Norihisa Hiromoto

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.

The Far-Infrared Surveyor (FIS) for AKARI

The Far-Infrared Surveyor (FIS) is one of two focal-plane instruments on the AKARI satellite. FIS has four photometric bands at 65, 90, 140, and 160 mu m, and uses two kinds of array detectors. The FIS arrays and optics are designed to sweep the sky with high spatial resolution and redundancy. The actual scan width is more than eight arcminutes, and the pixel pitch matches the diffraction limit of the telescope. Derived point-spread functions (PSFs) from observations of asteroids are similar to those given by the optical model. Significant excesses, however, are clearly seen around tails of the PSFs, whose contributions are about 30% of the total power. All FIS functions are operating well in orbit, and the performance meets the laboratory characterizations, except for the two longer wavelength bands, which are not performing as well as characterized. Furthermore, the FIS has a spectroscopic capability using a Fourier transform spectrometer (FTS). Because the FTS takes advantage of the optics and detectors of the photometer, it can simultaneously make a spectral map. This paper summarizes the in-flight technical and operational performance of the FIS.

Large-scale forbidden C II 158 micron emission from the Galaxy

A diffuse far-infrared [C II] emission line has been detected in an extensive region (30° ≤ l ≤ 51°) along the Galactic plane. The [C II] line is bright and extended far from discrete H II regions. Latitudinal and longitudinal profiles of the [C II] intensity distribution are quite similar to those of 12 CO (J = 1-0) and the 100 μm continuum, but are completely different from those of HI 21 cm. The diffuse [C II] emission probably comes from the photodissociated C + regions enveloping giant molecular clouds exposed to the general interstellar UV radiation field. The extended low-density ionized gas might also contribute to the diffuse [C II] emission.

Large-format and compact stressed Ge:Ga array for the ASTRO-F (IRIS) mission

Abstract We describe the development of a stressed Ge:Ga array detector with a large pixel format but a compact mechanical structure. The detector will be deployed in IRIS, a Japanese satellite to be launched in 2004, which will conduct an all-sky, far-infrared survey as well as spectroscopic observations of specific objects. The array has a 5 × 15 pixel format. Three of the five rows are for the 100 – 200 μm band, and the other two are for the 150 – 200 μm band. In the focal plane of the IRIS telescope, each pixel corresponds to a field of view of 50″ × 50″. A uniaxial mechanical stress is applied to the detector chips in each row using a single stressing mechanism. This results in a compact focal-plane array. Light pipes with entrance apertures of 0.9 × 0.9 mm size are distributed with a pitch of 1 mm. A charge-integrating amplifier (CIA) circuit is used as the readout. Bare-chip amplifiers and multiplexers are placed behind the Ge:Ga chips and are cooled to 1.8 K. The entire system, including chips and readouts, weighs 300 grams. We have thus succeeded in building a compact array detector suitable for a satellite payload.

Three-element stressed Ge:Ga photoconductor array for the infrared telescope in space

A stressed Ge:Ga photoconductor array with three elements applied to the Infrared Telescope in Space satellite was fabricated and tested in experiments at 2.0 K in very low-photon-influx conditions (~ 10(5) photons/s). Stress was applied to three Ge:Ga detectors in a series by a stable and compact stressing apparatus by using cone-disk springs. The cutoff wavelength was ~ 180 microm. Responsivity was ~ 100 A/W, and the product of quantum efficiency and photoconductive gain, etaG, was ~ 1 with a chopping frequency of 2 Hz. The noise equivalent power was <5 x 10(-18) W/Hz((1/2)) when low-noise transimpedance amplifiers were used. A slow transient response and a nonlinear response that was dependent on the background photon influx were observed in the experiments. The latter showed that the etaG had a time constant tau(c) that was proportional to N(ph)(-(1/2)).

Terahertz Spectroscopy: System and Sensitivity Considerations

Terahertz spectroscopy is a backbone method in many areas of research. We have analyzed typically employed THz spectroscopy systems and their sensitivity in a general comparative approach. Recent progress to reduce the data acquisition time by frequency multiplexing using a spectrometer with a THz quantum cascade laser is described. The performance of a spectrometer using a pulsed Ge THz laser with a few μs long integration time and recent progress to modulate the laser current within such a short pulse are presented. We also investigate the origin of random errors in intensity spectra of a THz TDS with the goal to identify common error sources in TDS systems to allow reduction of the total measurement time.

Terahertz-wave antireflection coating on Ge and GaAs with fused quartz

In the terahertz-wave region, fabrication of an antireflection (AR) coating is difficult because it must be as thick as several tens of micrometers, which is far thicker than that used in the optical region. We discuss a lapping method for fabricating an AR layer with a desired thickness for terahertz-wave optical devices. To demonstrate this method, we glued a thin fused-quartz plate to a surface of an undoped Ge or GaAs wafer and polished it to a thickness of one-quarter wavelength. This reduced the reflectivity of the AR surface to 1/720 of the reflection of an uncoated surface, as expected from optical theory.

FIS: far-infrared surveyor on board ASTRO-F (IRIS)

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.

Ge:Ga Far-Infrared Photoconductor with Low Compensation

A sensitive Ge:Ga photoconductor was developed, and the performance of the far-infrared detector was studied for application to astronomical and atmospheric observations using stratospheric balloons. The Ge:Ga photoconductor had a response up to 110-µm wavelength and a high responsivity of R = 8.0 A/W at 4.2 K. The product of quantum efficiency and photoconductive gain ηG was 0.11, and the photoconductive gain G was estimated to be 0.36 at a half bias field of breakdown. The NEP was 8.5 × 10-16 W/√Hz operated at 4.2 K. Both the resistivity and the breakdown field of the Ge:Ga detectors showed that the compensation of this crystal K=Nd/Na (donor concentration/acceptor concentration) was rather low, that is, less than 10-2. The Photoconductors high responsivity could be attributed to the high photoconductive gain produced by the low compensation.

Passive imaging and emissivity measurement with a 4K-cryocooled terahertz photoconductive detector

We have demonstrated terahertz (THz) passive imaging of room-temperature objects using a 4K-cryocooled THz photoconductive detector with background limited infrared performance (BLIP) at around 1.5-2.5THz. Images of a safety razor blade and a coin concealed in a plastic package or an envelope are successfully obtained with spatial resolutions of wavelength order using the THz passive imaging system. We have compared the measured THz intensity of several materials with emissivities calculated using the reported optical constants. The result shows that the THz intensity has a good linear relation to the emissivity, which means THz emissivity of an unknown material can be estimated at a room-temperature with the THz passive imaging system.