Akira Ohnishi

The Infrared Astronomical Mission AKARI

AKARI, the first Japanese satellite dedicated to infrared astronomy, was launched on 2006 February 21, and started observations in May of the same year. AKARI has a 68.5 cm cooled telescope, together with two focal-plane instruments, which survey the sky in six wavelength bands from mid- to far-infrared. The instruments also have a capability for imaging and spectroscopy in the wavelength range 2-180 mu m in the pointed observation mode, occasionally inserted into a continuous survey operation. The in-orbit cryogen lifetime is expected to be one and a half years. The All-Sky Survey will cover more than 90% of the whole sky with a higher spatial resolution and a wider wavelength coverage than that of the previous IRAS all-sky survey. Point-source catalogues of the All-Sky Survey will be released to the astronomical community. Pointed observations will be used for deep surveys of selected sky areas and systematic observations of important astronomical targets. These will become an additional future heritage of this mission.

A variable-emittance radiator based on a metal–insulator transition of (La,Sr)MnO3 thin films

Variable-emittance radiators based on the metal–insulator transition of (La,Sr)MnO3 thin films have been developed. The emittance property of the films was evaluated from infrared reflectance spectra; that is, the (La,Sr)MnO3 thin films show low emittance at low temperature but high emittance at high temperature. Moreover, the emittance property significantly changes at the metal–insulator transition temperature, where the material changes from a highly reflective (i.e., low emissive) metal to a less reflective (i.e., high emissive) insulator. The (La,Sr)MnO3 thin films fitted on a spacecraft surface can, therefore, be used to automatically control the emissive heat transfer from the spacecraft without the need for any electrical power. The developed radiators also greatly reduce the weight and production cost of the thermal control devices. The dependence of the emittance property on film thickness reveals that 1500-nm-thick films can be used for variable-emittance radiators.

Flight Performance of the AKARI Cryogenic System

We describe the flight performance of the cryogenic system of the infrared astronomical satellite AKARI, which was successfully launched on 2006 February 21 (UT). AKARI carries a 68.5 cm telescope together with two focal plane instruments, Infrared Cameras (IRC) and Far Infrared Surveyor (FIS), all of which are cooled down to cryogenic temperature to achieve superior sensitivity. The AKARI cryogenic system is a unique hybrid system, which consists of cryogen (liquid helium) and mechanical coolers (2-stage Stirling coolers). With the help of the mechanical coolers, 179 L (26.0 kg) of super-fluid liquid helium can keep the instruments cryogenically cooled for more than 500 days. The on-orbit performance of the AKARI cryogenics is consistent with the design and pre-flight test, and the boil-off gas flow rate is as small as 0.32 mg/s. We observed the increase of the major axis of the AKARI orbit, which can be explained by the thrust due to thermal pressure of vented helium gas.

Radiative and optical properties of La1-xSrxMnO3 (0 ≤ x ≤ 0.4) in the vicinity of metal-insulator transition temperatures from 173 to 413 K

Radiative and optical properties of polycrystalline La1−xSrxMnO3 (0≤x≤0.4) in the vicinity of the metal–insulator transition are presented. The temperature dependence of the total hemispherical emittance εH of La1−xSrxMnO3 was measured by the calorimetric method in the temperature range from 173 to 413K. It was confirmed that εH showed unexpected variation as a result of changes in the hole concentration (x). Especially in the case of La0.825Sr0.175MnO3, εH remains high above the transition temperature TC due to insulator-like behavior; on the other hand, it decreases sharply below TC because of metallic behavior. The spectral reflectance was measured by FT-IR in the wavelength range of 0.25 to 100 μm at room temperature. The optical constants were calculated by Kramers–Kronig analysis of the spectral reflectance data. An insulator-like character of the optical properties appears at lower Sr2+ doping levels while a metallic one exists at higher Sr2+ doping levels.

Development of Spectral Selective Multilayer Film for a Variable Emittance Device and Its Radiation Properties Measurements

A smart radiation device (SRD) that is a variable emittance radiator has been developed as a thermal control material for spacecraft. The SRD has the unique feature of large variation of the total hemispherical emittance εH near room temperature. The εH of the SRD changes depending on its temperature. However, there is a drawback of a large solar absorptance αS. It is too large to use as a thermal control material for spacecraft. In order to reduce the large αS, spectral selective multilayer film was developed. This multilayer film reflects solar radiation and transmits far-infrared radiation to maintain the variation in the εH of the SRD. This paper presents thermal radiative properties of the SRD with spectral selective multilayer film. The multilayer film was designed by using a genetic algorithm (GA). The designed multilayer film was evaporated on the surface of the SRD by the electron beam evaporation method. The experimental results of αS and εH of the SRD with the multilayer film agreed well with calculated results.

The Solar Optical Telescope onboard the Solar-B

The solar optical telescope onboard the Solar-B is aimed to perform a high precision polarization measurements of the solar spectral lines in visible wavelengths to obtain, for the first time, continuous sets of high spatial resolution (~0.2arcsec) and high accuracy vector-magnetic-field map of the sun for studying the mechanisms driving the fascinating activity phenomena occurring in the solar atmosphere. The optical telescope assembly (OTA) is a diffraction limited, aplanatic Gregorian telescope with an aperture of Φ500mm. With a collimating lens unit and an active folding mirror, the OTA provides a pointing-stabilized parallel beam to the focal plane package (FPP) with a field of view of about 360x200arcsec. In this paper we identify the key technical issues of OTA for achieving the mission goal and describe the basic concepts in its optical, mechanical and thermal designs. The strategy to verify the in-orbit performance of the telescope is also discussed.

Computational design of solar reflection and far-infrared transmission films for a variable emittance device

A smart radiation device (SRD) that is a variable emittance radiator has been studied as a method of thermal control for spacecraft. The SRD consists of manganese oxide with a perovskite-type structure, and the total hemispherical emittance of the SRD changes considerably depending on temperature. Here we propose an optimal method of designing multilayer films for the SRD by using agenetic algorithm. The multilayer films reflect solar radiation and transmit far-infrared radiation to maintain variation of the infrared optical properties of the SRD.

Variable Thermal Emittance Radiator Using Metal-Insulator Phase Transition in La1-xSrxMnO3

A variable-emittance radiator device, made of thin and light ceramic tiles, has been developed for thermal control applications on spacecraft. The ceramic material used is La1-xSrxMnO3 with a perovskite structure, and shows a phase transition from ferromagnetic metal to paramagnetic insulator at around 290 K (Tc). This device automatically controls a spacecrafts temperature without electrical or mechanical instruments. Below the Tc, the device is metallic with a low thermal emittance of 0.3, and above the Tc, it becomes insulative with a high thermal emittance of 0.7. For the ceramic tiles, two different fabrication processes were studied to reduce the cost and weight; one is a conventional ceramic wafer process and the other is a thick film process on zirconia substrates. Total thickness of the ceramic tiles obtained is less than 70 µm and the weight is 450 gr/m2.

Total Hemispherical Emittance of Polyimide Films for Space Use in the Temperature Range from 173 to 700 K

This paper describes the temperature dependence of the total hemispherical emittance ∈h in the temperature range from 173 to 700 K for three types of thermal control materials, which are based on a thin polymide film coated with aluminum on the back surface. The results obtained from the measurements are compared with calculated values from optical constants. The principle of the present measurement is based on the steady-state calorimetric method, and ∈h is obtained by measuring the equilibrium temperature of a sample corresponding to different heat inputs to a heater attached to the sample. On the other hand, the present calculation method is performed by using data for the optical constants of polyimide films and vapor-deposited metal in the wavelength region from 0.25 to 100 μm. These results agree with each other on the whole. It has been observed that the temperature dependence of ∈h is remarkable, and the values have a maximum around 410 K.

Space Vlbi Satellite Halca and its Engineering Accomplishments

Abstract The Institute of Space and Astronautical Science (ISAS), Japan, launched a satellite named HALCA in February 1997 by the ISASs new rocket M-V. It has become the first space Very Long Baseline Interferometry (VLBI) satellite of the world by accomplishing a series of engineering experiments, representative ones of which are deployment of 8 m diameter parabolic antenna, precise attitude control of spacecraft, transfer of phase reference signal, high data-rate telemetry, single dish telescope operation, and interferometry with ground radio telescopes. Following the successful engineering experiments, HALCA has been operated for science observations under the science program named “VSOP” (VLBI Space Observatory Programme) in cooperation with many organizations and radio telescopes around the world.