Shigeyuki Sako

The University of Tokyo, Tōkyō, Japan

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Publications (68)159.26 Total impact

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    ABSTRACT: We present infrared multi-epoch observations of the dust forming nova V1280 Sco over $\sim$2000 days from the outburst. The temporal evolution of the infrared spectral energy distributions at 1272, 1616 and 1947 days can be explained by the emissions produced by amorphous carbon dust of mass (6.6--8.7)$\times$10$^{-8}$M$_{\odot}$ with a representative grain size of 0.01$~\mu$m and astronomical silicate dust of mass (3.4--4.3)$\times$10$^{-7}$M$_{\odot}$ with a representative grain size of 0.3--0.5$~\mu$m. Both of these dust species travel farther away from the white dwarf without an apparent mass evolution throughout those later epochs. The dust formation scenario around V1280 Sco suggested from our analyses is that the amorphous carbon dust is formed in the nova ejecta followed by the formation of silicate dust in the expanding nova ejecta or as a result of the interaction between the nova wind and the circumstellar medium.
    Full-text · Article · Jul 2015 · The Astrophysical Journal
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    ABSTRACT: We have developed a near-infrared camera called ANIR (Atacama Near-InfraRed camera) for the University of Tokyo Atacama Observatory 1.0m telescope (miniTAO) installed at the summit of Cerro Chajnantor (5640 m above sea level) in northern Chile. The camera provides a field of view of 5'.1 $\times$ 5'.1 with a spatial resolution of 0".298 /pixel in the wavelength range of 0.95 to 2.4 $\mu$m. Taking advantage of the dry site, the camera is capable of hydrogen Paschen-$\alpha$ (Pa$\alpha$, $\lambda=$1.8751 $\mu$m in air) narrow-band imaging observations, at which wavelength ground-based observations have been quite difficult due to deep atmospheric absorption mainly from water vapor. We have been successfully obtaining Pa$\alpha$ images of Galactic objects and nearby galaxies since the first-light observation in 2009 with ANIR. The throughputs at the narrow-band filters ($N1875$, $N191$) including the atmospheric absorption show larger dispersion (~10%) than those at broad-band filters (a few %), indicating that they are affected by temporal fluctuations in Precipitable Water Vapor (PWV) above the site. We evaluate the PWV content via the atmospheric transmittance at the narrow-band filters, and derive that the median and the dispersion of the distribution of the PWV are 0.40+/-0.30 mm for $N1875$ and 0.37+/-0.21 mm for $N191$, which are remarkably smaller (49+/-38% for $N1875$ and 59+/-26% for $N191$) than radiometry measurements at the base of Cerro Chajnantor (5100 m alt.). The decrease in PWV can be explained by the altitude of the site when we assume that the vertical distribution of the water vapor is approximated at an exponential profile with scale heights within 0.3-1.9 km (previously observed values at night). We thus conclude that miniTAO/ANIR at the summit of Cerro Chajnantor indeed provides us an excellent capability for a "ground-based" Pa$\alpha$ observation.
    Full-text · Article · Feb 2015 · Publications- Astronomical Society of Japan
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    ABSTRACT: Luminous infrared galaxies (LIRGs) are enshrouded by a large amount of dust, produced by their active star formation, and it is difficult to measure their activity in the optical wavelength. We have carried out Pa$\alpha$ narrow-band imaging observations of 38 nearby star-forming galaxies including 33 LIRGs listed in $IRAS$ RBGS catalog with the Atacama Near InfraRed camera (ANIR) on the University of Tokyo Atacama Observatory (TAO) 1.0 m telescope (miniTAO). Star formation rates (SFRs) estimated from the Pa$\alpha$ fluxes, corrected for dust extinction using the Balmer Decrement Method (typically $A_V$ $\sim$ 4.3 mag), show a good correlation with those from the bolometric infrared luminosity of $IRAS$ data within a scatter of 0.27 dex. This suggests that the correction of dust extinction for Pa$\alpha$ flux is sufficient in our sample. We measure the physical sizes and the surface density of infrared luminosities ($\Sigma_{L(\mathrm{IR})}$) and $SFR$ ($\Sigma_{SFR}$) of star-forming region for individual galaxies, and find that most of the galaxies follow a sequence of local ultra luminous or luminous infrared galaxies (U/LIRGs) on the $L(\mathrm{IR})$-$\Sigma_{L(\mathrm{IR})}$ and $SFR$-$\Sigma_{SFR}$ plane. We confirm that a transition of the sequence from normal galaxies to U/LIRGs is seen at $L(\mathrm{IR})=8\times10^{10}$ $L_{\odot}$. Also, we find that there is a large scatter in physical size, different from those of normal galaxies or ULIRGs. Considering the fact that most of U/LIRGs are merging or interacting galaxies, this scatter may be caused by strong external factors or differences of their merging stage.
    Full-text · Article · Dec 2014 · The Astrophysical Journal Supplement Series
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    ABSTRACT: The Kiso Supernova Survey (KISS) is a high-cadence optical wide-field supernova (SN) survey. The primary goal of the survey is to catch the very early light of a SN, during the shock breakout phase. Detection of SN shock breakouts combined with multi-band photometry obtained with other facilities would provide detailed physical information on the progenitor stars of SNe. The survey is performed using a 2 $_{.}^{\circ}$2 × 2 $_{.}^{\circ}$2 field-of-view instrument on the 1.05-m Kiso Schmidt telescope, the Kiso Wide Field Camera (KWFC). We take a 3-min exposure in g-band once every hour in our survey, reaching magnitude g ∼ 20–21. About 100 nights of telescope time per year have been spent on the survey since 2012 April. The number of the shock breakout detections is estimated to be of the order of 1 during our three-year project. This paper summarizes the KISS project including the KWFC observing setup, the survey strategy, the data reduction system, and CBET-reported SNe discovered so far by KISS.
    Full-text · Article · Sep 2014 · Publications- Astronomical Society of Japan
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    ABSTRACT: We present our discovery of dramatic variability in SDSS J1100+4421 by the high-cadence transient survey Kiso Supernova Survey (KISS). The source brightened in the optical by at least a factor of three within about half a day. Spectroscopic observations suggest that this object is likely a narrow-line Seyfert 1 galaxy (NLS1) at z=0.840, however with unusually strong narrow emission lines. The estimated black hole mass of ~ 10^7 Msun implies bolometric nuclear luminosity close to the Eddington limit. SDSS J1100+4421 is also extremely radio-loud, with a radio loudness parameter of R ~ 4 x 10^2 - 3 x 10^3, which implies the presence of relativistic jets. Rapid and large-amplitude optical variability of the target, reminiscent of that found in a few radio- and gamma-ray loud NLS1s, is therefore produced most likely in a blazar-like core. The 1.4 GHz radio image of the source shows an extended structure with a linear size of about 100 kpc. If SDSS J1100+4421 is a genuine NLS1, as suggested here, this radio structure would then be the largest ever discovered in this type of active galaxies.
    Full-text · Article · Sep 2014
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    ABSTRACT: LISS (Line Imager and Slit Spectrograph) is an imager and spectrograph equipped with a liquid crystal etalon and a low resolution grism. It is specialized to observe and map the emission and absorption lines of astronomical objects. A fully depleted and back illuminated 2K x 1K Hamamatsu CCD which has high sensitivity at redder wavelengths in optical bands enables this instrument to give a good performance in imaging and spectroscopic observations of emission lines such as [SIII]λλ 906.9/953.2 nm. We successfully carried out commissioning observations at the 1.6-m Pirka telescope of Hokkaido University in September/October 2012 and June/July 2013. In this paper, we describe the design and performance of LISS as well as its early observational results and future prospects.
    No preview · Conference Paper · Aug 2014
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    ABSTRACT: Simultaneous Color Wide-field Infrared Multi-object Spectrograph, SWIMS, is one of the first generation in- struments for the University of Tokyo Atacama Observatory (TAO) 6.5m Telescope now under construction. A dichroic mirror being inserted in the collimated beam, it is capable of two-color simultaneous imaging with FoV of 9:16ϕ or R ∼ 1000 multi-object spectroscopy at 0.9–2.5μm wavelength range in one shot, and enables us to carry out efficient NIR imaging/spectroscopic survey of objects such as distant galaxies and young stellar objects. All the major components have been fabricated and we will start integration and laboratory cool-down test in the summer of 2014. After the engineering and initial science observations at the Subaru telescope, SWIMS will be transported to TAO telescope and see the first light in 2018.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: The telescope of the University of Tokyo Atacama Observatory has a 6.5-m primary mirror in diameter. In order to fabricate the reflecting film initially and to maintain its performance over a long period, we have mirror coating facility on site. We have chosen to leave the primary mirror in its cell with the mirror support system intact. Two major advantages of leaving the mirror in its cell are that the mirror does not have to be lifted or handled and the support system does not have to be removed or reinstalled for coating. The facility consists of a clean booth for stripping of the old film, an evaporation coating chamber, and a cart with a lifter for handling the primary mirror. A conventional evaporation system with a metal pre-wetted filament array is adopted for achieving various optical requests. The coating equipment has also a function of fabrication for film on secondary and tertiary mirrors.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: The F/1.25 light-weighted borosilicate (Ohara E6) honeycomb primary mirror is adopted and being fabricated by the Steward Observatory Mirror Laboratory. The primary mirror is supported by 104 loadspreaders bonded to the back surface of the mirror and 6 adjustable hardpoints. The mirror is actively controlled by adjusting the actuator forces based on the realtime wavefront measurement. The actuators are optimized for operation at high altitude of the site, 5640-m above the sea level, by considering the low temperature and low air pressure. The mirror is held in the primary mirror cell which is used as a part of the vacuum chamber when the mirror surface is aluminized without being detached from the cell. The pupil is set at the secondary mirror to minimize infrared radiation into instruments. The telescope has two Nasmyth foci for near-infrared and mid-infrared facility instruments (SWIMS and MIMIZUKU, respectively) and one folded-Caseggrain focus for carry-in instruments. At each focus, autoguider and wavefront measurement systems are attached to achieve seeing-limited image quality. The telescope mount is designed as a tripod-disk type alt-azimuth mount. Both the azimuthal and elevation axes are supported by and run on the hydrostatic bearings. Friction drives are selected for these axis drives. The telescope mount structure and primary mirror support as well as the mirrors are under thermal control and maintained at ambient air temperature to minimize the mirror seeing.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: We have developed a visible imager and spectrograph, Nayoro Optical Camera and Spectrograph (NaCS), in- stalled at the f/12 Nasmyth focus of the 1.6-m Pirka telescope of the Hokkaido University in Hokkaido, Japan. The optical and mechanical design is similar to that of WFGS2 of the University of Hawaii 2.2-m telescope (UH88), however the camera is newly designed. The spectral coverage is 380–970 nm, and the field of view is 8.4 × 4.5 arcmin with a pixel scale of 0.247 arcsec pixel-1. The SDSS (g', r', i', z') filters, Johnson (B, V ) filters and a replica grism (R ~300 at 650 nm) are equipped. The slit width can be selected from 2, 3, and 4 arcsec. We selected a 2kx1k fully-depleted back-illuminated Hamamatsu CCD as a detector, because it has a high quantum efficiency (≥ 80 %) over optical wavelength. The Kiso Array Controller (KAC) is used as a CCD controller. The first light observation was done on November 2011. NaCS is used mainly for long-term spectroscopic monitor of active galactic nuclei. It is also used for several astronomical observations such as light-curve measurements of asteroids and search of pre-main-sequence stars and brown dwarfs by slit-less spectroscopy as a major facility instrument of the Pirka telescope. We present the design, construction, integration, and performance of this instrument.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: We report the restraint deformation and the corrosion protection of gold deposited aluminum mirrors for mid-infrared instruments. To evaluate the deformation of the aluminum mirrors by thermal shrinkage, monitoring measurement of the surface of a mirror has been carried out in the cooling cycles from the room temperature to 100 K. The result showed that the effect of the deformation was reduced to one fourth if the mirror was screwed with spring washers. We have explored an effective way to prevent the mirror from being galvanically corroded. A number of samples have been prepared by changing the coating conditions, such as inserting an insulation layer, making a multi-layer and overcoating water blocking layer, or carrying out precision cleaning before coating. Precision cleaning before the deposition and protecting coat with SiO over the gold layer seemed to be effective in blocking corrosion of the aluminum. The SiO over-coated mirror has survived the cooling test for the mid-infrared use and approximately 1 percent decrease in the reflectance has been detected at 6-25 microns compared to gold deposited mirror without coating.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: We developed a new readout system for the near-infrared detector VIRGO-2K (2kx2k HgCdTe array) installed in the optical-infrared simultaneous camera, HONIR, for the 1.5 m Kanata telescope at Higashi-Hiroshima observatory. The main goal of this development is to read out one frame within ~ 1 second through 16 output readout mode of the detector, in order to reduce the overhead time per exposure. The system is based on a CCD controller, Kiso Array Controller (KAC). We redesigned the analog part of KAC to fit VIRGO-2K. We employed a fully differential input circuit and a third order Bessel low-pass filter for noise reduction and a constant current system to improve the linearity of the detector. We set the cutoff frequency of the Bessel low-pass filter at the readout clock rate (120 kHz). We also set the constant current at 200 μA according to the data sheet of VIRGO-2K. We tested the new readout system at room temperature and confirmed that the low-pass filter works well as designed. The fluctuation of the current level of the constant current system is less than 2% for the typical output voltage range of VIRGO-2K (3.2-4.4 V). We measured the readout noise caused by the new readout system (connected to cooled multiplexer) and found that it is 30-40 μV rms, being comparable to or slightly higher than the typical readout noise of VIRGO-2K, ∼ 37 μV rms.
    No preview · Conference Paper · Jul 2014
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    ABSTRACT: Formation of massive stars remains an open question in modern astronomy. How to prevent a massive molecular core from fragmenting into low-mass cores before it grows up to massive young stellar objects (MYSOs) is an important issue. A recent model predicts that a massive core grows up to a MYSO if it is heated by accreting radiation from less massive stars formed previously around the core(accretion-luminosity feedback model). To evaluate this model, it is important to observe interactions between objects in massive star forming regions with high spatial resolution because these regions exist far from us and are crowded. MYSOs are heavily obscured and their radiation is mainly emitted in the mid to far infrared wavelength. Therefore, the mid to far infrared observations are important to measure total fluxes and estimate luminosity and mass accurately.We have been carrying out mid-infrared survey observations of nearby massive star forming regions at 31 and 37 microns with the University of Tokyo Atacama 1.0-m Telescope with the mid-infrared camera MAX38. The MAX38 achieves high spatial resolutions of 8 arcsec at 31 micron which is better than that of any other space telescopes. We have observed three massive star forming regions and successfully obtained the first resolved images at 31 and 37 microns.We have found that each region consists of two components; an ultra compact HII region (UCHII) and a MYSO. We also have found that the masses of the MYSOs are larger than those of the UCHIIs in any observed regions. These suggest that the less massive objects began to collapse earlier because more massive objects evolved faster and UCHIIs are in the later evolutional stage than MYSOs. If the accretion-luminosity feedback works well, the less massive objects begin to collapse earlier. This corresponds to the results of our observations. Therefore, our results supports that the accretion-luminosity feedback works efficiently in the massive star forming regions.
    No preview · Article · Jul 2013
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    ABSTRACT: ANIR (Atacama Near InfraRed camera) is a near infrared camera for the University of Tokyo Atacama 1m telescope, installed at the summit of Co. Chajnantor (5,640 m altitude) in northern Chile. The high altitude and extremely low water vapor (PWV = 0.5 mm) of the site enable us to perform observation of hydrogen emission line at . Since its first light observation in June 2009, we have been carrying out a narrow-band imaging survey of nearby luminous infrared galaxies (LIRGs), and have obtained for 38 nearby LIRGs listed in AKARI/FIS-PSC at the velocity of recession between 2,800 km/s and 8,100 km/s. LIRGs are affected by a large amount of dust extinction (~ 3 mag), produced by their active star formation activities. Because is the strongest hydrogen recombination line in the infrared wavelength ranges, it is a good and direct tracer of dust-enshrouded star forming regions, and enables us to probe the star formation activities in LIRGs. We find that LIRGs have two star-forming modes. The origin of the two modes probably come from differences between merging stage and/or star-forming process.
    Full-text · Article · Nov 2012
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    ABSTRACT: We have evaluated on-sky performances of a mid-infrared camera MAX38 (Mid-infrared Astronomical eXploerer) on the miniTAO 1-meter telescope. A Strehl ratio at the N-band is estimated to be 0.7-0.8, and it reaches to 0.9 at the 37.7 micron, indicating that diffraction limited angular resolution is almost achieved at the wavelength range from 8 to 38 micron. System efficiencies at the N and the Q-band are estimated with photometry of standard stars. The sensitivity at the 30 micron cannot be exactly estimated because there are no standard stars bright enough. We use the sky brightness instead. The estimated efficiencies at the 8.9, 18.7, and 31.7 micron are 4%, 3%, 15% , respectively. One-sigma sensitivity in 1 sec integration of each filter is also evaluated. These give good agreements with the designed values. Preliminary scientific results are briefly reported.
    Full-text · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: TAO (The University of Tokyo Atacama Observatory) is planned to be constructed at the summit of Co. Chajnantor (5640 m altitude) in Chile. MIMIZUKU (Mid-Infrared Multi-field Imager for gaZing at the UnKnown Universe) is a mid-infrared imager (Field of View: 1' x 1'- 2' x 2') and spectrometer (Δλ/λ: 60-230) for the 6.5-m TAO telescope, covering the wavelength range of 2-38 μm. The MIMIZUKU has a unique equipment called Field Stacker (FS) which enables the simultaneous observation of target and reference object. The simultaneity is expected to improve photometric accuracy and to realize long-term monitoring observations. The development status of the MIMIZUKU is reported in this paper. The FS and the cryostat of the MIMIZUKU have been fabricated and under testing. The cold optics (550 mm x 750 mm x 2 floors) with 28 mirrors has been constructed. The mirrors were aligned with the positional precision of 0.1 mm and the angular precision of 0.1 deg. The evaluated optical performance is that the diffraction-limited image at λ <8 μm and the enough compact image (r <2 pix=0.22") at 2 λ ~2μm can be obtained. In the cold optics, the drive systems with backlash-less gears are employed and work well even in cryogenic environment. The grisms made with silicon and germanium have been fabricated by ultraprecision cutting. It was found that their surface roughness, grating constant, and blaze angle almost measure up to the designed values.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: Mid-infrared, 25 - 45 microns, is a very important wavelength region to investigate the physics of lower temperature environments in the universe. There are few transparent materials in the range of mid-infrared except silicon. However, the reflection on a silicon surface reaches 30 % because of its high refractive index (~3.4). To apply silicon to mid-infrared astronomical instruments, we need a way of antireflection and have adopted a moth-eye structure. This structure keeps durable under cryogenic environments, which is advantageous to mid-infrared instruments. We have fabricated three samples of the moth-eye structure on plane silicon surfaces by electron-beam photo-lithograph and reactive ion etching. The structures consist of many cones standing on silicon surfaces. We have substantiated the transmittance of 96 % or higher in the wide range of 20 - 50 microns and higher than 98 % at the maximum. The transmittance of moth-eye surfaces, however, is theoretically expected as 100 %. We have examined the discrepancy between the transmittance of the theory and fabrications with electromagnetic simulations. It has been revealed that shapes of the cones and gaps at the bottom of the cones seriously affect the transmittance. We have estimated a few tolerances for manufacturing the moth-eye structures achieving sufficient transmittance of nearly 100 %.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: The Kiso Wide Field Camera (KWFC) is a facility instrument for the 105-cm Schmidt telescope being operated by the Kiso Observatory of the University of Tokyo. This camera has been designed for wide-field observations by taking advantage of a large focal-plane area of the Schmidt telescope. Eight CCD chips with a total of 8k x 8k pixels cover a field-of-view of 2.2 degrees x 2.2 degrees on the sky. The dewar window works as a field flattener lens minimizing an image distortion across the field of view. Two shutter plates moving in parallel achieve uniform exposures on all the CCD pixels. The KWFC is equipped with a filter exchanger composed of an industrial robotic arm, a filter magazine capable of storing 12 filters, and a filter holder at the focal plane. Both the arm and the magazine are installed inside the tube framework of the telescope but without vignetting the beam. Wide-field survey programs searching for supernovae and late-type variable stars have begun in April 2012. The survey observations are performed with a management software system for facility instruments including the telescope and the KWFC. This system automatically carries out observations based on target lists registered in advance and makes appropriate decisions for implementation of observations by referring to weather conditions and status of the instruments. Image data obtained in the surveys are processed with pipeline software in real time to search for candidates of time-variable sources.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: ANIR (Atacama Near InfraRed camera) is a near infrared camera for the University of Tokyo Atacama 1.0m telescope installed at the summit of Co. Chajnantor (5640m altitude) in northern Chile. The high altitude and the extremely low water vapor (precipitable water vapor:PWV=0.5mm) of the site enables us to perform observation of hydrogen Paschen alpha (Paα) emission line at 1.8751 μm. Since the first light observation in June 2009, we have succesfully obtained Paα narrow-band images of Galactic objects and near-by Galaxies. However, as there are many atmospheric absorption features within the wavelength range of the narrow-band filters which vary temporally due to change of PWV, it is difficult to calibrate the emission line flux accurately. Therefore, we have developed a new method to restore Paα emission-line flux from ground-based narrow-band filter imaging observations. First, average atmospheric transmittance within the narrow-band filter is derived using 2MASS stars in a image. Second, PWV is then estimated by comparing the transmittance with that calculated by atmospheric transmittance model software, ATRAN. Finally, the atmospheric transmittance at the wavelength of Paα emission-line is obtained from the model atmosphere corresponding to the obtained PWV. By applying this method to the data of nearby Luminous Infrared Galaxies obtained by ANIR, the emission line strength is estimated within the accuracy of 10% relative to that observed by HST/NICMOS. In this paper, we describe details of the calibration method and its accuracy.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: A metal mesh filter is appropriate to a band-pass filter for astronomy in the long mid-infrared between 25 and 40 μm, where most of optical materials are opaque. The mesh filter does not require transparent dielectric materials unlike interference filters because the transmission characteristics bare determined by surface plasmon-polariton (SPP) resonances excited on a metal surface with a periodic structure. In this study, we have developed the mesh filters optimized to atmospheric windows at 31.8 and 37.5 μm accessible from the Chajnantor site of 5,640 m altitude. First, mesh filters made of a gold film of 2 μm thickness have been fabricated. Four identical film-type filters are stacked incoherently to suppress leakages at stop-bands. The transmissions of the stacked filters have been measured to be 0.8 at the peaks and below 1 x 10-3 at the stop-bands at 4 K. The ground-based mid-infrared camera MAX38 has been equipped with the stacked filters and successfully obtained diffraction-limited stellar images at the Chajnantor site. The film-type mesh filter does not have sufficient mechanical strength for a larger aperture and for use in space. We have developed mesh filters with higher strength by applying the membrane technology for x-ray optics. The membrane-type mesh filter is made of SiC and coated with a thin gold layer. The optical performance of the mesh filter is independent of internal materials in principle because the SPP resonances are excited only on the metal surface. The fabricated membrane-type mesh filter has been confirmed to provide comparable optical performance to the film-type mesh filter.
    No preview · Article · Sep 2012 · Proceedings of SPIE - The International Society for Optical Engineering

Publication Stats

512 Citations
159.26 Total Impact Points

Institutions

  • 2000-2015
    • The University of Tokyo
      • Department of Astronomy
      Tōkyō, Japan
  • 2014
    • Hiroshima University
      • Division of Physical Sciences
      Hirosima, Hiroshima, Japan
  • 2008
    • Ibaraki University
      • College of Science
      Mito-shi, Ibaraki, Japan
  • 2003
    • Kitasato University
      • Center for Natural Sciences
      Edo, Tōkyō, Japan