1,342 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
Apodization in high-contrast long-slit spectroscopy. Closer, deeper, fainter, cooler
The spectroscopy of faint planetary-mass companions to nearby stars is one of
the main challenges that new-generation high-contrast spectro-imagers are going
to face. In a previous work we presented a long slit coronagraph (LSC), for
which the presence of a slit in the coronagraphic focal plane induces a complex
distribution of energy in the Lyot pupil-plane that cannot be easily masked
with a binary Lyot stop. To alleviate this concern, we propose to use a pupil
apodization to suppress diffraction, creating an apodized long slit coronagraph
(ALSC). After describing how the apodization is optimized, we demonstrate its
advantages with respect to the CLC in the context of SPHERE/IRDIS long slit
spectroscopy (LSS) mode at low-resolution with a 0.12" slit and 0.18"
coronagraphic mask. We perform different sets of simulations with and without
aberrations, and with and without a slit to demonstrate that the apodization is
a more appropriate concept for LSS, at the expense of a significantly reduced
throughput (37%) compared to the LSC. Then we perform detailed end-to-end
simulations of the LSC and the ALSC that include realistic levels of
aberrations to obtain datasets representing 1h of integration time on stars of
spectral types A0 to M0 located at 10 pc. We insert spectra of planetary
companions at different effective temperatures (Teff) and surface gravities
(log g) into the data at angular separations of 0.3" to 1.5" and with contrast
ratios from 6 to 18 mag. Using the SD method to subtract the speckles, we show
that the ALSC brings a gain in sensitivity of up to 3 mag at 0.3" with respect
to the LSC, which leads to a much better spectral extraction below 0.5". In
terms of Teff, we demonstrate that at small angular separations the limit with
the ALSC is always lower by at least 100K, inducing an increase of sensitivity
of a factor up to 1.8 in objects' masses at young ages. [Abridged]Comment: 15 pages, 17 figures. Accepted for publication in A&
Solid–liquid transport in a modified co-rotating twin-screw extruder-dynamic simulator and experimental validations
This work presents a dynamic transport model of a solid–liquid media through a twin-screw extruder (TSE). The application under consideration is the solid–liquid extraction of solute from raw plant substrate. Dynamic experiments are performed and compared with the simulated results for step functions on the solid feed rate and on the screw rotating speed. Despite some imperfections, results allow to validate the simulator
Master of Science
thesisInternational community's commitment to achieve universal primary education triggered an increase in primary school enrollment in the developing world. Unfortunately, this increase in quantity (the number of students) led to the emergence of very big class sizes, multigrade, and double-shift schooling. Accordingly, this study fills a hole in the literature by investigating the impact of multigrade classrooms and doubleshifts on the dropout rate in Mali. Simultaneously, it also examines the impact of not completing the curriculum required on student test scores. Some surprising and interesting results show that schools that have libraries are less expected to have dropouts compared to schools that do not. I also found strong evidence that schools that do not collect fees on a consistent basis are likely to have higher dropout rates compared to schools that do
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
User Applications Driven by the Community Contribution Framework MPContribs in the Materials Project
This work discusses how the MPContribs framework in the Materials Project
(MP) allows user-contributed data to be shown and analyzed alongside the core
MP database. The Materials Project is a searchable database of electronic
structure properties of over 65,000 bulk solid materials that is accessible
through a web-based science-gateway. We describe the motivation for enabling
user contributions to the materials data and present the framework's features
and challenges in the context of two real applications. These use-cases
illustrate how scientific collaborations can build applications with their own
"user-contributed" data using MPContribs. The Nanoporous Materials Explorer
application provides a unique search interface to a novel dataset of hundreds
of thousands of materials, each with tables of user-contributed values related
to material adsorption and density at varying temperature and pressure. The
Unified Theoretical and Experimental x-ray Spectroscopy application discusses a
full workflow for the association, dissemination and combined analyses of
experimental data from the Advanced Light Source with MP's theoretical core
data, using MPContribs tools for data formatting, management and exploration.
The capabilities being developed for these collaborations are serving as the
model for how new materials data can be incorporated into the Materials Project
website with minimal staff overhead while giving powerful tools for data search
and display to the user community.Comment: 12 pages, 5 figures, Proceedings of 10th Gateway Computing
Environments Workshop (2015), to be published in "Concurrency in Computation:
Practice and Experience
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