Takayuki Kotani

National Astronomical Observatory of Japan, Edo, Tōkyō, Japan

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Publications (74)100.32 Total impact

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    ABSTRACT: We present the design, fabrication and test results for a dichroic mirror, which was primarily developed for the SPICA Coronagraph Instrument (SCI), but is potentially useful for various types of astronomical instrument. The dichroic mirror is designed to reflect near- and mid-infrared but to transmit visible light. Two designs, one with 3 layers and one with 5 layers on BK7 glass substrates, are presented. The 3-layer design, consisting of Ag and ZnS, is simpler, and the 5-layer design, consisting of Ag and TiO2 is expected to have better performance. Tape tests, evaluation of the surface figure, and measurements of the reflectivity and transmittance were carried out at ambient temperature in air. The reflectivity obtained from measurements made on mirrors with 5 layers were < 80 % for wavelengths, λ, from 1.2 to 22 μm and < 90 % for λ from 1.8 to 20 μm. The transmittance obtained from measurements made on mirrors with 5 layers were < 70 % for λ between 0.4 and 0.8 μm. Optical ghosting is estimated to be smaller than 10-4 at λ < 1.5 μm. A protective coating for preventing corrosion was applied and its influence on the reflectivity and transmittance evaluated. A study examining the trade-offs imposed by various configurations for obtaining a telescope pointing correction signal was also undertaken.
    SPIE Astronomical Telescopes + Instrumentation; 08/2014
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    ABSTRACT: The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is one of a handful of extreme adaptive optics systems set to come online in 2014. The extreme adaptive optics correction is realized by a combination of precise wavefront sensing via a non-modulated pyramid wavefront sensor and a 2000 element deformable mirror. This system has recently begun on-sky commissioning and was operated in closed loop for several minutes at a time with a loop speed of 800 Hz, on ~150 modes. Further suppression of quasi-static speckles is possible via a process called "speckle nulling" which can create a dark hole in a portion of the frame allowing for an enhancement in contrast, and has been successfully tested on-sky. In addition to the wavefront correction there are a suite of coronagraphs on board to null out the host star which include the phase induced amplitude apodization (PIAA), the vector vortex, 8 octant phase mask, 4 quadrant phase mask and shaped pupil versions which operate in the NIR (y-K bands). The PIAA and vector vortex will allow for high contrast imaging down to an angular separation of 1 λ/D to be reached; a factor of 3 closer in than other extreme AO systems. Making use of the left over visible light not used by the wavefront sensor is VAMPIRES and FIRST. These modules are based on aperture masking interferometry and allow for sub-diffraction limited imaging with moderate contrasts of ~100-1000:1. Both modules have undergone initial testing on-sky and are set to be fully commissioned by the end of 2014.
    SPIE Astronomical Telescopes + Instrumentation; 07/2014
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    ABSTRACT: We propose the application of coronagraphic techniques to the spectroscopic direct detection of exoplanets via the Doppler shift of planetary molecular lines. Even for an unresolved close-in planetary system, we show that the combination of a visible nuller and an extreme adaptive optics system can reduce the photon noise of a main star and increase the total signal-to-noise ratio (S/N) of the molecular absorption of the exoplanetary atmosphere: it works as a spectroscopic coronagraph. Assuming a 30 m telescope, we demonstrate the benefit of these high-contrast instruments for nearby close-in planets that mimic 55 Cnc b (0.6λ/D of the angular separation in the K band). We find that the tip-tilt error is the most crucial factor; however, low-order speckles also contribute to the noise. Assuming relatively conservative estimates for future wavefront control techniques, the spectroscopic coronagraph can increase the contrast ~50-130 times and enable us to obtain ~3-6 times larger S/N for warm Jupiters and Neptunes at 10 pc than those without it. If the tip-tilt error can be reduced to 0.3 mas (rms), it gains ~10-30 times larger S/N and enables us to detect warm super-Earths with an extremely large telescope. This paper demonstrates the concept of spectroscopic coronagraphy for future spectroscopic direct detection. Further studies of the selection of coronagraphs and tip-tilt sensors will extend the range of application of the spectroscopic direct detection beyond the photon collecting area limit.
    The Astrophysical Journal Supplement Series 06/2014; 212(2):27. · 16.24 Impact Factor
  • 03/2014;
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    Dataset: aa21894-13
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    ABSTRACT: FIRST is a prototype instrument built to demonstrate the capabilities of the pupil remapping technique, using single-mode fibers and working at visible wavelengths. We report on observations of the binary system Capella at three epochs over a period of 14 months with FIRST-18 (that recombines 2 sets of 9 fibers) mounted on the 3-m Shane telescope at Lick Observatory. The binary separation during our observations ranges from 0.8 to 1.2 times the diffraction limit of the telescope at the central wavelength. We successfully resolved the Capella binary system with an astrometric precision as good as 1mas under the best observing conditions. FIRST also gives access to the spectral flux ratio between the two components directly measured with an unprecedented spectral resolution (around 300) over the 600-850nm range. In particular, our data allow to detect the well-known overall slope of the flux ratio spectrum, leading to an estimation of the pivot wavelength of 0.64+/-0.01um, at which the cooler component becomes the brightest. Spectral features arising from the difference in effective temperature (specifically the Halpha line, TiO and CN bands) have been used to constrain the stellar parameters. The effective temperatures we derive for both components are slightly lower (5-7%) than the well-established properties for this system. This difference mainly originates from deeper molecular features than those predicted by state-of-the-art stellar atmospheric models, suggesting that molecular line lists used in the photospheric models are incomplete and/or oscillator strengths are underestimated (most likely concerning the CN molecule). These results demonstrate the power of FIRST, a fibered pupil remapping based instrument, in terms of high angular resolution and show that the direct measurement of the spectral flux ratio provides valuable information to characterize little known companions.
    Astronomy and Astrophysics 10/2013; · 5.08 Impact Factor
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    ABSTRACT: Most exoplanets detected by direct imaging so far have been characterized by relatively hot (> ~1000 K) and cloudy atmospheres. A surprising feature in some of their atmospheres has been a distinct lack of methane, possibly implying non-equilibrium chemistry. Recently, we reported the discovery of a planetary companion to the Sun-like star GJ 504 using Subaru/HiCIAO within the SEEDS survey. The planet is substantially colder (<600 K) than previously imaged planets, and has indications of fewer clouds, which implies that it represents a new class of planetary atmospheres with expected similarities to late T-type brown dwarfs in the same temperature range. If so, one might also expect the presence of significant methane absorption, which is characteristic of such objects. Here, we report the detection of deep methane absorption in the atmosphere of GJ 504 b, using the Spectral Differential Imaging mode of HiCIAO to distinguish the absorption feature around 1.6 um. We also report updated JHK photometry based on new Ks-band data and a re-analysis of the existing data. The results support the notion that GJ 504 b has atmospheric properties distinct from other imaged exoplanets, and will become a useful reference object for future planets in the same temperature range.
    The Astrophysical Journal Letters 10/2013; 778(1). · 6.35 Impact Factor
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    ABSTRACT: We successfully generated a 12.5-GHz-spacing astro-comb ranging over 500 nm from 1250 to 1750 nm in an HN-DSF where the OPS-synthesized pump pulse was launched after the amplification and compression under noise suppression.
    Nonlinear Optics; 07/2013
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    ABSTRACT: Several exoplanets have recently been imaged at wide separations of >10 AU from their parent stars. These span a limited range of ages (<50 Myr) and atmospheric properties, with temperatures of 800--1800 K and very red colors (J - H > 0.5 mag), implying thick cloud covers. Furthermore, substantial model uncertainties exist at these young ages due to the unknown initial conditions at formation, which can lead to an order of magnitude of uncertainty in the modeled planet mass. Here, we report the direct imaging discovery of a Jovian exoplanet around the Sun-like star GJ 504, detected as part of the SEEDS survey. The system is older than all other known directly-imaged planets; as a result, its estimated mass remains in the planetary regime independent of uncertainties related to choices of initial conditions in the exoplanet modeling. Using the most common exoplanet cooling model, and given the system age of 160 [+350, -60] Myr, GJ 504 b has an estimated mass of 4 [+4.5, -1.0] Jupiter masses, among the lowest of directly imaged planets. Its projected separation of 43.5 AU exceeds the typical outer boundary of ~30 AU predicted for the core accretion mechanism. GJ 504 b is also significantly cooler (510 [+30, -20] K) and has a bluer color (J-H = -0.23 mag) than previously imaged exoplanets, suggesting a largely cloud-free atmosphere accessible to spectroscopic characterization. Thus, it has the potential of providing novel insights into the origins of giant planets, as well as their atmospheric properties.
    The Astrophysical Journal 07/2013; 774(1). · 6.73 Impact Factor
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    ABSTRACT: Imaging the direct light signal from a faint exoplanet against the overwhelming glare of its host star presents one of the fundamental challenges to modern astronomical instrumentation. Achieving sufficient signal-to-noise for detection by direct imaging is limited by three basic physical processes: aberration of the wavefronts (both instrumental and atmospheric), photon noise, and detector noise. In this paper, we advance a novel optical setup which synthesizes the advantages of two different techniques: nulling interferometry to mitigate photon noise, and closure phase to combat optical aberrations. Our design, which employs technology from integrated optics and photonics, is intended to combine the advantageous aspects of both a coronagraph and a non-redundant interferometer. We show that such an instrument would allow readout noise limited detection of exoplanets, even in the presence of residual co-phasing errors. As a result, this concept would be ideal for space interferometry and for ground based observations of bright stellar hosts (apparent magnitude below 10). The method has applicability as a beam-combiner for a long baseline interferometer, or as a competitive alternative to coronagraphy on a large single-mirror telescope.
    06/2013;
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    ABSTRACT: We successfully generated a 12.5-GHz-spacing astro-comb ranging over 400 nm from 1350 to 1750 nm. A synthesized pulse whose repetition rate directly linked to the spacing was compressed for nonlinearity enhancement and broad astro-comb generation.
    CLEO: Science and Innovations; 06/2013
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    ABSTRACT: The Cosmic Infrared Background (CIB) as an integrated history of the early universe is important for the study of the Dark Ages, and it may include the light from the first stars at z~10. However, previous CIB measurements suffer from residual contamination from the strong foreground emission (e.g. the zodiacal light). We propose to observe Ganymede eclipsed in the shadow of Jupiter at 3.6 microns to detect the absolute CIB intensity without any zodiacal light subtraction error. The zodiacal light originates inside the orbit of Jupiter; since Ganymede in eclipse shields all light beyond the Jovian orbit, it should be detected as a 'dark spot' if the strong CIB implied by previous observations exists. The intensity deficit of this dark spot relative to the surrounding sky directly measures the strength of the CIB, free from any assumptions about the zodiacal light. A previous DDT program demonstrated the efficacy of the major observational components of this program, including the severity of the Jovian straylight. The new observations are required to further refine the measurements to reach the required S/N.
    Spitzer Proposal. 12/2012;
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    ABSTRACT: We have designed fiber coupling optics between a telescope focus and a high precision IR Doppler spectrograph. A multi-mode fiber with 60-um core diameter has been tested both for image and pupil plane coupling.
    Frontiers in Optics; 10/2012
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    ABSTRACT: We propose a novel calibration system for astronomical spectrographs using optical frequency shifter and laser frequency comb generator. We successfully confirmed comb spectral lines and gas absorption spectrum are precisely frequency-shifted by optical frequency shifters.
    Frontiers in Optics; 10/2012
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    ABSTRACT: This paper presents the results of a laboratory experiment on a new free-standing pupil mask coronagraph for the direct observation of exoplanets. We focused on a binary-shaped pupil coronagraph, which is planned for installation in the next-generation infrared space telescope SPICA. Our laboratory experiments on the coronagraph were implemented inside a vacuum chamber (HOCT) to achieve greater thermal stability and to avoid air turbulence, and a contrast of 1.3×10-9 was achieved with PSF subtraction. We also carried out multi-color/broadband experiments to demonstrate that the pupil mask coronagraph works, in principle, at all wavelengths. We had previously manufactured a checker-board mask, a type of binary-shaped pupil mask, on a glass substrate, which had the disadvantages of light loss by transmission, ghosting from residual reflectance and a slightly different refractive index for each wavelength. Therefore, we developed a new free-standing mask in sheet metal, for which no substrate was needed. As a result of a He-Ne laser experiment with the free-standing mask, a contrast of 1.0×10-7 was achieved for the raw coronagraphic image. We also conducted rotated mask subtractions and numerical simulations of some errors in the mask shape and WFEs. Speckles are the major limiting factor. The free-standing mask exhibited about the same ability to improve contrast as the substrate mask. Consequently, the results of this study suggest that the binary-shaped pupil mask coronagraph can be applied to coronagraphic observations by SPICA and other telescopes.
    Proc SPIE 09/2012;
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    ABSTRACT: We present the Prototype-testbed for Infrared Optics and Coronagraphs (PINOCO) which is a large, multi-purpose cryogenic chamber. At present, the priority for PINOCO is to evaluate binary pupil mask coronagraphs in the mid-infrared wavelength region, which are planned to be adopted for the SPICA coronagraph instrument. In addition, various other experiments are possible using PINOCO: testing diverse high dynamic-range techniques, mirrors, active optics, infrared detectors, filters and spectral dispersion devices, the mechanics of the instruments, measurement of material properties, and so on. PINOCO provides a work space of 1m × 1m × 0.3m, of which inside is cooled to <5K. Flexible access to the work surface is possible by removing detachable plates at the four sides and on the top of the chamber. At the interface to the exterior, PINOCO is currently equipped with an optical window, electric connectors, and an interferometer stage. PINOCO is cooled by two GM-cycle cryo-coolers, so no cryogen is needed. A cooling test of PINOCO was successfully completed.
    Proc SPIE 09/2012;
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    ABSTRACT: A stellar coronagraph system for direct observations of extra solar planets is under development by combining unbalanced nulling interferometer (UNI), adaptive optics, and a focal plane coronagraph 1, 2, 3, 4, 5. It can reach a high contrast as using lambda/10000 precision optics by lambda/1000 quality ones. However, a sufficiently high contrast has yet to be obtained for the experiment. It is thought that the remaining speckle noise at the final coronagraph focal plane detector is produced by a “non-common path error” of lambda/100 level, which is a wavefront error of differences between the coronagraph and a wavefront sensor (WFS) of adaptive optics, even when the WFS indicates lambda/1000 conversion. The non-common path error can be removed by the focal plane sensing method of wavefront correction by wavefront sensing at the final focal plane detector, although it has an issue of operation for very faint targets because of a slow feedback loop. In the present paper, we describe how our coronagraph system becomes practically higher contrast by upgrading the control method of adaptive optics with the WFS assisted by a focal plane wavefront sensing. Then, we control a wavefront error by two feedback loops, the first of which uses a WFS to make fast control for telescope optics deformation and the second of which uses a focal plane detector to compensate for the non-common path error with slow control. We show experiment results of the coronagraph system performance with both wavefront sensing methods.
    Proc SPIE 09/2012;
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    ABSTRACT: We have studied a coronagraph system with an unbalanced nulling interferometer (UNI). It consists of the UNI, adaptive optics, and a coronagraph. An important characteristic is a magnification of the wavefront aberrations in the UNI stage, which enables us to compensate for the wavefront aberrations beyond the AO systems capabilities. In our experiments, we have observed the stable aberration magnification of about 6 times and compensation to about λ/100 rms corresponding to λ/600 rms virtually. As a result, at the final focal plane of a 3-dimensional Sagnac interferometric nulling coronagraph, we have obtained the extra speckle reduction of better than 0.07 by the advantage of the UNI-PAC system. In order to obtain better contrast, we consider improvement of the optics with an 8OPM coronagraph, a dual feedback control, an unbalanced nulling interferometer with 4QPM or VVM, and a wavefront correction inside the UNI.
    Proc SPIE 09/2012;
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    ABSTRACT: We present on-sky results obtained with the visible light prototype of the Fibered Imager foR Single Telescope (FIRST) mounted on the 3-m Shane Telescope at Lick Observatory and using its Adaptive Optics system. This instrument is dedicated to high angular resolution and high dynamic range imaging. Its principle combines both techniques of single-mode fiber interferometry and pupil remapping. Simulations predict a dynamic range up to 106 at /D, or at a few tens of milliarcseconds at 630nm using an 8-m telescope. Laboratory experiments based on a 9-fiber prototype working in the 600nm-900nm spectral band successfully demonstrated the power of the concept. The same prototype has been set-up on the 3-m Shane telescope in July 2010. In this paper, we present the on-sky results obtained in October 2011 with an improved version of the instrument using 18 fibers. They clearly show the detection of the binary star Capella at the diffraction limit of the telescope.λ
    Proc SPIE 09/2012;
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    ABSTRACT: Thirty Meter Telescope (TMT) will see the first light in 2019. We propose Second-Earth Imager for TMT (SEIT) as a future instrument of TMT. The central science case of SEIT is direct imaging and characterization of habitable planets around nearby late-type stars. Focusing on simultaneous spectroscopy of the central star and the planet, SEIT allows us to remove an impact from the telluric absorption and then reveal the presence of oxygen molecules on the Earth-like planets. In order to achieve such a science goal, an extreme AO, a coronagraph, and a post-process technique for achieving high contrast at the small inner working angle are key components. The combination of a shearing nulling interferometer and a pupil remapping interferometer is applied to the first SEIT concept. The shearing nulling interferometer suppresses the diffracted starlight after the extreme AO wavefront correction, and then the pupil remapping interferometer tackles the speckle noise from starlight. Focusing on a fact that the pupil remapping interferometer has difficulty reconstructing the wavefront from only the speckle noise, we found an unbalnced nulling technique enhances the performance of the pupil remapping interferometer. We performed a numerical simulation to validate this concept and found this concept achieves the 5-sigma detection contrast down to 8x10-8 at 10 mas for 5 hours. Thus, the SEIT concept detects habitable planets with a radius two times that of the Earth around ten nearby M stars.
    Proc SPIE 09/2012;

Publication Stats

69 Citations
100.32 Total Impact Points

Institutions

  • 2012–2014
    • National Astronomical Observatory of Japan
      Edo, Tōkyō, Japan
  • 2011
    • Japan Aerospace Exploration Agency
      • Institute of Space and Astronautical Science (ISAS)
      Chōfu, Tōkyō, Japan
  • 2010
    • SETI Institute
      Mountain View, California, United States
  • 2004–2009
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 2002
    • The University of Tokyo
      • Department of Astronomy
      Tokyo, Tokyo-to, Japan