Duk-Hang Lee

OBSERVATIONS OF THE NEAR-INFRARED SPECTRUM OF THE ZODIACAL LIGHT WITH CIBER

Interplanetary dust (IPD) scatters solar radiation which results in the zodiacal light that dominates the celestial diffuse brightness at optical and near-infrared wavelengths. Both asteroid collisions and cometary ejections produce the IPD, but the relative contribution from these two sources is still unknown. The low resolution spectrometer (LRS) onboard the Cosmic Infrared Background ExpeRiment (CIBER) observed the astrophysical sky spectrum between 0.75 and 2.1 μm over a wide range of ecliptic latitude. The resulting zodiacal light spectrum is redder than the solar spectrum, and shows a broad absorption feature, previously unreported, at approximately 0.9 μm, suggesting the existence of silicates in the IPD material. The spectral shape of the zodiacal light is isotropic at all ecliptic latitudes within the measurement error. The zodiacal light spectrum, including the extended wavelength range to 2.5 μm using Infrared Telescope in Space (IRTS) data, is qualitatively similar to the reflectance of S-type asteroids. This result can be explained by the proximity of S-type asteroidal dust to Earths orbit, and the relatively high albedo of asteroidal dust compared with cometary dust.

FAR-ULTRAVIOLET OBSERVATIONS OF THE OPHIUCHUS REGION WITH SPEAR

We present the first far-ultraviolet (FUV; 1370-1670 A) image of the Ophiuchus molecular cloud region, observed with the SPEAR imaging spectrograph. The flux levels of the diffuse FUV continuum are in reasonable agreement with those of the Voyager observations in the shorter FUV wavelengths (912-1216 A), provided that the diffuse FUV emission is dominated by the spectra from late O- and early B-type stars. The observed region of the present study was divided into five subregions according to their FUV intensities, and the spectrum was obtained for each subregion with prominent H_2 fluorescent emission lines. A synthetic model of the H_2 fluorescent emission indicates that the molecular cloud has more or less uniform physical parameters over the Ophiuchus region, with a hydrogen density n_H of 500 cm^−3 and a H2 column density N(H_2) of 2 × 10^(20) cm^−2. It is notable that the observed diffuse FUV continuum is well reproduced by a single-scattering model with scattered starlight from the dust cloud located at ~120-130 pc, except at a couple of regions with high optical depth. The model also gives reasonable properties of the dust grains of the cloud with an albedo a of 0.36 ± 0.20 and a phase function asymmetry factor g of 0.52 ± 0.22.

MEASUREMENTS OF THE MEAN DIFFUSE GALACTIC LIGHT SPECTRUM IN THE 0.95-1.65 μm BAND FROM CIBER

We report measurements of the diffuse galactic light (DGL) spectrum in the near-infrared, spanning the wavelength range 0.95–1.65 μm by the Cosmic Infrared Background ExpeRiment. Using the low-resolution spectrometer calibrated for absolute spectro-photometry, we acquired long-slit spectral images of the total diffuse sky brightness toward six high-latitude fields spread over four sounding rocket flights. To separate the DGL spectrum from the total sky brightness, we correlated the spectral images with a 100 μm intensity map, which traces the dust column density in optically thin regions. The measured DGL spectrum shows no resolved features and is consistent with other DGL measurements in the optical and at near-infrared wavelengths longer than 1.8 μm. Our result implies that the continuum is consistently reproduced by models of scattered starlight in the Rayleigh scattering regime with a few large grains.

Molecular Hydrogen Fluorescence in the Eridanus Superbubble

The first far-ultraviolet (1350-1750 A) spectral imaging observations of the Eridanus superbubble, obtained with the SPEAR/FIMS mission, have revealed distinct fluorescent emission from molecular hydrogen. Here we compare the observed emission features with those from a photodissociation region model with assumed illuminating stellar fields. The result shows rather high line ratios I1580/I1610, which may imply the existence of high-temperature molecular clouds in the region. The H2 fluorescence intensity shows a proportional correlation with Hα emission, indicating that the fluorescence and the recombination emission have similar physical origins.

Diffuse Far-Ultraviolet Observations of the Taurus Region

Diffuse far-ultraviolet (FUV; 1370-1670 A) flux from the Taurus molecular cloud region has been observed with the SPEAR/FIMS imaging spectrograph. An FUV continuum map of the Taurus region, similar to the visual extinction maps, shows a distinct cloud core and halo region. The dense cloud core, where the visual extinction Av > 1.5, obscures the background diffuse FUV radiation, while scattered FUV radiation is seen in and beyond the halo region, where Av < 1.5. The total intensity of H2 fluorescence in the cloud halo is I = 6.5 × 104 photons cm-2 s-1 sr-1 in the 1370-1670 A wavelength band. A synthetic model of the H2 fluorescent emission fits the present observation best with a hydrogen density nH = 50 cm-3, H2 column density N(H2) = 0.8 × 1020 cm-2, and incident FUV intensity IUV = 0.2. H2 fluorescence is not seen in the core, presumably because the required radiation flux to induce fluorescence is unable to penetrate the core region.

FAR-ULTRAVIOLET OBSERVATION OF THE DRACO CLOUD WITH FIMS/SPEAR

We report an observation of the Draco cloud region using the Far-ultraviolet IMaging Spectrograph (FIMS/SPEAR). The spectra show important ionic lines, such as C IV {lambda}{lambda}1548, 1551, Si IV+O IV] {lambda}1394, Si II* {lambda}1533, and Al II {lambda}1671, indicating the existence of hot and warm interstellar gases toward the region. The map of the continuum is generally in accord with the infrared map, which indicates far-ultraviolet continuum is mostly the starlight scattered off by the dust grains in the Draco cloud. Enhanced C IV emission is seen inside the Draco cloud region and attributed to the turbulent mixing of the interacting cold and warm/hot media. This interpretation is supported by the detection of O III] {lambda}1661 emission line and the H{alpha} feature in this region. We found slightly fainter C IV and far brighter Si II* emissions covering the rest of the region outside the Draco cloud, in agreement with previous observations of Galactic halos. Additionally, we also found that the molecular hydrogen fluorescence map is consistent with the morphology of the atomic neutral hydrogen and dust emission of the Draco cloud, direct evidence supporting the notion that substantial amounts of hydrogen nuclei exist in molecular form in the cloud.

Conceptual Design of the NISS onboard NEXTSat-1

The NISS onboard NEXTSat-1 is being developed by Korea astronomy and space science institute (KASI). For the study of the cosmic star formation history, the NISS performs the imaging spectroscopic observation in the near-infrared range for nearby galaxies, low background regions, star-forming regions and so on. It is designed to cover a wide field of view ( deg) and a wide wavelength range from 0.95 to by using linear variable filters. In order to reduce the thermal noise, the telescope and the infrared sensor are cooled down to 200 K and 80 K, respectively. Evading a stray light outside the field of view and making the most use of limited space, the NISS adopts the off-axis reflective optical system. The primary and the secondary mirrors, the opto-mechanical part and the mechanical structure are designed to be made of aluminum material. It reduces the degradation of optical performance due to a thermal variation. This paper presents the study on the conceptual design of the NISS.

System design of the compact IR space imaging system MIRIS

Multi-purpose Infra-Red Imaging System (MIRIS) is the main payload of the Korea Science and Technology Satellite-3 (STSAT-3), which is being developed by Korea Astronomy & Space Science Institute (KASI). MIRIS is a small space telescope mainly for astronomical survey observations in the near infrared wavelengths of 0.9~2 μm. A compact wide field (3.67 x 3.67 degree) optical design has been studied using a 256 x 256 Teledyne PICNIC FPA IR sensor with a pixel scale of 51.6 arcsec. The passive cooling technique is applied to maintain telescope temperature below 200 K with a cold shutter in the filter wheel for accurate dark calibration and to reach required sensitivity, and a micro stirling cooler is employed to cool down the IR detector array below 100K in a cold box. The science mission of the MIRIS is to survey the Galactic plane in the emission line of Paschen-α (Paα, 1.88 μ;m) and to detect the cosmic infrared background (CIB) radiation. Comparing the Paα map with the Hα data from ground-based surveys, we can probe the origin of the warm-ionized medium (WIM) of the Galaxy. The CIB is being suspected to be originated from the first generation stars of the Universe and we will test this hypothesis by comparing the fluctuations in I (0.9~1.2 um) and H (1.2~2.0 um) bands to search the red shifted Lyman cutoff signature. Recent progress of the MIRIS imaging system design will be presented.

Development of mechanical structure for the compact space IR camera MIRIS

MIRIS is a compact near-infrared camera with a wide field of view of 3.67°×3.67° in the Korea Science and Technology Satellite 3 (STSAT-3). MIRIS will be launched warm and cool the telescope optics below 200K by pointing to the deep space on Sun-synchronous orbit. In order to realize the passive cooling, the mechanical structure was designed to consider thermal analysis results on orbit. Structural analysis was also conducted to ensure safety and stability in launching environments. To achieve structural and thermal requirements, we fabricated the thermal shielding parts such as Glass Fiber Reinforced Plastic (GFRP) pipe supports, a Winston cone baffle, aluminum-shield plates, a sunshade, a radiator and 30 layers of Multi Layer Insulation (MLI). These structures prevent the heat load from the spacecraft and the earth effectively, and maintain the temperature of the telescope optics within operating range. A micro cooler was installed in a cold box including a PICNIC detector and a filter-wheel, and cooled the detector down to a operating temperature range. We tested the passive cooling in the simulated space environment and confirmed that the required temperature of telescope can be achieved. Driving mechanism of the filter-wheel and the cold box structure were also developed for the compact space IR camera. Finally, we present the assembly procedures and the test result for the mechanical parts of MIRIS.

Optical design and performance of MIRIS near-infrared camera

Multi-purpose Infra-Red Imaging System (MIRIS) is a near-infrared camera onboard on the Korea Science and Technology Satellite 3 (STSAT-3). The MIRIS is a wide-field (3.67° × 3.67°) infrared imaging system which employs a fast (F/2) refractive optics with 80 mm diameter aperture. The MIRIS optics consists of five lenses, among which the rear surface of the fifth lens is aspheric. By passive cooling on a Sun-synchronous orbit, the telescope will be cooled down below 200 K in order to deliver the designed performance. As the fabrication and assembly should be carried out at room temperature, however, we convert all the lens data of cold temperature to that of room temperature. The sophisticated opto-mechanical design accommodates the effects of thermal contraction after the launch, and the optical elements are protected by flexure structures from the shock (10 G) during the launch. The MIRIS incorporates the wide-band filters, I (1.05 μm) and H (1.6 μm), for the Cosmic Infrared Background observations, and also the narrow-band filters, Paα (1.876 μm) and a specially designed dual-band continuum, for the emission line mapping of the Galactic interstellar medium. We present the optical design, fabrication of components, assembly procedure, and the performance test results of the qualification model of MIRIS near-infrared camera.