707 research outputs found
Apodized-pupil Lyot coronagraphs: multistage designs for extremely large telescopes
Earlier apodized-pupil Lyot coronagraphs (APLC) have been studied and
developed to enable high-contrast imaging for exoplanet detection and
characterization with present-day ground-based telescopes. With the current
interest in the development of the next generation of telescopes, the future
extremely large telescopes (ELTs), alternative APLC designs involving
multistage configuration appear attractive. The interest of these designs for
application to ELTs is studied. Performance and sensitivity of multistage APLC
to ELT specificities are analyzed and discussed, taking into account several
ineluctable coronagraphic telescope error sources by means of numerical
simulations. Additionally, a first laboratory experiment with a two-stages-APLC
in the near-infrared (H-band) is presented to further support the numerical
treatment. Multistage configurations are found to be inappropriate to ELTs. The
theoretical gain offered by a multistage design over the classical single-stage
APLC is largely compromised by the presence of inherent error sources occurring
in a coronagraphic telescope, and in particular in ELTs. The APLC remains an
attractive solution for ELTs, but rather in its conventional single-stage
configuration.Comment: A&A accepte
Detection and Characterization of Exoplanets and Disks using Projections on Karhunen-Loeve Eigenimages
We describe a new method to achieve point spread function (PSF) subtractions
for high- contrast imaging using Principal Component Analysis (PCA) that is
applicable to both point sources or extended objects (disks). Assuming a
library of reference PSFs, a Karhunen-Lo`eve transform of theses references is
used to create an orthogonal basis of eigenimages, on which the science target
is projected. For detection this approach provides comparable suppression to
the Locally Optimized Combination of Images (LOCI) algorithm, albeit with
increased robustness to the algorithm parameters and speed enhancement. For
characterization of detected sources the method enables forward modeling of
astrophysical sources. This alleviates the biases in the astrometry and
photometry of discovered faint sources, which are usually associated with LOCI-
based PSF subtractions schemes. We illustrate the algorithm performance using
archival Hubble Space Telescope (HST) images, but the approach may also be
considered for ground-based data acquired with Angular Differential Imaging
(ADI) or integral-field spectrographs (IFS).Comment: 12 pages, 4 figure
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
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
Shaped pupil design for the Gemini Planet Imager
The Gemini Planet Imager (GPI) is an instrument designed for the Gemini South
telescope to image young Jupiter-mass planets in the infrared. To achieve the
high contrast needed for this, it employs an apodized pupil Lyot coronagraph
(APLC) to remove most of the starlight. Current designs use a
partially-transmitting apodizer in the pupil; we examine the use of binary
apodizations in the form of starshaped shaped pupils, and present a design that
could achieve comparable performance, along with a series of design guidelines
for creating shaped pupil versions of APLCs in other systems.Comment: 20 pages, 7 figures, accepted for publication in Ap
Reference-less detection, astrometry, and photometry of faint companions with adaptive optics
We propose a complete framework for the detection, astrometry, and photometry
of faint companions from a sequence of adaptive optics corrected short
exposures. The algorithms exploit the difference in statistics between the
on-axis and off-axis intensity. Using moderate-Strehl ratio data obtained with
the natural guide star adaptive optics system on the Lick Observatory's 3-m
Shane Telescope, we compare these methods to the standard approach of PSF
fitting. We give detection limits for the Lick system, as well as a first guide
to expected accuracy of differential photometry and astrometry with the new
techniques. The proposed approach to detection offers a new way of determining
dynamic range, while the new algorithms for differential photometry and
astrometry yield accurate results for very faint and close-in companions where
PSF fitting fails. All three proposed algorithms are self-calibrating, i.e.
they do not require observation of a calibration star thus improving the
observing efficiency.Comment: Astrophysical Journal 698 (2009) 28-4
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
Optimization of Apodized Pupil Lyot Coronagraph for ELTs
We study the optimization of the Apodized Pupil Lyot Coronagraph (APLC) in
the context of exoplanet imaging with ground-based telescopes. The APLC
combines an apodization in the pupil plane with a small Lyot mask in the focal
plane of the instrument. It has been intensively studied in the literature from
a theoretical point of view, and prototypes are currently being manufactured
for several projects. This analysis is focused on the case of Extremely Large
Telescopes, but is also relevant for other telescope designs.
We define a criterion to optimize the APLC with respect to telescope
characteristics like central obscuration, pupil shape, low order segment
aberrations and reflectivity as function of the APLC apodizer function and mask
diameter. Specifically, the method was applied to two possible designs of the
future European-Extremely Large Telescope (E-ELT).
Optimum configurations of the APLC were derived for different telescope
characteristics. We show that the optimum configuration is a stronger function
of central obscuration size than of other telescope parameters. We also show
that APLC performance is quite insensitive to the central obscuration ratio
when the APLC is operated in its optimum configuration, and demonstrate that
APLC optimization based on throughput alone is not appropriate.Comment: 9 pages, 17 figures, accepted for publication in Astronomy &
Astrophysic
High resolution imaging with Fresnel interferometric arrays: suitability for exoplanet detection
We propose a new kind of interferometric array that yields images of high
dynamic range and large field. The numerous individual apertures in this array
form a pattern related to a Fresnel zone plate. This array can be used for
astrophysical imaging over a broad spectral bandwidth spanning from the U.V.
(50 nanometers) to the I.R. (20 microns). Due to the long focal lengths
involved, this instrument requires formation-flying of two space borne vessels.
We present the concept and study the S/N ratio in different situations, then
apply these results to probe the suitability of this concept to detect
exoplanets.Comment: 12 pages, 19 figures, to be published in A&
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