134 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
New Completeness Methods for Estimating Exoplanet Discoveries by Direct Detection
We report new methods for evaluating realistic observing programs that search
stars for planets by direct imaging, where observations are selected from an
optimized star list, and where stars can be observed multiple times. We show
how these methods bring critical insight into the design of the mission & its
instruments. These methods provide an estimate of the outcome of the observing
program: the probability distribution of discoveries (detection and/or
characterization), & an estimate of the occurrence rate of planets (eta). We
show that these parameters can be accurately estimated from a single mission
simulation, without the need for a complete Monte Carlo mission simulation, &
we prove the accuracy of this new approach. Our methods provide the tools to
define a mission for a particular science goal, for example defined by the
expected number of discoveries and its confidence level. We detail how an
optimized star list can be built & how successive observations can be selected.
Our approach also provides other critical mission attributes, such as the
number of stars expected to be searched, & the probability of zero discoveries.
Because these attributes depend strongly on the mission scale, our methods are
directly applicable to the design of such future missions & provide guidance to
the mission & instrument design based on scientific performance. We illustrate
our new methods with practical calculations & exploratory design reference
missions for JWST operating with a distant starshade to reduce scattered and
diffracted starlight on the focal plane. We estimate that 5 habitable
Earth-mass planets would be discovered & characterized with spectroscopy, with
a probability of 0 discoveries of 0.004, assuming a small fraction of JWST
observing time (7%), eta=0.3, and 70 observing visits, limited by starshade
fuel.Comment: 27 pages, 4 figures, 6 tables, accepted for publication by Ap
The Strehl Ratio in Adaptive Optics Images: Statistics and Estimation
Statistical properties of the intensity in adaptive optics images are usually
modeled with a Rician distribution. We study the central point of the image,
where this model is inappropriate for high to very high correction levels. The
central point is an important problem because it gives the Strehl ratio
distribution. We show that the central point distribution can be modeled using
a non-central Gamma distribution.Comment: 8 pages, 5 figure
Apodized pupil Lyot coronagraphs for arbitrary apertures. V. Hybrid Shaped Pupil designs for imaging Earth-like planets with future space observatories
We introduce a new class of solutions for Apodized Pupil Lyot Coronagraphs
(APLC) with segmented aperture telescopes to remove broadband diffracted light
from a star with a contrast level of . These new coronagraphs provide
a key advance to enabling direct imaging and spectroscopy of Earth twins with
future large space missions. Building on shaped pupil (SP) apodization
optimizations, our approach enables two-dimensional optimizations of the system
to address any aperture features such as central obstruction, support
structures or segment gaps. We illustrate the technique with a design that
could reach contrast level at 34\,mas for a 12\,m segmented telescope
over a 10\% bandpass centered at a wavelength 500\,nm. These
designs can be optimized specifically for the presence of a resolved star, and
in our example, for stellar angular size up to 1.1\,mas. This would allow
probing the vicinity of Sun-like stars located beyond 4.4\,pc, therefore fully
retiring this concern. If the fraction of stars with Earth-like planets is
\eta_{\Earth}=0.1, with 18\% throughput, assuming a perfect, stable wavefront
and considering photon noise only, 12.5 exo-Earth candidates could be detected
around nearby stars with this design and a 12\,m space telescope during a
five-year mission with two years dedicated to exo-Earth detection (one total
year of exposure time and another year of overheads). Our new hybrid APLC/SP
solutions represent the first numerical solution of a coronagraph based on
existing mask technologies and compatible with segmented apertures, and that
can provide contrast compatible with detecting and studying Earth-like planets
around nearby stars. They represent an important step forward towards enabling
these science goals with future large space missions.Comment: 9 pages, 6 figures, ApJ accepted on 01/04/201
ALICE Data Release: A Revaluation of HST-NICMOS Coronagraphic Images
The Hubble Space Telescope NICMOS instrument was used from 1997 to 2008 to perform coronagraphic observations of about 400 targets. Most of them were part of surveys looking for substellar companions or resolved circumstellar disks to young nearby stars, making the NICMOS coronagraphic archive a valuable database for exoplanets and disks studies. As part of the Archival Legacy Investigations of Circumstellar Environments program, we have consistently reprocessed a large fraction of the NICMOS coronagrahic archive using advanced starlight subtraction methods. We present here the high-level science products of these re-analyzed data, which we delivered back to the community through the Mikulski Archive for Space Telescopes: doi:10.17909/T9W89V. We also present the second version of the HCI-FITS format (for High-Contrast Imaging FITS format), which we developed as a standard format for data exchange of imaging reduced science products. These re-analyzed products are openly available for population statistics studies, characterization of specific targets, or detected point-source identification
Apodized Pupil Lyot Coronagraphs for Arbitrary Telescope Apertures
In the context of high dynamic range imaging, this study presents a
breakthrough for the understanding of Apodized Pupil Lyot Coronagraphs, making
them available for arbitrary aperture shapes. These new solutions find
immediate application in current, ground-based coronagraphic studies (Gemini,
VLT) and in existing instruments (AEOS Lyot Project). They also offer the
possiblity of a search for an on-axis design for TPF. The unobstructed aperture
case has already been solved by Aime et al. (2002) and Soummer et al. (2003).
Analytical solutions with identical properties exist in the general case and,
in particular, for centrally obscured apertures. Chromatic effects can be
mitigated with a numerical optimization. The combination of analytical and
numerical solutions enables the study of the complete parameter space (central
obstruction, apodization throughput, mask size, bandwidth, and Lyot stop size).Comment: 7 pages 4 figures - ApJL, accepte
Active compensation of aperture discontinuities for WFIRST-AFTA: analytical and numerical comparison of propagation methods and preliminary results with a WFIRST-AFTA-like pupil
The new frontier in the quest for the highest contrast levels in the focal
plane of a coronagraph is now the correction of the large diffractive artifacts
effects introduced at the science camera by apertures of increasing complexity.
The coronagraph for the WFIRST/AFTA mission will be the first of such
instruments in space with a two Deformable Mirrors wavefront control system.
Regardless of the control algorithm for these multi Deformable Mirrors, they
will have to rely on quick and accurate simulation of the propagation effects
introduced by the out-of-pupil surface. In the first part of this paper, we
present the analytical description of the different approximations to simulate
these propagation effects. In Annex A, we prove analytically that, in the
special case of surfaces inducing a converging beam, the Fresnel method yields
high fidelity for simulations of these effects. We provide numerical
simulations showing this effect. In the second part, we use these tools in the
framework of the Active Compensation of Aperture Discontinuities technique
(ACAD) applied to pupil geometries similar to WFIRST-AFTA. We present these
simulations in the context of the optical layout of the High-contrast imager
for Complex Aperture Telescopes, which will test ACAD on a optical bench. The
results of this analysis show that using the ACAD method, an apodized pupil
lyot coronagraph and the performance of our current deformable mirrors, we are
able to obtain, in numerically simulations, a dark hole with an AFTA-like
pupil. Our numerical simulation shows that we can obtain contrast better than
in monochromatic light and better than 3.e-8 with 10% bandwidth
between 5 and 14 lambda/D.Comment: 16 pages, 5 figures, Accepted for publication (Oct. 23, 2015) in
Journal of Astronomical Telescopes, Instruments, and Systems, special
WFIRST-AFTA coronagrap
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