Direct detection of exoplanets requires a high-contrast instrument called a coronagraph to reject bright light from the central star. However, a coronagraph cannot perfectly reject the starlight if the incoming stellar wave front is distorted by aberrations due to the Earth’s atmospheric turbulence and/or the telescope instrumental optics. Wave-front aberrations cause residual stellar speckles that prevent detection of faint planetary light. In this paper, we report a laboratory demonstration of a speckle-nulling wave-front control using a spatial light modulator (SLM) to suppress the residual speckles of a common-path visible nulling coronagraph. Because of its large format, the SLM potentially has the ability to generate a dark hole over a large region or at a large angular distance from a star of up to hundreds of λ/D. We carry out a laboratory demonstration for three cases of dark hole generation: (1) in an inner region (3–8 λ/D in horizontal and 5–15 λ/D in vertical directions), (2) in an outer region (70–75 λ/D in horizontal and 65–75 λ/D in vertical directions), and (3) in a large region (5–75 λ/D in both directions). As a result, the residual speckles are rejected to contrast levels on the order of 10−8 in cases 1 and 2. In cases 2 and 3, we can generate dark holes at a large distance (up to >100 λ/D) and with a large size (70 λ/D square), both of which are out of the Nyquist limit of currently available deformable mirrors.

Shift-and-add (SAA) is a simple image processing procedure. SAA was devised to reconstruct a diffraction-limited image from atmospherically degraded stellar images. Recently SAA has been applied to biological imaging. There are several variants of SAA. Here proposed is an SAA procedure incorporated with unsharp masking (USM). The SAA procedure proposed here encompasses an extended version of USM. The proposed SAA method retains the simplicity and easiness, and the basic features of SAA. The effectiveness of the proposed method is examined by restoring atmospherically degraded solar images. It is shown that the USM SAA reconstructed image exhibits high contrast and reveals fine structures blurred by atmospheric turbulence. It is also shown that the USM SAA performs better with a data frame selection scheme.

In the astronomical speckle interferometry, low-light CCD cameras such as the electron bombarded/multiplying CCDs (EB/EMCCDs) are being widely used for taking the faint speckle images. The detector noise affects the power spectral estimate of the astronomical object as well as the speckle noise does. The estimation errors of the object power spectrum are results of the propagation of the random fluctuations due to those noises. We have formulated a method for correcting and evaluating the estimation errors based on our detection model developed for EB/EMCCDs and the conventional model of the speckle noise. In our method, the error correction and evaluation are accomplished with the aid of the auxiliary data known as the dark and flat frames. The unbiased estimator of the object power spectrum and its SNR evaluator as derived above are verified using simulated data, where we have adopted the values of the detector parameters as obtained by measuring the actually used EMCCD camera in our observations. The results of the simulated experiments show that, within the diffraction cutoff frequency, our power spectral estimator is unbiased, and our SNR evaluator for that is useful except below the seeing cutoff.

Direct detection of faint exoplanets is challenging due to a high-contrast ratio between a primary star and a planet. A high-contrast imaging system has an important role in directly detecting exoplanets. The system consists of coronagraph and speckle reduction technique. A common-path visible nulling coronagraph (VNC) is one of the attracting methods for high-contrast observation because of a simple optical configuration and achromatic stellar elimination. We introduced a spatial light modulator (SLM) into the common-path VNC to suppress residual speckles caused by wavefront aberrations. The SLM can potentially generate a huge dark hole thanks to its large pixel format. As a focal plane wavefront sensor, we utilized the self-coherent camera (SCC) method for the common-path VNC. We carried out the laboratory demonstration of the speckle reduction technique for the common-path VNC combined with the SCC methods. The experimental results show an initial contrast of 2.2 × 10-5 and a final contrast of 1.3 × 10-6 in monochromatic light at a wavelength of 633 nm. We discuss the limiting factors of the contrast for improvement of our demonstration to achieve the higher contrast.

We report experiments of solar ground-layer (GL) adaptive optics (AO) using the 60cm domeless solar telescope of the Hida Observatory, Japan. We developed an averaging-type GL wavefront sensor and confirmed that it properly worked in computer simulations. We set the wavefront sensor behind a conventional AO system and modified AO software so as to drive a deformable mirror using the GL sensor. We conducted solar observations with the GLAO system in September, 2017. It worked to improve observational images over wide fields.

Atmospheric turbulence degrades the image resolution of a ground-based telescope, and a speckle imaging technique can restore the image with high resolution up to the diffraction limit. We have obtained the speckle data of Io ( a Jupiter's satellite) using the 2 m telescope in Nishi-Harima Astronomical Observatory. The speckle data are reduced by the shift-and-add method followed by a background-subtraction procedure. The reduced image is then deconvolved by the similarly reduced image of the unresolved binary star HR6168. The comparison with the close-up image taken by the Voyager 2 spacecraft reveals that the restored image shows well-resolved structural features on the surface of Io.

The Savart-Plate Lateral-shearing Interferometric Nuller for Exoplanets (SPLINE) is a kind of a visible nulling coronagraph for directly detecting exoplanets. The SPLINE consists of two crossed polarizers and a Savart plate placed between them. Theoretically the SPLINE realizes perfect cancellation of starlight. However, achievable contrast is limited by residual stellar speckles due to wavefront aberration caused by imperfect optical surfaces of the optical elements. For reducing the residual stellar speckles of the SPLINE, we propose a speckle nulling technique using a Liquid-Crystal Spatial Light Modulator (LCSLM) to create a dark hole. For the speckle nulling, we apply the Self-Coherent Camera (SCC) technique to the SPLINE for wavefront sensing in the focal plane. We report our recent progress on computer simulation and preliminary laboratory experiments of the speckle nulling technique applied to the SPLINE.

An adaptive optics (AO) system is developed for the 60cm domeless solar telescope of the Hida Observatory, Japan. Its performances are analyzed by the computer simulations, and improved by replacing the Zernike polynomials by Karhunen-Loeve functions. Also, a tomographic wavefront sensor is developed for a ground-layer AO system. From test data acquired at the Hida observatory, wavefront-phase maps both in the ground-layer and in an upper layer are successfully derived.

An effective aperture with several tens or more kilometers is needed to resolve exoplanets. A hypertelescope consists of multiple elemental telescopes like an interferometric array. Light beams from the elemental telescopes are collected and densified and used to form a snap-shot image. Thus formed image, however, does not exhibit high quality features, because the spatial frequency sampling is not dense enough to image properly exoplanets. Some kind of image restoration should be implemented to reveal the surface features of exoplanets. We conduct the image restoration and show the results and the effectiveness of the image restoration through computer simulations.

A technique for restoring solar images partially compensated with adaptive optics is developed. An additional wavefront sensor is installed in an adaptive optics system to acquire residual wavefront information simultaneously to a solar image. A point spread function is derived from the wavefront information and used to deconvolve the solar image. Successful image restorations are demonstrated when the estimated point spread functions have relatively high Strehl ratios.

We designed and fabricated an achromatic eight-octant phase mask (8OPM) for broadband coronagraphic observations of exoplanets. The 8OPM is composed of three-layer eight-octant half-wave plates based on photonic crystals. By using Jones calculus, it is shown that the three-layer 8OPM achieves much higher contrast over broad wavelength range than that of the previous single-layer design. We carry out preliminary laboratory experiments of the coronagraph using the fabricated three-layer 8OPM. As a model star, we use several visible laser light sources for characterizing the coronagraphic performance. As a result, we obtain higher contrasts than theoretical ones of the single-layer 8OPM. However, the achieved contrasts are lower than the theoretical values of the three-layer one. At present we suspect that fabrication errors of the half-wave plates in the 8OPM limit the achieved contrasts.

A technique of restoring solar images partially compensated with adaptive optics is developed. It is based on deconvolution using an additional wavefront sensor. Results of solar image improvement with the technique are shown.

Phase-mask coronagraph holds the ability to detect exoplanets very close to their parent star. We report a new kind of phase mask that performs the contrast ratio of more than the tenth power of 10 for a circular aperture with shades of a secondary mirror and spiders. The phase distribution of the phase mask is numerically obtained by making the leaked light distribute outside the transparent part of the pupil. We applied the hybrid input-output algorithm, one of phase retrieval methods, to find the phase distribution of the phase mask. We show the characteristics of thus obtained phase mask.

The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a multipurpose high-contrast imaging platform designed for the discovery and detailed characterization of exoplanetary systems and serves as a testbed for high-contrast imaging technologies for ELTs. It is a multi-band instrument which makes use of light from 600 to 2500nm allowing for coronagraphic direct exoplanet imaging of the inner 3 lambda/D from the stellar host. Wavefront sensing and control are key to the operation of SCExAO. A partial correction of low-order modes is provided by Subaru's facility adaptive optics system with the final correction, including high-order modes, implemented downstream by a combination of a visible pyramid wavefront sensor and a 2000-element deformable mirror. The well corrected NIR (y-K bands) wavefronts can then be injected into any of the available coronagraphs, including but not limited to the phase induced amplitude apodization and the vector vortex coronagraphs, both of which offer an inner working angle as low as 1 lambda/D. Non-common path, low-order aberrations are sensed with a coronagraphic low-order wavefront sensor in the infrared (IR). Low noise, high frame rate, NIR detectors allow for active speckle nulling and coherent differential imaging, while the HAWAII 2RG detector in the HiCIAO imager and/or the CHARIS integral field spectrograph (from mid 2016) can take deeper exposures and/or perform angular, spectral and polarimetric differential imaging. Science in the visible is provided by two interferometric modules: VAMPIRES and FIRST, which enable sub-diffraction limited imaging in the visible region with polarimetric and spectroscopic capabilities respectively. We describe the instrument in detail and present preliminary results both on-sky and in the laboratory.

We have been developing focal-plane phase-mask coronagraphs ultimately aiming at direct detection and characterization of Earth-like extrasolar planets by future space coronagraph missions. By utilizing photonic-crystal technology, we manufactured various coronagraphic phase masks such as eight-octant phase masks (8OPMs), 2nd-order vector vortex masks, and a 4th-order discrete (32-sector) vector vortex mask. Our laboratory experiments show that the 4th-order vortex mask reaches to higher contrast than the 2nd-order one at inner region on a focal plane. These results demonstrate that the higher-order vortex mask is tolerant of low-order phase aberrations such as tip-tilt errors. We also carried out laboratory demonstration of the 2nd-order vector vortex masks in the High-Contrast Imaging Testbed (HCIT) at the Jet Propulsion Laboratory (JPL), and obtained 10(-8)-level contrast owing to an adaptive optics system for creating dark holes. In addition, we manufactured a polarization-filtered 8OPM, which theoretically realizes achromatic performance. We tested the manufactured polarization-filtered 8OPM in the Infrared Coronagraphic Testbed (IRCT) at the JPL. Polychromatic light sources are used for evaluating the achromatic performance. The results suggest that 10(-5)-level peak-to-peak contrasts would be obtained over a wavelength range of 800-900 nm. For installing the focal-plane phase-mask coronagraph into a conventional centrally-obscured telescope with a secondary mirror, pupil-remapping plates have been manufactured for removing the central obscuration to enhance the coronagraphic performance. A result of preliminary laboratory demonstration of the pupil-remapping plates is also reported. In this paper, we present our recent activities of the photonic-crystal phase coronagraphic masks and related techniques for the high-contrast imaging.

We have developed the Savart-Plate Lateral-shearing Interferometric Nuller for Exoplanets (SPLINE), which is a kind of a nulling interferometer, for directly imaging exoplanets. The SPLINE consists of two polarizers and a Savart plate between them. The SPLINE can theoretically obtain fully achromatic and stable nulled output. However, a drawback of the SPLINE is its low system throughput due to the polarizers. For improving the system throughput, we propose a dualchannel SPLINE using polarization beam splitters instead of the polarizers. We have carried out laboratory demonstration of the dual-channel SPLINE. The achievable contrast of the SPLINE is limited by residual speckles caused by surface roughness of optical elements. For improving the achievable contrast, we propose a method of wavefront correction using a liquid-crystal spatial light modulator (LCSLM). We have carried out preliminary laboratory demonstration using a liquid-crystal variable retarder (LCVR), instead of the LCSLM, for simulating the proposed wavefront correction method. We report the laboratory demonstration in this paper.

Focal-plane phase-mask coronagraphs, such as eight-octant phase-mask coronagraphs (8OPM), are one of the most promising tools for high contrast observations. However, coronagraphic performance would be degraded when combined with a centrally-obscured telescope. We propose pupil-remapping optics for removing the shade of a secondary mirror to generate a clear, circular pupil for the phase-mask coronagraph. First, we show the design of the pupil-remapping optics, called central-obscuration removal plates (CRPs). Next, we report laboratory experiments on the 8OPM coronagraph using manufactured CRPs. We also evaluate off-axis point-spread functions via both laboratory experiments and numerical simulations. Finally, we evaluate, via numerical simulations, limiting factors for coronagraphic performance, such as phase aberrations introduced by the CRPs, the effect of Fresnel diffraction, and chromatic behavior. The numerical simulations suggest that the phase aberrations could be a dominant limiting factor of the achievable contrast in the current laboratory experiments.

In astronomical speckle imaging, deconvolving a shift-and-add (SAA) image has an advantage over deconvolving noisy specklegrams, because an SAA image is an integration of many specklegrams and has a relatively enhanced signal-tonoise ratio. In this paper, to reinforce the deconvolution of a single SAA image, we propose a multiframe deconvolution applied to multiple SAA images that are obtained by diversely recombining the same set of specklegrams to have different point spread functions. We have found that such diverse SAA images can be easily produced by permuting specklegrams to be processed by SAA. The results of experiments using simulated and observational data have shown a robustness of our present approach: in the previous approach of deconvolving a single SAA frame, the resulting object estimate is apt to be influenced by the given SAA frame and the estimation sometimes fails, whereas in the present approach, a reliable object image is stably reconstructed regardless of the given SAA frames.

Using the framework of conversation analysis, this paper examines the communication problems which international graduate students in engineering encounter when they have a Q&A session in Japanese after giving a presentation. This analysis focuses on (1) the phenomenon where international students fail to initiate a ‘repair sequence’ (Schegloff et al. 1977) to attain mutual understanding among participants when they do not understand a question from the floor ; and (2) why the conversational procedure of ‘repair’ is not employed by these students. Finally, this paper discusses what kinds of classroom activities can be introduced to solve these students’ communication problems. These issues are also applicable to situations in which native speakers of Japanese use English or other foreign languages in engineering communities.

We extend our solar SCIDAR technique to measuring wind velocities on turbulent layers. We apply our technique to data observed with the 60 cm domeless solar telescope at the Hida Observatory in Japan, and confirm that notable peaks corresponding to wind flows appear on a temporal-correlation plane.