174 research outputs found
Apodized Pupil Lyot Coronagraphs for Arbitrary Apertures. IV. Reduced Inner Working Angle and Increased Robustness to Low-Order Aberrations
The Apodized Pupil Lyot Coronagraph (APLC) is a diffraction suppression
system installed in the recently deployed instruments Palomar/P1640,
Gemini/GPI, and VLT/SPHERE to allow direct imaging and spectroscopy of
circumstellar environments. Using a prolate apodization, the current
implementations offer raw contrasts down to at 0.2 arcsec from a star
over a wide bandpass (20\%), in the presence of central obstruction and struts,
enabling the study of young or massive gaseous planets. Observations of older
or lighter companions at smaller separations would require improvements in
terms of inner working angle (IWA) and contrast, but the methods originally
used for these designs were not able to fully explore the parameter space. We
here propose a novel approach to improve the APLC performance. Our method
relies on the linear properties of the coronagraphic electric field with the
apodization at any wavelength to develop numerical solutions producing
coronagraphic star images with high-contrast region in broadband light. We
explore the parameter space by considering different aperture geometries,
contrast levels, dark-zone sizes, bandpasses, and focal plane mask sizes. We
present an application of these solutions to the case of Gemini/GPI with a
design delivering a raw contrast at 0.19 arcsec and offering a
significantly reduced sensitivity to low-order aberrations compared to the
current implementation. Optimal solutions have also been found to reach
contrast in broadband light regardless of the telescope aperture
shape (in particular the central obstruction size), with effective IWA in the
range, therefore making the APLC a suitable option for the
future exoplanet direct imagers on the ground or in space.Comment: 14 pages, 10 figures, accepted in Ap
Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor
Context. Several exoplanet direct imaging instruments will soon be in
operation. They use an extreme adaptive optics (XAO) system to correct the
atmospheric turbulence and provide a highly-corrected beam to a near-infrared
(NIR) coronagraph for starlight suppression. The performance of the coronagraph
is however limited by the non-common path aberrations (NCPA) due to the
differential wavefront errors existing between the visible XAO sensing path and
the NIR science path, leading to residual speckles in the coronagraphic image.
Aims. Several approaches have been developed in the past few years to
accurately calibrate the NCPA, correct the quasi-static speckles and allow the
observation of exoplanets at least 1e6 fainter than their host star. We propose
an approach based on the Zernike phase-contrast method for the measurements of
the NCPA between the optical path seen by the visible XAO wavefront sensor and
that seen by the near-IR coronagraph. Methods. This approach uses a focal plane
phase mask of size {\lambda}/D, where {\lambda} and D denote the wavelength and
the telescope aperture diameter, respectively, to measure the quasi-static
aberrations in the upstream pupil plane by encoding them into intensity
variations in the downstream pupil image. We develop a rigorous formalism,
leading to highly accurate measurement of the NCPA, in a quasi-linear way
during the observation. Results. For a static phase map of standard deviation
44 nm rms at {\lambda} = 1.625 {\mu}m (0.026 {\lambda}), we estimate a possible
reduction of the chromatic NCPA by a factor ranging from 3 to 10 in the
presence of AO residuals compared with the expected performance of a typical
current-generation system. This would allow a reduction of the level of
quasi-static speckles in the detected images by a factor 10 to 100 hence,
correspondingly improving the capacity to observe exoplanets.Comment: 11 pages, 14 figures, A&A accepted, 2nd version after language-editor
correction
k-nearest neighbors prediction and classification for spatial data
We propose a nonparametric predictor and a supervised classification based on
the regression function estimate of a spatial real variable using k-nearest
neighbors method (k-NN). Under some assumptions, we establish almost complete
or sure convergence of the proposed estimates which incorporate a spatial
proximity between observations. Numerical results on simulated and real fish
data illustrate the behavior of the given predictor and classification method
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