Abstract
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
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D
in horizontal and 5–15
λ
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D
in vertical directions), (2) in an outer region (70–75
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D
in horizontal and 65–75
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D
in vertical directions), and (3) in a large region (5–75
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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
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/
D
) and with a large size (70
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/
D
square), both of which are out of the Nyquist limit of currently available deformable mirrors.
A vector vortex coronagraph (VVC) is one of promising means for imaging extremely faint objects around bright stars such as exoplanets. We present a design of an achromatic VVC, in which an ...axially-symmetric half-wave plate (AHP) is placed between crossed polarization filters (circular polarizer and analyzer). The circular polarizer and the analyzer are both composed of a polarizer and a quarter-wave plate (QWP). We demonstrate, via Jones calculus and Fourier analysis, that the achromatic stellar elimination can theoretically be realized by optimal polarization filters, even when chromatic AHP and QWPs are used. We carried out laboratory demonstrations of the designed VVC using a photonic-crystal AHP. As a result, we observed achromatic coronagraphic performance, a light suppression level of 7 × 10(-5), over a wavelength from 543 nm to 633 nm.
A solar SCIDAR (scintillation detection and ranging) technique is proposed for measuring the heights of turbulent layers using the Sun itself, instead of the binary stars used in night-time SCIDAR. A ...formula for the technique is derived under various assumptions: uniform intensity distributions on the solar surface and sparse speckle distributions on the image plane. It indicates that the cross-correlation of scintillation shadows yields peaks at positions corresponding to layer heights, although the shapes of peaks are blurred by both an extended seeing disc and a finite-sized field stop. A knife-edge effect caused by field stops in an observational system is also described, which yields another peak at the centre of the correlation plane. Observations were conducted using a solar SCIDAR system developed at the Hida Observatory in Japan. In many results, peaks on correlation planes demonstrated a contrast high enough for them to be distinguished from the background. Most of the distances to turbulent layers derived from the scintillation peaks were found to be between 2.5 and 3.5 km. Use of a high-performance adaptive-optics system upstream of the SCIDAR system is suggested in order to provide better results.
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 theVoyager 2spacecraft reveals that the restored image shows well-resolved structural features on the surface of Io.
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.
Shift-and-add (SAA) is a simple method for celestial speckle imaging. However, the raw SAA-image, a direct output of the SAA operations, is not useful, because a seeing-dependent huge background is ...superimposed on the high-resolution image of the object. To obtain the latter, a background subtraction (BGS) is applied on the raw SAA-image. The BGS-image so obtained includes negativities due to an over-subtraction that causes brightnesses of the object’s image biased downward. The negativity in the BGS-image can be removed by a deconvolution with a point-spread-function (PSF) that has negative values. In this paper, we model the PSF with a shape that possesses peak and concave portions, and perform a deconvolution by iteratively estimating the object’s image and the model parameters. The simulated experiment has shown that the present algorithm can restore the object’s image with unbiased brightnesses. Processing the observed speckle data of Io (a Jupiter satellite) by the present method has yielded a feasible Io image with reduced negativities.
A bispectral method for astronomical speckle imaging utilizes an average speckle bispectrum of an object to derive its Fourier phase. There has been, however, a problem in conventional bispectral ...algorithm owing to difficulty in processing bispectral data in a four-dimensional (4D) space. In this paper, we propose an implementation to overcome this problem, where a one-dimensional (1D) object projection is reconstructed from a two-dimensional (2D) average bispectrum of speckle projections, and object projections so obtained at various angles are then tomographically combined into a 2D object image. In this tomographic approach, processes are separable into those for individual projection angles, implying that bispectral data required to be stored at a time are from 4D to 2D and computation time can be substantially reduced by parallelizing angle-by-angle processes. We have performed experiments using simulated and observed data, and have demonstrated the feasibility of the present approach with an achievable accuracy comparable to that of a conventional approach.
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