Naoshi Baba

The Subaru Coronagraphic Extreme Adaptive Optics System: Enabling High-Contrast Imaging on Solar-System Scales

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 Subarus 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.

An Eight-Octant Phase-Mask Coronagraph

We present numerical simulations and laboratory experiments on an eight-octant phase-mask (EOPM) coronagraph. The numerical simulations suggest that an achievable contrast for the EOPM coronagraph can be greatly improved as compared to that of a four-quadrant phase-mask (FQPM) coronagraph for a partially resolved star. On-sky transmission maps reveal that the EOPM coronagraph has relatively high optical throughput, a small inner working angle, and large discovery space. We have manufactured an eight-segment phase mask utilizing a nematic liquid-crystal device, which can be easily switched between the FQPM and the EOPM modes. The la- boratory experiments demonstrate that the EOPM coronagraph has a better tolerance of the tip-tilt error than the FQPM one. We also discuss feasibility of a fully achromatic and high-throughput EOPM coronagraph utilizing a polarization interferometric technique.

Combination of nulling interferometer and modified pupil for observations of exo-planets

We consider a combination of two techniques, namely the nulling interferometer and halo suppression by modified pupil such as shaped pupils and apodized square apertures, which achieves very high dynamic range, in order to detect faint extra-solar planets around nearby stars. The effects of the nulling interferometer and the modified pupil are independent and in combination hence multiplied. We show that one can achieve higher dynamic range in the case of a resolved star than with either method alone. By numerical simulations, we show that the combination method can achieve dynamic range levels of 10 −10 at 3λ/D. Used alone, the two-telescope interferometer would give the nulling depth of 10 −3 whilst the halo suppression by a shaped aperture would do 10 −7 , with a point-spread-function core radius less than 3λ/D for a shaped aperture. The introduction of the modified pupil has the same effect whether it is made at the entrance apertures of the interferometer or at a re-imaged common pupil plane after the nulling interferometry. From another point of view, a nulling interferometer works as pre-optics in front of any single telescope methods, which reduces the intensity of a resolved source transmitting some uniform wavefront residuals.

Pupil‐remapping Mirrors for a Four‐Quadrant Phase Mask Coronagraph

A coronagraph with a four-quadrant phase mask (FQPM) can perfectly eliminate stellar light under ideal conditions. However, the coronagraphic performance is severely degraded in a telescope with a central obscuration. We show that a high-performance FQPM coronagraph can be realized with a centrally obscured telescope by using a two-mirror pupil-remapping system. We calculate the optimal shapes of the remapping mirrors to transform a centrally obscured pupil into clear circular one. The mirror shapes become parabola-like, and the remapping mirrors are shown to provide good coronagraphic performance. The expected performance with adaptive optics systems is also evaluated.

Polarization degree analysis of objective spectrum in polarization differential stellar coronagraph

It is shown that the degree of polarization analysis is useful to find objective spectra of exoplanets immersed in noisy stellar spectra. We report the laboratory experiment of polarization differential objective spectroscopy with a four-quadrant polarization mask coronagraph, where partially polarized planetary signal is expected to be discerned from unpolarized stellar noise. The detection of the planet signal is impeded by the stellar noise remained after subtracting mutually orthogonally polarized components of light. We distinguish clearly the planetary spectrum by use of the degree of polarization. We also show the refinement of the spectrum of the planet model.

Solar adaptive optics system at the Hida Observatory

A solar adaptive optics system is developed for the 60 cm domeless solar telescope of the Hida Observatory in Japan. It is designed for compensating low order turbulence in G-band using a 52-electromagnetic-actuator deformable mirror, a 6x6 Shack-Hartmann wavefront sensor and standard personal computers. The details of the system, particularly features of the deformable mirror are described. Laboratory experiments show that the use of adaptive optics raises the Strehl ratio by a factor of five for turbulence of under 99Hz. In solar observations, the improvement of resolution in long-exposure images with the adaptive optics system is demonstrated.

Four-Quadrant Phase Mask Coronagraph with a Jacquinot-Lyot Stop

We report numerical simulations and laboratory demonstrations of a four-quadrant phase mask (FQPM) coronagraph equipped with a Jacquinot pupil as a Lyot stop. We demonstrate that the Jacquinot-Lyot stop can effectively suppress the residual stellar intensity due to tip-tilt errors for unresolved stars. We also show that the achievable contrast with the Jacquinot-Lyot stop depends on the direction of the tip-tilt errors, which suggests that the contrast can be improved if the direction of these errors is monitored simultaneously with the data acquisition. Furthermore, we conduct laboratory experiments with a polarization differential imager in which the Jacquinot-Lyot stop is introduced into a four-quadrant polarization mask (FQPoM) coronagraph. In these experiments we also demonstrate two image processing techniques, a cross-correlation technique and a polarization-degree analysis, to extract planetary signal from residual stellar speckle noise.

Polarization Differential Objective Spectroscopy with a Nulling Coronagraph

ABSTRACT We report the laboratory demonstration of polarization differential objective spectroscopy using a four‐quadrant polarization mask coronagraph. The optical setup consists of a liquid crystal polarization modulator, coronagraphic optics, and an objective spectrometer. The experimental results show the detection of not only images but also objective spectra of faint companions. We also show that the proposed technique can be used as a high‐contrast spectropolarimeter. It is possible to investigate the atmospheric compositions of exoplanets more clearly by using the proposed method.

Deconvolution of partially compensated solar images from additional wavefront sensing

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.

Common-Path Spectropolarimetric Differential Imager Using Variable Channeled Spectrum

We propose a method for a differential imager that makes use of a variable channeled spectrum. The channeled spectrum is an oscillating spectrum with respect to wavelength, generated by two polarizers with a retarder between them. A variable retarder, inserted between these polarizers, modulates the channeled spectrum. By restricting a spectral band with a bandpass filter, the whole optical system acts as a common-path variable bandpass filter with two specific central wavelengths. Provided that the temporal modulation of the channeled spectrum is fast enough compared to that of the evolution of phase aberrations, it is possible to obtain spectral differential images free from non-common-path aberrations. Furthermore, polarization differential images are also available by inserting another variable retarder in front of this optical system. Thus, both common-path spectral and polarization differential images can be obtained. We propose using a liquid-crystal variable retarder, which has been successfully demonstrated in a polarization differential imager equipped with a four-quadrant polarization mask coronagraph. In order to achieve a higher system throughput, we suggest using Wollaston prisms instead of polarizers. In this paper, we describe the principle of this technique. Results of a preliminary laboratory experiment and a numerical simulation of the spectral differential imager are also given.