Various types of high-contrast imaging instruments have been proposed and developed for direct detection of exoplanets by suppressing nearby stellar light. Stellar speckles due to wavefront aberration can be suppressed by the appropriate wavefront control, called the dark hole control. However, the speckles, which fluctuate faster than the dark hole control due to atmospheric turbulence in ground-based telescopes or instrument deformation caused by temperature changes in space telescopes, cannot be suppressed by the control and remain in focal plane images. The Coherent Differential Imaging on Speckle Area Nulling (CDI-SAN) method was proposed to overcome such fast fluctuating speckles and detect exoplanetary light. We constructed an optical setup in a laboratory to demonstrate the CDI-SAN method. With the dark hole control and the CDI-SAN method, we achieved 10−8 level of contrasts.
High-contrast imaging techniques are essential for future space missions aimed at directly detecting and characterizing exoplanets. We constructed a new testbed called the facility for coronagraphic elemental technologies (FACET) for the development of high-contrast imaging techniques. FACET has three optical paths (A, B, and C). Path A is used for developing focal-plane phase mask (FPM) coronagraphs. Currently, we have been developing photonic-crystal multi-layer phase masks for suppressing stellar light over a broad wavelength range. In path B, we demonstrate high-contrast observation combining FPM coronagraphs with a dark hole (DH) control technique. A spatial light modulator (SLM) is used as a wavefront control device. We are developing DH control techniques that take advantage of the large pixel format of the SLM. In path C, we demonstrate DH control for detecting exoplanets not only around single stars but also around binary-star systems. We install the SLM into the common-path visible nulling coronagraph to test the multiple-star DH control. We report details of FACET and recent progress of our activities at FACET. We have recently made significant progress with the demonstration of observation for a single star for which we achieved a monochromatic contrast of 2.9 × 10−9 at path C.
The mid-infrared spectrometer and camera transit spectrometer (MISC-T) is one of the three baseline instruments for Origins Space Telescope (Origins) and provides the capability to assess the habitability of nearby exoplanets and search for signs of life. MISC-T employs a densified pupil optical design, and HgCdTe and Si:As detector arrays. This optical design allows the instrument to be relatively insensitive to minor line-of-sight pointing drifts and telescope aberrations, and the detectors do not require a sub-Kelvin refrigerator. MISC-T has three science spectral channels that share the same field-of-view by means of beam splitters, and all channels are operated simultaneously to cover the full spectral range from 2.8 to 20 μm at once with exquisite stability and precision (<5 ppm between 2.8 to 11 μm, <20 ppm between 11 and 20 μm). A Lyot-coronagraph-based tip–tilt sensor located in the instrument fore-optics uses the light reflected by a field stop, which corresponds to 0.3% of the light from the target, to send fine pointing information to the field steering mirror in the Origins telescope. An additional MISC Wide Field Imager (WFI) is studied as an upscope option for the Origins. MISC-WFI offers a wide field imaging (3 ′ × 3 ′ ) and low-resolution spectroscopic capability with filters and grating-prisms (grisms) covering 5 to 28 μm. The imaging capability of the MISC-WFI will be used for general science objectives. The low-resolution spectroscopic capability in MISC-WFI with a resolving power R ( = λ / Δλ) of a few hundreds will be used to measure the mid-infrared dust features and ionic lines at z up to ∼1 in the Origins mission’s Rise of Metals and Black Hole Feedback programs. The MISC-WFI also serves as a focal plane pointing and guiding instrument for the observatory, including when the MISC-T channel is performing its exoplanet spectroscopy observations.
Development of a high-contrast imaging system, especially toward direct detection of habitable Earth-like exoplanets, would be one of the most challenging themes in modern astronomy. For direct detection of exo-Earth, coronagraphic devices are required for suppressing bright diffracted light from a parent star. In addition, residual stellar speckles, caused by imperfection of optical components, have to be also rejected by wavefront control such as the speckle nulling technique. It is important to construct a dedicated testbed at which we can comprehensively develop the high-contrast imaging system for future era of space coronagraphs aimed at searching for exo-Earths. Recently, we have started construction of a new testbed in Japan which is called EXIST (Exoplanet Imaging System Testbed). The EXIST is planned to be compatible with various types of coronagraphs, such as phase-mask coronagraphs based on the photoniccrystal technology, common-path visible nulling coronagraph, and so on. In addition, optimally designed pupil apodizers will be installed into the testbed for maximizing the performance of the phase-mask coronagraphs with arbitrary aperture telescopes. We plan to utilize a spatial light modulator (SLM) for conducting the speckle nulling control. Thanks to a large pixel format of the SLM, we expect that a huge dark hole can be created against the residual speckles. Here, we report our recent progress on the construction of the new testbed and results of some preparatory experiments related to the coronagraphs and the speckle nulling control using the SLM.
Subaru telescope has been operating a high-contrast imaging instruments called Subaru coronagraphic extreme adaptive optics (SCExAO) which is used for exoplanet research. We are developing phase mask coronagraphs using photonic crystal wave plates inside the SCExAO. An eight-octant phase mask (8OPM) of three-layer achromatic structure has been fabricated as a second generation. It was designed for J and H band to reach 10-5 contrast, and Ks band to 10-4. A retardation and a coronagraphic performance of the 8OPM were confirmed almost as designed at 1550nm. An apodised (binary shaped) pupil to be used with the 8OPM was also studied to suppress diffracted light by the secondary shadow and spiders. We confirmed a performance of the combination of the apodizer and the 8OPM at visible wavelengths in a lab. We optimized the apodizer for a pupil of the SCExAO where we obtained a transmission of 50 % and a contrast of 10-4 the center and 10-6 at outer region. We manufactured the designed apodizer to be installed in SCExAO for infrared observations.
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.
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument is a high-contrast imaging system installed at the 8-m Subaru Telescope on Maunakea, Hawaii. Due to its unique evolving design, SCExAO is both an instrument open for use by the international scientific community, and a testbed validating new technologies, which are critical to future high-contrast imagers on Giant Segmented Mirror Telescopes (GSMTs). Through multiple international collaborations over the years, SCExAO was able to test the most advanced technologies in wavefront sensors, real-time control with GPUs, low-noise high frame rate detectors in the visible and infrared, starlight suppression techniques or photonics technologies. Tools and interfaces were put in place to encourage collaborators to implement their own hardware and algorithms, and test them on-site or remotely, in laboratory conditions or on-sky. We are now commissioning broadband coronagraphs, the Microwave Kinetic Inductance Detector (MKID) Exoplanet Camera (MEC) for high-speed speckle control, as well as a C-RED ONE camera for both polarization differential imaging and IR wavefront sensing. New wavefront control algorithms are also being tested, such as predictive control, multi-camera machine learning sensor fusion, and focal plane wavefront control. We present the status of the SCExAO instrument, with an emphasis on current collaborations and recent technology demonstrations. We also describe upgrades planned for the next few years, which will evolve SCExAO —and the whole suite of instruments on the IR Nasmyth platform of the Subaru Telescope— to become a system-level demonstrator of the Planetary Systems Imager (PSI), the high-contrast instrument for the Thirty Meter Telescope (TMT).
The Mid-infrared Imager, Spectrometer, Coronagraph (MISC) is one of the instruments studied both for the Origins Space Telescope (OST) Mission Concept 1 and 2. The MISC for OST Mission Concept 1 consists of the MISC imager and spectrometer module (MISC I and S), the MISC coronagraph module (MISC COR) and the MISC transit spectrometer module (MISC TRA). The MISC I and S offers (1) a wide field (3 arcminx3 arcmin) imaging and low-resolution spectroscopic capability with filters and grisms for 6-38 μm, (2) a medium-resolution (R~1,000) Integral Field Unit (IFU) spectroscopic capability for 5- 38 μm and (3) a high-resolution (R~25,000) slit spectroscopic capability for 12-18 μm and 25-36 μm. The MISC COR module employs PIAACMC coronagraphy method and covers 6-38 μm achieving 10-7 contrast at 0.5 arcsec from the central star. The MISC TRA module employs a densified pupil spectroscopic design to achieve 3-5 ppm of spectro-photometric stability and covers 5-26 μm with R=100-300. The MISC for OST Mission Concept 2 consists of the MISC wide field imager module (MISC WFI) and the MISC transit Spectrometer module (MISC TRA). The MISC WFI offers a wide field (3 arcmin ×3 arcmin) imaging and low-resolution spectroscopic capabilities with filters and grisms for 6-28μm. The MISC TRA module in the OST Mission Concept 2 also employs the densified pupil spectroscopic design to achieve <5 ppm of spectro-photometric stability and covers 4-22 μm with R=100-300. The highest ever spectrophotometric stability achieved by MISC TRA enables to detect bio-signatures (e.g., ozone, water, and methane) in habitable worlds in both primary and secondary transits of exoplanets and makes the OST a powerful tool to bring an revolutionary progress in exoplanet sciences. Combined with the spectroscopic capability in the FIR provided by other OST instruments, the MISC widens the wavelength coverage of OST down to 5μm, which makes the OST a powerful tool to diagnose the physical and chemical condition of the ISM using dust features, molecules lines and atomic and ionic lines. The MISC also provides the OST with a focal plane guiding function for the other OST science instruments as well as its own use.
The Mid-infrared Imager, Spectrometer Coronagraph (MISC) instrument studied for the Origins Space Telescope (OST) Mission Concept 1 is designed to observe at mid-infrared (MIR) wavelengths ranging from 5 to 38 microns for OST. In the OST Mission Concept 1 study, MISC consists of three separate optical modules providing imaging, spectroscopy, and coronagraph capabilities. The MISC Coronagraph module (MISC COR) employs Phase-Induced Amplitude Apodization (PIAA) coronagraph (Guyon et al. 2014) in which pupil apodization is modified by reflection on mirrors and central starlight is blocked by focal plane mask and Lyot mask. The performance target of MISC COR is to achieve 10-7 contrast at 0.5” from the central star with covering wavelength of 6-38 microns using 2 optical channels. MISC COR will be a powerful tool to bring a revolutionary progress in exoplanet sciences. In this paper, we present detailed design of its optics and optomechanics, and discuss expected performances for a variety of combination of focal plane mask and Lyot mask.
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.
We are studying a coronagraph system with an imperfect pre-coronagraph in the field of direct detection of exoplanets which can provide additional contrast to a main coronagraph. It is a kind of an unbalanced nulling interferometer (UNI) concept which consists of the first deformable mirror (DM), the pre-coronagraph, the second DM, and a main coronagraph. The pre-coronagraph and the DM1 reduce the star light and the speckle noise to about one-hundreds which would be added to the main coronagraph contrast. The DMs can be controlled by the dark-hole algorithm by changing the masks at the coronagraph foci.
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.
We designed and fabricated an achromatic eight-octant phase mask (8OPM) for broadband coronagraphic observations of exoplanets. The fabricated 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 manufacturing errors of the half-wave plates in the 8OPM limit the achieved contrasts.
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.
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.
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 mask coronagraph1,2,3,4,5,6. It can reach a high contrast as using λ/10000 precision optics by λ/1000 quality ones. However, a sufficient high contrast is not obtained yet in the experiment before. It is thought that the remained speckle noise at the final coronagraph focal plane detector are produced by a “non-common path error” of λ/100 level, which is a wavefront error of the coronagraph different from that of a wavefront sensor (WFS) of adaptive optics, even when the WFS indicates λ/1000 conversion. The non-common path error can be removed by the dark zone method that is the way 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 that our coronagraph system becomes practically higher contrast by upgrading the control method of deformable mirror (DM) with the WFS assisted by final focal plane wavefront sensing method. We accomplished contrast of 8×10-7 relative to the star in experiment.
We have studied a coronagraph system with an unbalanced nulling interferometer (UNI). An important characteristic is a pre-reduction of the star light to 1/100 at the UNI stage which enables to enhance the final contrast. In other point of view, the UNI stage magnifies the wavefront aberrations, which lead us to compensate for the wavefront aberrations beyond the AO systems capabilities. It consists of the UNI, adaptive optics, and a coronagraph. In our experiments, we have observed the extra speckle reduction of better than 0.07 by the advantage of the UNI system. In order to obtain better contrast, we planned to reconstruct all of the optics, which use UNI with 4QPM, a coronagraph with 8OPM or VVM, a dual feedback control method, and a wavefront correction inside the UNI by an upstream AO.
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.
We propose a new high contrast imager for Kyoto 4m segmented telescope called SEICA (Second-generation
Exoplanet Imager with Coronagraphic Adaptive optics), aiming at detection and characterization of selfluminous
gas giants within 10AU around nearby stars. SEICA is aggressively optimized for high performance
at very small inner working angle, 10-6 detection contrast at 0".1 in 1-hour integration. We start the on-sky
commissioning test in 2016 and the science observations in 2017. Since it is the first time to realize the highcontrast
imaging on the segmented telescope, SEICA is an important step toward future high contrast
sciences on Extremely Large Telescopes (ELTs). This paper presents an overall of the SEICA program and
the conceptual design for ultimate performance under given atmospheric conditions.
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.
One of the problems for direct observation of extrasolar planets is the speckle noise due to a wave-front error.
Therefore, high-accuracy adaptive optics is required for realizing a wavefront quality of λ/10000 rms. An unbalanced
nulling interferometer has a possibility to assist high-accuracy correction. In this paper, we propose the interferometer
with a four-quadrant phase mask in which an optical path is common. By using the mask, we succeed in stabilizing the
interference and taking measurements of wavefront errors with 10-times higher sensitivity. In this way, we expect to
construct high-accuracy adaptive optics which is more stable.
The Savart-Plate Lateral-shearing Interferometric Nuller for Exoplanet (SPLINE) is a stable and fully achromatic nulling
interferometer proposed for direct detection of extrasolar planets with segmented-mirror telescopes like the Thirty Meter
Telescope (TMT). The SPLINE uses a Savart plate, a kind of polarizing beam splitter, to split a light beam into two
orthogonally polarized ones with a lateral shift. The Savart plate placed between crossed polarizers causes fully
achromatic destructive interference for an on-axis star light. On the other hand, planetary light from an off-axis direction
does not destructively interfere due to the lateral shift. The SPLINE provides a stable interferometric output because of
its simple common-path optical design without an optical-path difference control system. We carried out laboratory
demonstrations of the SPLINE to evaluate its stability, achromaticity, and achievable contrast. As a result, a high
contrast of >104 (peak-to-peak contrast) is achieved using a broadband light source as a star model. In addition, we also
propose to apply a differential imaging technique to the SPLINE for improving achievable contrast. We report our recent
activities and show the results of the laboratory demonstrations.
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.
We present the current status of the development of the SPICA Coronagraph Instrument (SCI). SPICA is a next-generation
3-meter class infrared telescope, which will be launched in 2022. SCI is high-contrast imaging, spectroscopic
instrument mainly for direct detection and spectroscopy of extra-solar planets in the near-to-mid infrared wavelengths to
characterize their atmospheres, physical parameters and evolutionary scenarios. SCI is now under the international
review process. In this paper, we present a science case of SCI. The main targets of SCI, not only for direct imaging but
also for spectroscopy, are young to matured giant planets. We will also show that some of known exoplanets by ground-based
direct detection are good targets for SCI, and a number of direct detection planets that are suitable for SCI will be
significantly increased in the next decade. Second, a general design of SCI and a key technology including a new high-throughput
binary mask coronagraph, will be presented. Furthermore, we will show that SCI is potentially capable of
achieving 10-6 contrast by a PSF subtraction method, even with a telescope pointing error. This contrast enhancement
will be important to characterize low-mass and cool planets.
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.
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.
Photonic crystal, an artificial periodic nanostructure of refractive indices, is one of the attractive technologies for
coronagraph focal-plane masks aiming at direct imaging and characterization of terrestrial extrasolar planets. We
manufactured the eight-octant phase mask (8OPM) and the vector vortex coronagraph (VVC) mask very precisely using
the photonic crystal technology. Fully achromatic phase-mask coronagraphs can be realized by applying appropriate
polarization filters to the masks. We carried out laboratory experiments of the polarization-filtered 8OPM coronagraph
using the High-Contrast Imaging Testbed (HCIT), a state-of-the-art coronagraph simulator at the Jet Propulsion
Laboratory (JPL). We report the experimental results of 10-8-level contrast across several wavelengths over 10%
bandwidth around 800nm. In addition, we present future prospects and observational strategy for the photonic-crystal
mask coronagraphs combined with differential imaging techniques to reach higher contrast. We proposed to apply the
polarization-differential imaging (PDI) technique to the VVC, in which we built a two-channel coronagraph using
polarizing beam splitters to avoid a loss of intensity due to the polarization filters. We also proposed to apply the
angular-differential imaging (ADI) technique to the 8OPM coronagraph. The 8OPM/ADI mode mitigates an intensity
loss due to a phase transition of the mask and provides a full field of view around central stars. We present results of
preliminary laboratory demonstrations of the PDI and ADI observational modes with the phase-mask coronagraphs.
Small-angle coronagraphy is technically and scientifically appealing because it enables the use of smaller telescopes,
allows covering wider wavelength ranges, and potentially increases the yield and completeness of circumstellar
environment – exoplanets and disks – detection and characterization campaigns. However, opening up
this new parameter space is challenging. Here we will review the four posts of high contrast imaging and their
intricate interactions at very small angles (within the first 4 resolution elements from the star). The four posts
are: choice of coronagraph, optimized wavefront control, observing strategy, and post-processing methods. After
detailing each of the four foundations, we will present the lessons learned from the 10+ years of operations of
zeroth and first-generation adaptive optics systems. We will then tentatively show how informative the current
integration of second-generation adaptive optics system is, and which lessons can already be drawn from this
fresh experience. Then, we will review the current state of the art, by presenting world record contrasts obtained
in the framework of technological demonstrations for space-based exoplanet imaging and characterization mission
concepts. Finally, we will conclude by emphasizing the importance of the cross-breeding between techniques
developed for both ground-based and space-based projects, which is relevant for future high contrast imaging
instruments and facilities in space or on the ground.
In an unbalanced nulling interferometer (UNI) of our coronagraph system, the incidence light is divided into two, and they interfere by a reverse phase with different amplitude. Thereby, phase errors are magnified and we can correct a wavefront with higher precision. But phase errors of the incident wave will be magnified together with the wavefront errors inside UNI. Now, I am developing a control algorithm of the adaptive optics which removes the wavefront errors inside the interferometer by operating the phase of the light to a suitable value before dividing. In a simulation, wavefront accuracy improved by about 3 times with this technique, and also a comparable effect was acquired experimentally.
The Vector Vortex Coronagraph (VVC) is one of the most attractive new-generation coronagraphs for ground- and
space-based exoplanet imaging/characterization instruments, as recently demonstrated on sky at Palomar and
in the laboratory at JPL, and Hokkaido University. Manufacturing technologies for devices covering wavelength
ranges from the optical to the mid-infrared, have been maturing quickly. We will review the current status of
technology developments supported by NASA in the USA (Jet Propulsion Laboratory-California Institute of
Technology, University of Arizona, JDSU and BEAMCo), Europe (University of Li`ege, Observatoire de Paris-
Meudon, University of Uppsala) and Japan (Hokkaido University, and Photonics Lattice Inc.), using liquid
crystal polymers, subwavelength gratings, and photonics crystals, respectively. We will then browse concrete
perspectives for the use of the VVC on upcoming ground-based facilities with or without (extreme) adaptive
optics, extremely large ground-based telescopes, and space-based internal coronagraphs.
We have proposed a four-stage coronagraph system with an unbalanced nulling interferometer (UNI). It consists of a
first adaptive optics (AO), the UNI, a second AO, and a coronagraph. An important feature 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 aberration magnification of about 6 times and
compensated to about lambda/100 rms corresponding to lambda/600 rms virtually, and its performance is becoming
stable. We have put a 3-dimensional Sagnac interferometric nulling coronagraph at the final stage of the system and tried
to see the speckle reduction with the UNI-PAC system.
We report laboratory demonstrations of an eight-octant phase-mask (EOPM) coronagraph for direct detection of
exoplanets. The EOPM coronagraph is a family of a four-quadrant phase-mask (FQPM) one, and shows better
coronagraphic performance for partially resolved stars. We manufactured an eight-octant ferroelectric liquid-crystal
(FLC) mask. The FLC mask is composed of eight-segmented half-wave plates whose principal axes are different
between adjacent segments. The mask operates as a fully achromatic EOPM when the FLC mask is placed between
crossed polarizers. We carried out laboratory experiments on the EOPM coronagraph by using partially resolved whitelight
source, and compared the performance with that of the FQPM one. As a result, we confirmed that the EOPM shows
higher contrast than the FQPM. A drawback of the proposed method is that the FLC mask can be used only for one
component of polarization of incoming light because it is necessary to use the polarizer in front of the FLC mask. To
solve this problem, a two-channel coronagraph, based on two polarizing beam splitters instead of the polarizers, is
proposed. Observational efficiency can significantly be improved because the two-channel coronagraph enables us to
detect both components of polarizations from exoplanets. We also report preliminary experimental results of laboratory
demonstrations of the two-channel coronagraph.
An eight-octant phase-mask (EOPM) coronagraph is one of the highest performance coronagraphic concepts, and attains
simultaneously high throughput, small inner working angle, and large discovery space. However, its application to
ground-based telescopes such as the Subaru Telescope is challenging due to pupil geometry (thick spider vanes and large
central obstruction) and residual tip-tilt errors. We show that the Subaru Coronagraphic Extreme Adaptive Optics
(SCExAO) system, scheduled to be installed onto the Subaru Telescope, includes key technologies which can solve these
problems. SCExAO uses a spider removal plate which translates four parts of the pupil with tilted plane parallel plates.
The pupil central obstruction can be removed by a pupil remapping system similar to the PIAA optics already in the
SCExAO system, which could be redesigned with no amplitude apodization. The EOPM is inserted in the focal plane to
divide a stellar image into eight-octant regions, and introduces a π-phase difference between adjacent octants. This
causes a self-destructive interference inside the pupil area on a following reimaged pupil plane. By using a reflective
mask instead of a conventional opaque Lyot stop, the stellar light diffracted outside the pupil can be used for a
coronagraphic low-order wave-front sensor to accurately measure and correct tip-tilt errors. A modified inverse-PIAA
system, located behind the reimaged pupil plane, is used to remove off-axis aberrations and deliver a wide field of view.
We show that this EOPM coronagraph architecture enables high contrast imaging at small working angle on the Subaru
Telescope. Our approach could be generalized to other phase-mask type coronagraphs and other ground-based telescopes.
The MIRA-I.2 is a 30m baseline optical interferometer located at the Mitaka campus of the National Astronomical
Observatory of Japan. After the detection of the first fringes with Vega in 2002, we have continued
improvement of system performance and have demonstrated stellar diameter measurement in wide band
(600nm-1000nm). Recently, we begin on development of two scientific detectors: spectrometer with separate
fringe tracking system and interfeometric polarimetry. Recent progress and performance of our two system is
reported.
We have developed a dichroic beam splitter for the Subaru AO188, which reflects optical light (0.4-0.9 &mgr;m) for
wavefront sensing and transmits near-infrared light (0.93-5.2 &mgr;m) for science observations. The beam splitter
is made of 145mm × 200mm calcium fluoride substrate coated by fluoride and metal chalcogen compound
multilayer, which should be a best way to realize high transmittance over wide wavelength range in the near
infrared. However, since typical fluoride soft coating is less resistant to the moisture in the air, the fluoride
coating become damaged as we use on the AO188 optical bench which is placed in the room temperature
condition. We have performed several accelerated endurance tests of the beam splitter under high-humidity
condition by changing the design of the coatings, and found an optimal solution with an oxide protection layer
which prevents the damage of the dichroic coating and keeps high transmittance at near-infrared wavelength. In
this paper, we report the results of the endurance tests and the performance of our dichroic beam splitter.
The Subaru laser guide star adaptive optics (AO) system was installed at the Nasmyth focus of the Subaru
Telescope, and had the first light with natural guide star on October 2006. The AO system has a 188-element
curvature based wavefront sensor with photon-counting avalanche photodiode (APD) modules. It measures high-order
terms of wavefront using either of a single laser (LGS) or natural guide star (NGS) within a 2' diameter
field. The AO system has also a source simulator. It simulates LGS and NGS beams, simultaneously, with and
without atmospheric turbulence by two turbulent layer at about 0 and 6 km altitudes, and reproduces the cone
effect for the LGS beam. We describe the design, construction, and integration of the curvature wavefront sensor
and calibration source unit.
A four-quadrant phase-mask (FQPM) coronagraph can suppress perfectly stellar light when a star can be regarded as
a point-like source. However, the FQPM coronagraph is highly sensitive to partially resolved stars, and shows
second-order sensitivity to tip-tilt error leakage. Higher-order sensitivity is required for extremely high-contrast imaging
of nearby stars.
We propose an eight-octant phase-mask (EOPM) for achieving fourth-order sensitivity to tip-tilt errors. We
manufactured the phase-mask utilizing a nematic liquid crystal (LC) device, which is composed of eight segments. A
phase retardation of the LC can be adjustable by an applied voltage to the device. The LC phase-mask can be switched
between FQPM-mode and EOPM-mode by applying appropriate voltages to the segments. We carry out experiments on
the phase-mask coronagraph with various tip-tilt errors. The experimental results show the higher-order behavior of the
EOPM compared to the FQPM.
We present a current status of the laboratory experiments on the EOPM coronagraph, and also show coronagraphic
performance of the EOPM derived from numerical simulations.
The SPace Infrared telescope for Cosmology and Astrophysics (SPICA) is a infrared space-borne telescope mission of
the next generation following AKARI. SPICA will carry a telescope with a 3.5 m diameter monolithic primary mirror
and the whole telescope will be cooled to 5 K. SPICA is planned to be launched in 2017, into the sun-earth L2 libration
halo orbit by an H II-A rocket and execute infrared observations at wavelengths mainly between 5 and 200 micron. The
large telescope aperture, the simple pupil shape, the capability of infrared observations from space, and the early launch
gives us with the SPICA mission a unique opportunity for coronagraphic observation. We have started development of a
coronagraphic instrument for SPICA. The primary target of the SPICA coronagraph is direct observation of extra-solar
Jovian planets. The main wavelengths of observation, the required contrast and the inner working angle (IWA) of the
SPICA coronagraph are set to be 5-27 micron (3.5-5 micron is optional), 10-6, and a few λ/D (and as small as possible),
respectively, in which λ is the observation wavelength and D is the diameter of the telescope aperture (3.5m). For our
laboratory demonstration, we focused first on a coronagraph with a binary shaped pupil mask as the primary candidate
for SPICA because of its feasibility. In an experiment with a binary shaped pupil coronagraph with a He-Ne laser
(λ=632.8nm), the achieved raw contrast was 6.7×10-8, derived from the average measured in the dark region without
active wavefront control. On the other hand, a study of Phase Induced Amplitude Apodization (PIAA) was initiated in an
attempt to achieve better performance, i.e., smaller IWA and higher throughput. A laboratory experiment was performed
using a He-Ne laser with active wavefront control, and a raw contrast of 6.5×10-7 was achieved. We also present recent
progress made in the cryogenic active optics for SPICA. Prototypes of cryogenic deformable by Micro Electro
Mechanical Systems (MEMS) techniques were developed and a first demonstration of the deformation of their surfaces
was performed with liquid nitrogen cooling. Experiments with piezo-actuators for a cryogenic tip-tilt mirror are also
ongoing.
We present a method of the polarization degree analysis of exoplanets' objective-prism spectra. The polarization
analysis of the objective spectra can be used for discerning planet signal from noisy stellar light. The light reflected from
the planet is expected to be partially polarized, while the direct stellar light can be considered to be unpolarized. For
measuring objective spectra we use a four-quadrant polarization mask (FQPoM) coronagraph and a prism. The primary
suppression of starlight is achieved by destructive interference of the light passing through the central region of FQPoM.
For further suppression of starlight we use a polarization differential technique. By taking the difference between two
orthogonally polarized components of incoming light we can further suppress unpolarized starlight and reveal the
spectrum of the exoplanet. However, when the intensity contrast between the star and its planet is high, the starlight
noise impedes detection of the planetary spectrum. The analysis of the degree of polarization relieves the separation of
the planetary spectrum from the stellar noise. Moreover, any peculiar features in the objective spectra would be useful to
find out the location of the exoplanet. We obtained the experimental results under an intensity contrast of 3.5×10-5 and an
angular separation of 4.9 λ/D.
We proposed a novel method based on a pre-optics setup that behaves partly as a low-efficiency coronagraph, and partly
as a high-sensitivity wavefront aberration compensator (phase and amplitude). The combination of the two effects results
in a highly accurate corrected wavefront. First, an (intensity-) unbalanced nulling interferometer (UNI) performs a
rejection of part of the wavefront electric field. Then the recombined output wavefront has its input aberrations
magnified. Because of the unbalanced recombination scheme, aberrations can be free of phase singular points (zeros) and
can therefore be compensated by a downstream phase and amplitude correction (PAC) adaptive optics system, using two
deformable mirrors. In the image plane, the central star's peak intensity and the noise level of its speckled halo are
reduced by the UNI-PAC combination: the output-corrected wavefront aberrations can be interpreted as an improved
compensation of the initial (eventually already corrected) incident wavefront aberrations. The important conclusion is
that not all the elements in the optical setup using UNI-PAC need to reach the lambda/10000 rms surface error quality. In
the experiments, we observed the aberration magnification of more than 5 times and compensated to about lambda/70
rms which is the current limit of the AO system. This means that we reached to lambda/350 level virtually. We observed
the speckle reduction in the focal plane with a coronagraph.
The current status and recent results, since last SPIE conference at Orlando in 2006, for the laser guide star adaptive optics system for Subaru Telescope is presented. We had a first light using natural guide star and succeed to launch the sodium laser beam in October 2006. The achieved Strehl ratio on the 10th magnitude star was around 0.5 at K band. We confirmed that the full-width-half-maximum of the stellar point spread function is smaller than 0.1 arcsec even at the 0.9 micrometer wavelehgth. The size of the artificial guide star by the laser beam tuned at the wavelength of 589 nm was estimated to be 10 arcsec. The obtained blurred artificial guide star is caused by the wavefront error on the laser launching telescope. After the first light and first launch, we found that we need to modify and to fix the components, which are temporarily finished. Also components, which were postponed to fabricate after the first light, are required to build newly. All components used by the natural guide star adaptive optics system are finalized recently and we are ready to go on the sky. Next engineering observation is scheduled in August, 2008.
KEYWORDS: Coronagraphy, Space telescopes, Telescopes, Point spread functions, Mirrors, Infrared telescopes, Binary data, James Webb Space Telescope, Photomasks, Wavefronts
We present the status of the development of a coronagraph for the Space Infrared telescope for Cosmology and
Astrophysics (SPICA). SPICA is the next generation of infrared space-borne telescope missions following to AKARI,
led by Japan. SPICA will carry a telescope that has a 3.5 m diameter monolithic primary mirror and the whole telescope
will be cooled to 4.5 K. It is planned to launch SPICA into the sun-earth L2 libration halo orbit using H II-A rocket in the
middle of the 2010s and execute infrared observations at wavelengths mainly between 5 and 200 micron. The SPICA
mission gives us a unique opportunity for coronagraph observations, because of the large telescope aperture, the simple
pupil shape, the capability of infrared observations from space, and the early launch. We have started development of the
SPICA coronagraph in which the primary target is direct observation of extra-solar Jovian planets. The main
wavelengths of observation, the required contrast and the inner working angle (IWA) of the SPICA coronagraph
instrument are set to be 5-27 micron, 10-6, and a few λ/D (and as small as possible), respectively, in which λ is the
observation wavelength and D is the diameter of the telescope aperture (3.5m). We focused on a coronagraph with a
binary shaped pupil mask as the primary candidate for SPICA because of its feasibility. Nano-fabrication technology
using electron beam lithography was applied to manufacture a high precision mask and a laboratory experiment with a
He-Ne laser (λ=632.8nm) was performed in air without active wavefront control. The raw contrast derived from the
average measured in the dark region reached 6.7×10-8. On the other hand, a study of Phase Induced Amplitude
Apodization (PIAA) was started in an attempt to achieve higher performance, i.e., smaller IWA and higher throughput. A
hybrid solution using PIAA and a shaped pupil mask was proposed. A laboratory experiment was performed using a He-
Ne laser with active wavefront control via a 32×32 channel deformable mirror. A raw contrast of 6.5×10-7 was achieved.
Designs of binary shaped pupil mask are presented for the actual SPICA pupil which is obstructed by the telescope's
secondary mirror and its support. Subtraction of point spread function (PSF) was also evaluated.
We present the status of the development of a coronagraph for the Space Infrared telescope for Cosmology and
Astrophysics (SPICA). SPICA is the next generation infrared space-borne telescope missions led by Japan. The SPICA
satellite will be equipped with a telescope that has a 3.5 m diameter monolithic primary mirror and the whole telescope
will be cooled to 4.5 K. The satellite is planed be launched early in the 2010s into the sun-earth L2 libration halo orbit
and execute infrared observations at wavelengths mainly between 5 and 200 micron. The SPICA mission gives us a
unique opportunity for coronagraph observations, because of the large telescope aperture, a simple pupil shape,
capability of infrared observations from space and the early launch. We have started development of the SPICA
coronagraph in which the primary target is direct observation of extra-solar Jovian planets. The main wavelengths of
observation, the required contrast and the inner working angle (IWA) of the SPICA coronagraph instrument are set to be
5-20 micron, 106, and approximately 5 λ/D respectively, whereλ is the observation wavelength and D is the diameter of
the telescope aperture. Coronagraphs using a checkerboard mask and a concentric ring mask have been investigated. We
found some solutions for the SPICA pupil, which has a large obstruction due to the secondary mirror and its supports.
We carried out laboratory experiments to examine coronagraphs obtained using checkerboard-type pupil masks with a
central obstruction. Nano-fabrication technology with electron beam was applied to manufacture a high precision mask
consisting of a patterned aluminum film on a glass substrate and its performance was confirmed by experiments with
visible light. Contrast higher than 106 was achieved. In the future, we will be developing a cryogenic mid-infrared
test-bed to investigate the SPICA coronagraphs.
Direct exploration of exoplanets is one of the most exciting topics in astronomy. Our current efforts in this field are concentrated on the Subaru 8.2m telescope at Mauna Kea, Hawaii. Making use of the good observing site and the excellent image quality, the infrared coronagraph CIAO (Coronagraphic Imager with Adaptive Optics) has been used for various kinds of surveys, which is the first dedicated cold coronagraph on the 8-10m class telescopes. However, its contrast is limited by the low-order adaptive optics and a limited suppression of the halo speckle noise.
HiCIAO is a new high-contrast instrument for the Subaru telescope. HiCIAO will be used in conjunction with the new adaptive optics system (188 actuators and/or its laser guide star - AO188/LGSAO188) at the Subaru infrared Nasmyth platform. It is designed as a flexible camera comprising several modules that can be configured into different modes of operation. The main modules are the AO module with its future extreme AO capability, the warm coronagraph module, and the cold infrared camera module. HiCIAO can combine coronagraphic techniques with either polarization or spectral simultaneous differential imaging modes. The basic concept of such differential imaging is to split up the image into two or more images, and then use either different planes of polarization or different spectral filter band-passes to produce a signal that distinguishes faint objects near a bright central object from scattered halo or residual speckles.
In this contribution, we will outline the HiCIAO instrument, its science, and performance simulations. The optical and mechanical details are described by Hodapp et al. (2006)1. We also present a roadmap of Japanese facilities and future plans, including ASTRO-F (AKARI), SPICA, and JTPF, for extrasolar planet explorations.
We present a method to remove the central obscuration and spiders, or any kind of geometry inside a telescope
pupil. The technique relies on the combination of a first focal plane diffracting mask, and a complex amplitude
pupil mask. In this combination, the central obscuration and eventual spider arms patterns in the re-imaged
pupil (after the diffracting mask) are filled with coherent light. Adding an appropriate complex amplitude pupil
mask allows virtually any kind of pupil shaping (in both amplitude and/or phase). We show that the obtained
output pupil can feed a high efficiency coronagraph (any kind) with a very reasonable overall throughput and
good performance even when considering pointing errors. In this paper, we specifically assess the performance
of this technique when using apodized entrance pupils. This technique is relevant for ground based telescopes
foreseeing the advent of higher order (so called ExAO) adaptive optics systems providing very high Strehl ratios.
Some feasibility points are also discussed. adaptive optics systems providing very high Strehl ratios. Some
feasibility points are also discussed.
MIRA-I.2 is a 30m-baseline two-aperture stellar interferometer working in the visible band (from 600 to 1000 nm).
In this article are presented the up-to-date progress and performance of MIRA-I.2 as well as some ongoing and future
plans. The fast and coarse delay lines are now both evacuated, and the maximum OPD (optical path delay)
compensations are about 16 m and 4 m long, respectively, for the fast and coarse delay lines. The current limiting
magnitude is about I=4.5mag, and stars within the declination range from +8 to +51 degree is possible to be observed
longer than one hour at the elevation angle of 60 degrees and higher. The OPD of the coarse delay line is modulated
by about 128 micrometers around the expected fringe center with the use of PZT, and 187 fringe packets are scanned
during one shot (= 60 seconds duration) to yield the mean visibility of about 10 % internal errors for each shot. The
thermal environment of the building that houses the delay lines and interference optics has been improved very much,
and readjustments of the optical alignment are not necessary for a whole night. The assembly and the setup of the optics
to be used for the fringe tracking experiment are nearly completed.
We have started demonstrating a technique for high dynamic range observations in the lab. This method, proposed by
Nishikawa et al., combines a nulling interferometer, a wavefront compensator, and a coronagraph. In the experiments,
two beams are generated by a beam splitter and they are combined by another beam splitter under an intensity-unbalanced
nulling condition. After the unbalanced nulling interferometer (UNI), normal wavefront sensor and two
deformable mirrors are applied for phase and amplitude correction (PAC). Wavefront errors of the two original beams,
large errors after the UNI, and compensated errors after the PAC by the deformable mirrors will be measured. After the
UNI-PAC method is applied, a downstream coronagraph optics will be set to see that the peak intensity of the central star
is dimmed and speckle noise level is also reduced relative to off-axis planet intensity. Possible applications of the
method are also discussed.
The light from an exoplanet is expected to be different from that of its parent star in regard to polarization and spectral features. Thus, polarization or spectral differential technique will be a powerful tool for direct detection of exoplanets. We propose a novel technique that enables to detect both polarization and spectral differential images. This technique will be used in combination with the other high-contrast imagers to enhance the performance.
This approach uses two Wollaston prisms (WPs) and a thick retarder (TR) inserted between the WPs. Two WPs generate four images and make it possible to obtain polarization differential images. Spectra of these four images have sinusoidal patterns (channeled spectra) due to the WP-TR-WP system. By inserting an interference filter, this system works as bandpass filters whose center wavelengths are different between the images. Therefore, both polarization and spectral differential images can be obtained. Furthermore, we propose to use two variable retarders (VRs) to remove differential aberrations caused by different optical paths of these four images. One VR is inserted in front of WP-TR-WP system to modulate the polarized light beams, and the other one is inserted between WPs to modulate the channeled spectra. By modulating the retardations of the VRs, it becomes possible to remove the differential aberrations.
We present the principle of this method. Currently, we are conducting the preliminary laboratory experiment. The results of the laboratory demonstrations will be also reported.
The light from exoplanets is expected to be partially polalized and the image intensity becomes different with the polarization direction. Based on this expectation we have reported the laboratory experiment of two-channel nulling stellar coronagraph for direct imaging of exoplanets, where a differential imaging with respect to mutually orthogonally polarized light is conducted. We show that this differential technique is also useful for obtaining objective spectra of exoplanets. Several experimental results on the differential objective spectrometer are reported.
A dual-channel nulling coronagraph to improve the detectability of
extrasolar planets is demonstrated. We have been developing a nulling
coronagraph with a four-quadrant polarization mask, and confirmed
its performance with monochromatic and polychromatic light sources.
However, the imperfections of the mask cause the leakage of the starlight, which is obstructive to the detection of faint companions. Here, we propose a two-channel nulling coronagraph, where s- and p-polarized components of the incident light are separated.
The light scattered and reflected from the atmosphere of an
extrasolar planet is expected to be partially polarized, while
the light from the parent star is usually unpolarized. Thus, the differential method, in which subtraction is taken between coronagraphic images of s- and p-polarized lights, is very useful for direct detection of extrasolar planets. In this approach, cancellation of the residual unpolarized starlight is realized when the coronagraphic performance of the two channels is identical.
We constructed the two-channel instrument. The experimental results confirm that the two-channel coronagraph can suppress the residual
stellar noise, and improve the detectability of faint companions.
The effects of the difference between two channels on the detectability are also discussed.
Nulling stellar coronagraph has been proposed to detect faint
objects very close to a bright point-like star, especially extra-solar planets. The principle of the nulling stellar coronagraph is to cause destructive interference for the light from a star. There have been proposed several methods for nulling interferometry. The key point of the nulling interferometry is the way to produce π-phase shift over wide range of wavelength. Here we propose a method for realizing achromatic π-phase shift utilizing polarization interference. The phase difference between two light beams that pass through different polarizers is π radians when these polarizers are placed between mutually orthogonal polarizer and analyzer. We adopt a ferroelectric liquid-crystal (FLC) device to convert the polarization direction of the incident beam. The FLC device is regarded as a birefringent device with retardation π, namely a half wave plate. The FLC device forms four-quadrant structure and is placed between the polarizer and the analyzer. By fixing the optic axes of the four-quadrant FLC suitably, it can rotate the incident linearly polarized light in parts by plus/minus 45°.
Geometric phase modulation is used to realize achromatic phase shift in nulling interferometer, where Fresnel rhombs are key components of the geometric phase modulator. Experimental results show high extinction ratio in visible region. Extension of our scheme to the infrared region is also discussed.
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