Suk Minn Kwon

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.

Transfer of diffuse astronomical light and airglow in scattering Earth atmosphere

To understand an observed distribution of atmospheric diffuse light (ADL) over an entire meridian, we have solved rigorously, with the quasi-diffusion method, the problem of radiative transfer in an anisotropically scattering spherical atmosphere of the earth. In addition to the integrated starlight and the zodiacal light we placed a narrow layer of airglow emission on top of the scattering earth atmosphere. The calculated distribution of the ADL brightness over zenith distance shows good agreement with the observed one. The agreement can be utilized in deriving the zodiacal light brightness at small solar elongations from the night sky brightness observed at large zenith distances.

Three-dimensional infrared models of the interplanetary dust distribution

We have calculated the brightness of zodiacal emission by using the three dimensional optical models of zodiacal cloud. By comparing the calculated brightness distribution with the IRAS observations, we found that the cosine model is the best out of the three for describing the helioecliptic latitude dependence of dust distribution. We also found best parameters for the heliocentric variations of the dust temperature and volumetric absorption cross-section. Inclination and ascending node of the symmetry plane were deduced from annual variations of i) the peak offset latitude and ii) the pole brightness difference. Longitudes of the ascending node derived from i) and ii) are shown to be significantly different from each other.

The influence of the brightness of the asteroidal dust bands on the Gegenschein

The position and shape of the Gegenschein’s maximum brightness provide information on the structure of the interplanetary dust cloud. We show that the asteroidal dust bands, extended near the anti-solar point, play an important role in determining both the position of the maximum brightness and the shape of the Gegenschein. After removing the asteroidal dust bands from an observation of the Gegenschein on November 2, 1997, it was found that the maximum brightness point shifted −0.4° in ecliptic latitude, i.e., to the south of the ecliptic plane, at an ecliptic longitude of 180°, in contrast to a latitude value of +0.1° when the dust bands were included. Furthermore, the part of the Gegenschein to the south of the ecliptic plane was brighter than the northern part at the time of observation. Referring to the cloud model of T. Kelsall et al. (1998, Astrophy. J. 508, 44–73), it can be estimated that the ascending node of the symmetry plane of the dust cloud is 57°−3°+7° when its inclination is 2.03° ∓ 0.50°.

Development of zodiacal light observation system: WIZARD

We describe a new system (WIZARD: Wide-field Imager of Zodiacal light with ARray Detector) for the zodiacal light observation developed by a Korean and Japanese zodiacal light observation group. Since the zodiacal light is faint and wide-spread all over the sky, it consists of a very sensitive CCD camera of a quantum efficiency of 90% at 460(nm) and a wide angle lens with the field-of-view of 49x98 (degree). WIZARD is designed to measure the absolute brightness of diffuse sky in visible wavelengths. The zodiacal component will be separated from the integrated starlight, the airglow continuum and the scattered light in the atmosphere in the data reduction procedure. We got a first image by WIZARD in 2001 at Mauna Kea (4200m, Hawaii) under the collaboration with SUBARU Telescope. We observed the zodiacal light and the gegenschein in 2002 again, and got the excellent images. In this paper, we describe the design of WIZARD and report the performance examined by the laboratory measurements and the observations at Mauna Kea in 2002.