal light emission and the diffuse
background emission are prepared and the geometries of the diffuse structures are obtained by the imaging data, we can
reproduce the slit-less spectroscopic patterns made by a uniform zodiacal emission and the diffuse background emission
by a convolution of those template profiles. This technique enables us to obtain the spectra of infrared sources in highly
complicated diffuse background and/or foreground structures, such as in the Galactic plane and in nearby galaxies.
The AKARI satellite carries a cryogenically cooled telescope of an F/6 Richey-Chetien system with a sandwich-type
silicon carbide (SiC) primary mirror of 685mm in effective diameter. The AKARI satellite ran out of the liquid helium
(LHe) cryogen on 26 August 2006. With LHe the telescope system was kept around 6K, whereas it is kept around 40K
by the on-board cryocoolers after the LHe exhaustion. The telescope system has a focus adjustment mechanism in the
secondary mirror assembly. The telescope focus on orbit was adjusted referring to images taken with the Infrared
Camera (IRC) on board. The focus adjustment was made both at 6 and 40K. The in-orbit imaging performance at 6K
was estimated to be diffraction limited at 7.3μm, a little worse than the laboratory measurements prior to the launch. It
was slightly degraded to be approximately diffraction limited at 8μm at 40K as expected from the laboratory test, but the
movement of the focus position was in the opposite sense to the ground test. The AKARI mission provided us data of the
focus shift with temperature on orbit for the first time. We report an overview of the AKARI telescope system and the
focus adjustment operations at 6 and 40K.
We present the in-orbit performance of slow-scan observation of the Far-Infrared Surveyor (FIS) onboard the
AKARI satellite. The FIS, one of the two focal-plane instruments of AKARI, has four photometric bands from
50-180 μm with two kinds of Ge:Ga array detectors. In addition to the All-Sky Survey, the FIS also took images
of specific targets by the slow-scan. Because of the longer exposure time on a targeted source, the sensitivity
in the slow-scan mode is 1-2 orders of magnitude better than that in the All-Sky Survey mode. In order to
evaluate the point spread functions (PSFs), several bright point-like objects such as asteroids, stars, and galaxies
were observed. Though significant enhancements are seen at the tails of the PSFs, the derived full width at
the half maximum (FWHM) are consistent with those expected from the optical simulation and the laboratory
measurements; ~40" for two shorter wavelength bands and ~60" for two longer wavelength bands, respectively.
The absolute photometric calibration has been performed by observing well established photometric calibration
standards (asteroids and stars) in a wide range of fluxes. After the establishment for the method of the aperture
photometry, the photometric accuracy for point sources is less than 10% in all bands.
We report the in-orbit performance of the AKARI/Far-Infrared Surveyor Ge:Ga photoconductors, focusing on the
transient response and the radiation effects, to perform the characterization of these effects for data analyses. The
behavior for these effects is found to be significantly different between the Short-Wavelength and Long-Wavelength
array detectors of the FIS, most probably due to the difference in the array configuration. We discuss cosmic-ray
radiation effects, referring to the results of pre-flight proton-beam irradiation measurements. We also describe our efforts
to correct the slow transient response of the detectors by adopting a physical approach.
We have developed an imaging Fourier transform spectrometer (iFTS) for space-based far-infrared astronomical
observations. The iFTS employs newly developed photoconductive detector arrays with a capacitive transimpedance
amplifier, which makes the iFTS a completely unique instrument. The iFTS was installed as a
function of the far-infrared instrument (FIS: Far-Infrared Surveyor) on the Japanese astronomical satellite,
AKARI, which was launched on February 21, 2006 (UT) from the Uchinoura Space Center. The iFTS had
worked properly in the space environment as well as in laboratory for more than one year before liquid helium
ran out on August 26, 2007. The iFTS was operated nearly six hundreds of pointed observations. More than
one hundred hours of astronomical observations and almost the same amount of time for calibrations have been
carried out in the mission life. Meanwhile, it becomes clear that the detector transient effect is a considerable
factor for FTSs with photoconductive detectors. In this paper, the instrumentation of the iFTS and interesting
phenomena related to FTSs using photoconductive detectors are described, and the calibration strategy of the
iFTS is discussed briefly.
The SPICA mission has been proposed to JAXA as the second Japanese IR space telescope to be launched in 2017. The
SPICA spacecraft, launched with an H-IIA launch vehicle, is to be transferred into a halo orbit around the Sun-Earth L2,
where effective radiant cooling is feasible owing to solar rays and radiant heat fluxes from the Earth constantly coming
from the same direction. That optimal thermal environment enables this IR space telescope to use a large 3.5-mdiameter-
single-aperture primary mirror cooled to 4.5 K with advanced mechanical cryocoolers and effective radiant
cooling instead of a massive and short-lived cryogen. As a result of thermal and structural analyses, the thermal design of
cryogenic system was obtained. Then, mechanical cryocoolers have been developed to meet cooling requirement at 1.7
K, 4.5 K and 20 K. The latest results of upgrading of the 20 K-class two-stage Stirling cooler, the 4K-class JT cooler,
and the 1K-class JT cooler indicate that all cryocoolers gain a sufficient margin of cooling capacity with unprecedentedly
low power consumption for the cooling requirement. It is concluded that the feasibility of the SPICA mission was
confirmed for the critical cryogenic system design, while some attempts to achieve higher reliability, higher cooling
capacity and less vibration have been continued for stable operations throughout the entire mission period.
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.
In this paper, we describe our recent activities on wave-front measurement of space infrared telescopes. Optical performance of the 685-mm lightweight telescope on board the Japanese infrared astronomical satellite, ASTRO-F, has been evaluated at cryogenic temperatures. The mirrors of the ASTRO-F telescope are made of sandwich-type silicon carbide (SiC) material, comprising porous core and CVD coat of SiC on the surface. The total wavefront errors of the telescope were measured with an interferometer from outside a liquid-helium chamber; a 75-cm reflecting flat mirror was used for auto-collimating the light from the interferometer. The cryogenic deformation of the flat mirror was derived independently by shifting it in the chamber and its contribution to the wavefront error was removed. In addition to the ASTRO-F telescope, we are currently developing a 3.5-m telescope system for SPICA, the next Japanese infrared astronomical satellite project. Details of our methodology for the ASTRO-F telescope, together with our optical test plan for the SPICA telescope, are reported.
KEYWORDS: Space telescopes, Telescopes, Mirrors, Silicon carbide, Infrared telescopes, Cryogenics, James Webb Space Telescope, Optical instrument design, Silicon, Far infrared
The Space Infrared Telescope for Cosmology and Astrophysics (SPICA) mission is the third Japanese astronomical infrared satellite project of a 3.5m cooled telescope optimized for mid- to far-infrared observations, following the Infrared Telescope in Space (IRTS) and the ASTRO-F missions. It will employ mechanical coolers and an efficient radiative cooling system, which allow us to have a cooled (4.5K) telescope of the aperture much larger than previous missions in space. The SPICA will attack a number of key problems in present-day astrophysics, ranging from the origin of the universe to the formation of planetary systems, owing to its high spatial resolution and unprecedented sensitivity in the mid- to far-infrared. The large aperture size for cryogenically use is, however, a great challenge and demands substantial technology developments for the telescope system. We adopt monolithic mirror design in the baseline model because of the technical feasibility and reliability. We set the optical performance requirement as being diffraction limited at 5μm at the operating temperature of 4.5K. The total weight attributed to the telescope system is 700kg, which requires a very light 3.5m primary mirror together with the mirror support structure. At present we are working on two candidate materials for the SPICA telescope: silicon carbide (SiC) and carbon-fiber reinforced silicon carbide (C/SiC). This presentation gives a general overview of the SPICA mission and reports the current design and status of the SPICA telescope system, including recent progress of the development of C/SiC 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.
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.
We present pre-flight performance of a monolithic Ge:Ga array detector for Far-Infrared Surveyor (FIS) onboard the ASTRO-F satellite. The primary purpose of the ASTRO-F mission is to perform an all-sky survey in four photometric bands form 50-200 um. For shorter half of this spectral range, 50-110 um, we have developed the monolithic Ge:Ga array which is directly connected to a cryogenic readout electronics (CRE) with the indium-bump technology. In order to investigate the point-source detectability in the survey observation, we carry out a simulation experiment. The experiment was done by taking a image of moving pinholes located on the focal plane of the FIS optics. A clear image without any distortion was obtained, but the size of point source image is slightly larger than expected. We estimate the detection limit in the survey observation by taking account of all detector properties including the imaging performance. The results show that the detector sensitivity is sufficiently high to meet the requirement of the ASTRO-F mission.
Placed on the L2 Lagrangian point, the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) will operate in the 5 to 200 μm wavelength range, at 4.5K. The large aperture telescope (3.5m diameter in a single piece) requires a strong manufacturing mastering, associated with high technical performances. The background acquired by EADS-Astrium (France) on the 3.5m Silicone Carbide Herschel Telescope is a key for the success of the SPICA development. EADS-Astrium has been awarded by the Japan Aerospace Exploration Agency (JAXA) and Sumitomo Heavy Industries to assess the feasibility of the 3.5m all SiC telescope through a design phase contract. The Telescope driving requirements are the large diameter of 3.5m especially critical for the manufacturing aspects, and the Wave Front Error which has to be kept below 350nm rms over a large temperature range from ambient to the operational temperature of 4.5K which requires a strong mastering of the distortions.
The Far-Infrared Surveyor (FIS) is a focal plane instrument of the ASTRO-F satellite, and is designed primarily to achieve far-infrared all sky survey with four photometric bands in wavelength range of 50 - 200um. Compared to IRAS, the FIS has higher sensitivity, higher spatial resolution, and longer wavelength coverage. The FIS also has spectroscopic capability with a Fourier transform spectrometer (FTS). In order to assemble these two kinds of instrument into a small and light body, we have developed new compact detector arrays and adopt the unique optical design. In the first half year of the ASTRO-F mission, the all sky survey is performed intensively, and is completed in the following half year. In addition to this survey, the telescope can be kept to a specific direction during 10 minutes for pointing observations. In pointing observations, we can take deep photometric images by using the photometric mode, or can take spectra by using the FTS. According to the laboratory calibration, it is expected that the detection limit of the all sky survey is almost one order of magnitude better than the IRAS one. The FTS could take spectra with full spectral resolution for about two orders of magnitude brighter sources than the detection limit of the all sky survey for one pointing observation. Due to the imaging FTS, the observing efficiency is much improved for the extended sources. The FIS will provide us unique and valuable observational data in the far-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.
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.
ASTRO-F is a Japanese infrared satellite, which is scheduled for launch in early 2004. Far-infrared instrument that will be onboard ASTRO-F, Far-Infrared Surveyor (FIS), will perform the four-color all sky survey in the 50-200 um wavelength range with the diffraction-limited spatial resolution for 67-cm diameter telescope. For short-wave photometric bands of 50-110 um, we have developed a monolithic Ge:Ga two-dimensional array detector with no light cavity. This top-illumination type array design is promising for making future large-format array. The monolithic Ge:Ga is directly attached onto cryogenic readout electronics, capacitive trans-impedance amplifier composed of silicon p-MOSFETs, designed specially for low-temperature use. Results of the detector measurements show that the device works properly and sensitive enough for astronomical applications. Complex behavior of the detector, such as non-linearity of the integration ramp, transient response, non-uniform responsivity in the array, and cross-talk response, which may cause systematic error in the photometry, have been found. But, these effects are ~10% of major part of the signal and correctable with accuracy of a few %.
The telescope onboard Japanese infrared astronomical satellite, ASTRO-F, forms an F/6 Ritchey-Chretien system with a primary mirror of 670 mm in diameter, the total weight of which is about 42 kg. The primary and secondary mirrors are made of a sandwich-type SiC material, consisting of light porous core and dense CVD coat of SiC. The whole system will be cooled down to 5.8 K with a combined use of super-fluid liquid helium and mechanical coolers on orbit. In order to estimate optical performance of the flight-model telescope at operating cryogenic temperatures, the primary mirror alone was first cooled and tested, and then the whole telescope assembly was tested at cryogenic temperatures. In both cases, the changes in the surface figure were measured from outside the cryostat by an interferometer for the temperature range of 10 K to 300 K. As a result, non-negligible degradation in wave-front errors of the primary mirror and the telescope assembly was detected at low temperatures. The deformation of the primary mirror was found to be mainly due to the thermal contraction of support structures and heat anchors, and degradation by the SiC mirror itself was much smaller. The observed wave-front error of the telescope assembly at 13 K, which was found to originate mainly from the distortion of the primary mirror, marginally meets the requirement to achieve the diffraction-limited performance at 5 microns. This paper summarizes the optical performances thus achieved at cryogenic temperatures for the ASTRO-F telescope.
The ASTRO-F project is currently in its final stage of proto-model, which is constructed same as flight-model. Since instrument goals of the Far-Infrared Surveyor (FIS) are unprecedented achievement of high sensitivity and high spatial resolution in far-infrared wavelength, the proto- model stage is important to prove the performance as the flight instrument. We mainly present here the latest optical, thermal, and mechanical properties of the proto- model of the FIS.
We have developed the new balloon-borne telescope, Far Infrared Balloon-Borne Experiment (FIRBE), to survey the far-infrared radiations of star-forming regions. The primary mirror is an offset parabolid with a diameter of 50 cm (F/2) and telescope structure is made from Carbon Fiber Reinforced Plastics to lighten the whole telescope and hold the strain of image at the focal position minimum since its thermal contraction is very small. The telescope optics is off-axis system with on second mirror and no warm support structure in its optical path in order to reduce the infrared emission from the telescope structure itself.
ASTRO-E is the next Japanese x-ray satellite to be launched in the year 2000. It carries three high-energy astrophysical experiments, including the hard x-ray detector (HXD) which is unique in covering the wide energy band from 10 keV to 700 keV with an extremely low background. The HXD is a compound-eye detector, employing 16 GSO/BGO well-type phoswich scintillation counters together with 64 silicon PIN detectors. The scintillation counters cover an energy range of 40 - 700 keV, while the PIN diodes fill the intermediate energy range from 10 keV to 70 keV with an energy resolution about 3 keV. In this paper, we report on the developments of the large area, thick silicon PIN diodes. In order to achieve a high quantum efficiency up to 70 keV with a high energy resolution, we utilize a double stack of silicon PIN diodes, each 20 by 20 mm2 in size and 2 mm thick. Signals from the two diodes are summed into a single output. Four of these stacks (or eight diodes) are placed inside the deep BGO active-shield well of a phoswich counter, to achieve an extremely low background environment. Thus, the HXD utilizes 64 stacked silicon PIN detectors, achieving a total geometrical collecting area of 256 cm2. We have developed the 2 mm thick silicon PIN diodes which have low leakage current, a low capacitance, and a high breakdown voltage to meet the requirements of our goal. Through various trials in fabricating PIN diodes with different structures, we have found optimal design parameters, such as mask design of the surface p+ layer and the implantation process.
Astro-E is the x-ray satellite to be launched in the year 2000 by Inst. of Space & Astronautical Science. This report deals with the design and expected performance of the hard x-ray detector (HXD), one of the 3 experiments aboard Astro- E. The HXD is a combination of GSO/BGO well-type phoswich counters and silicon PIN diodes: the two combined will cover a wide energy band of 10 - 700 keV. The detector is characterized by its low background of approximately 10-5/s/cm2/keV and its sensitivity higher than any past missions between a few 10 keV and several 100 keV. Combined with the other 2 experiments, a micro-calorimeter array (XRS) and 4 CCD arrays (XIS), both with x-ray mirrors, the mission will cover the soft and hard x-ray range at a highest sensitivity.
The ASTRO-E satellite is scheduled for launch in 2000 by the Institute of Space and Astronautical Science (ISAS). In this paper the design and performance of the hard x ray detector (HXD) developed for ASTRO-E are described. The HXD is a combination of YAP/BGO phoswich scintillators and silicon PIN diodes covering a wide energy band of 10 - 700 keV. The detector background is reduced down to several times 10-6c/s/cm2/keV, and the sensitivity of the HXD is more than one order of magnitude higher than any other past missions in the range of a few 10 keV to several 100 keV. Thus ASTRO-E HXD is expected to achieve an extreme high performance for detecting cosmic hard x rays and low-energy gamma rays. Astrophysics to be explored with the HXT are expected to be extremely widespread and rich.
The fourth Japanese x-ray astronomy satellite, ASCA, carries two imaging gas scintillation proportional counters (GIS) on its focal plane. Extensive ground calibration has established its position resolution to be 0.5 mm and FWHM energy resolution to be 8.0% both at 6 keV. When combined with the x-ray telescope, a sensitivity range becomes 0.7 - 10 keV. These properties have been confirmed through in-orbit calibrations. The in-orbit background of the GIS has been confirmed to be as low as (5 - 7) X 10-4 c s-1cm-2keV-1 over the 1 - 10 keV range. The long-term detector gain is stable within a few % for two years. Gain dependence on the position and temperature has been calibrated down to 1%. The overall energy response is calibrated very accurately. Thus the GIS is working as an all-round cosmic x-ray detector.
Yoshiki Kohmura, Yasushi Fukazawa, Yasushi Ikebe, Yoshitaka Ishisaki, S. Kamijo, Hidehiro Kaneda, Kazuo Makishima, Kyoko Matsushita, K. Nakagawa, Makoto Tashiro, Takaya Ohashi, Hiroyuki Inoue, Manabu Ishida, F. Makino, Toshio Murakami, Y. Ogawara, Y. Tanaka, Yoshihiro Ueda, K. Ebisawa, Teruyoshi Mihara, T. Takeshima, Takeshi Tsuru, K. Hiyoshi, M. Horii, R. Shomura, Koji Taguchi
The fourth Japanese X-ray astronomy satellite, ASTRO-D, was launched successfully by the Institute of Space and Astronautical Science on February 20, 1993 and was named ASCA. Two of the focal plane detectors are imaging gas scintillation proportional counters (Gas Imaging Spectrometer:GIS). The GIS sensors performed the energy resolution of 8% FWHM at 6 keV, and position resolution of 0.5 mm FWHM on-board, which confirmed their ultimate capability as gas counters. The non-Xray background counting rate was approximately 6 X 10-4 c/s/cm2/keV in the energy range of 2 - 10 keV, which was as low as that achieved by the Ginga instrument. The scientific results obtained by the GIS sensors are also presented.
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