252 research outputs found
Simultaneous exoplanet detection and instrument aberration retrieval in multispectral coronagraphic imaging
High-contrast imaging for the detection and characterization of exoplanets
relies on the instrument's capability to block out the light of the host star.
Some current post-processing methods for calibrating out the residual speckles
use information redundancy offered by multispectral imaging but do not use any
prior information on the origin of these speckles. We investigate whether
additional information on the system and image formation process can be used to
more finely exploit the multispectral information. We developed an inversion
method in a Bayesian framework that is based on an analytical imaging model to
estimate both the speckles and the object map. The model links the instrumental
aberrations to the speckle pattern in the image focal plane, distinguishing
between aberrations upstream and downstream of the coronagraph. We propose and
validate several numerical techniques to handle the difficult minimization
problems of phase retrieval and achieve a contrast of 10^6 at 0.2 arcsec from
simulated images, in the presence of photon noise. This opens up the the
possibility of tests on real data where the ultimate performance may override
the current techniques if the instrument has good and stable coronagraphic
imaging quality. This paves the way for new astrophysical exploitations or even
new designs for future instruments
Exoplanets detection limits using spectral cross-correlation with spectro-imaging. An analytical model applied to the case of ELT-HARMONI
The combination of high-contrast imaging and medium to high spectral
resolution spectroscopy offers new possibilities for the detection and
characterization of exoplanets. The molecular mapping technique uses the
difference between the planetary and stellar spectra. While traditional
post-processing techniques are quickly limited by speckle noise at short
angular separation, it efficiently suppresses speckles. Its performance depends
on multiple parameters such as the star magnitude, the adaptive optics residual
halo, the companion spectrum, the telluric absorption, as well as the telescope
and instrument properties. Exploring this parameter space through end-to-end
simulations to predict potential science cases and to optimize future
instrument designs is very time-consuming, making it difficult to draw
conclusions. We propose to define an efficient methodology for such an
analysis. Explicit expressions of the estimates of signal and noise are
derived, and they are validated through comparisons with end-to-end
simulations. They provide an understanding of the instrumental dependencies,
and help to discuss optimal instrumental choices with regard to the targets of
interest. They are applied in the case of ELT/HARMONI, as a tool to predict the
contrast performance in various observational cases. We confirm the potential
of molecular mapping for high-contrast detections, especially for cool planets
at short separations. We provide guidelines based on quantified estimates for
design trade-offs of future instruments. We discuss the planet detection
performances of HARMONI observing modes. While they nicely cover the
appropriate requirements for high detection capability of warm exoplanets, a
transmission extended down to J band would be beneficial. A contrast of a few
1E-7 at 50mas should be within reach on bright targets in photon noise regime
with molecular mapping.Comment: 19 page
Frame Combination Techniques for Ultra High-Contrast Imaging
We summarize here an experimental frame combination pipeline we developed for
ultra high-contrast imaging with systems like the upcoming VLT SPHERE
instrument. The pipeline combines strategies from the Drizzle technique, the
Spitzer IRACproc package, and homegrown codes, to combine image sets that may
include a rotating field of view and arbitrary shifts between frames. The
pipeline is meant to be robust at dealing with data that may contain non-ideal
effects like sub-pixel pointing errors, missing data points, non-symmetrical
noise sources, arbitrary geometric distortions, and rapidly changing point
spread functions. We summarize in this document individual steps and
strategies, as well as results from preliminary tests and simulations.Comment: 9 pages, 4 figures, SPIE conference pape
Direct Detection of Exoplanets
Invited review at the "Protostars and Planets V" Conference, October 24â28, 2005, Hilton Waikoloa Village, Hawai'i. Proceedings of the PPV Conference, in press, B. Reipurth and D. Jewitt Eds.Direct detection of exoplanets from the ground is now within reach of existing astronomical instruments. Indeed, a few planet candidates have already been imaged and analyzed and the capability to detect (through imaging or interferometry) young, hot, Jupiter-mass planets exists. We present here an overview of what such detection methods can be expected to do in the near and far term. These methods will provide qualitatively new information about exoplanets, including spectroscopic data that will mature the study of exoplanets into a new field of comparative exoplanetary science. Spectroscopic study of exoplanet atmospheres promises to reveal aspects of atmospheric physics and chemistry as well as internal structure. Astrometric measurements will complete orbital element determinations partially known from the radial velocity surveys. We discuss the impact of these techniques, on three different time scales, corresponding to the currently available instruments, the new ``Planet Finder'' systems under development for 8 to 10-m telescopes, foreseen to be in operation in 5 to 10 years, and the more ambitious but more distant projects at the horizon of 2020
Apodized Lyot Coronagraph for VLT-SPHERE: Laboratory tests and performances of a first prototype in the visible
We present some of the High Dynamic Range Imaging activities developed around
the coronagraphic test-bench of the Laboratoire A. H. Fizeau (Nice). They
concern research and development of an Apodized Lyot Coronagraph (ALC) for the
VLT-SPHERE instrument and experimental results from our testbed working in the
visible domain. We determined by numerical simulations the specifications of
the apodizing filter and searched the best technological process to manufacture
it. We present the results of the experimental tests on the first apodizer
prototype in the visible and the resulting ALC nulling performances. The tests
concern particularly the apodizer characterization (average transmission radial
profile, global reflectivity and transmittivity in the visible), ALC nulling
performances compared with expectations, sensitivity of the ALC performances to
misalignments of its components
NICMOS Imaging of the HR 4796A Circumstellar Disk
We report the first near infrared (NIR) imaging of a circumstellar annular
disk around the young (~8 Myr), Vega-like star, HR 4796A. NICMOS coronagraph
observations at 1.1 and 1.6 microns reveal a ring-like symmetrical structure
peaking in reflected intensity 1.05 arcsec +/- 0.02 arcsec (~ 70 AU) from the
central A0V star. The ring geometry, with an inclination of 73.1 deg +/- 1.2
deg and a major axis PA of 26.8 deg +/- 0.6 deg, is in good agreement with
recent 12.5 and 20.8 micron observations of a truncated disk (Koerner, et al.
1998). The ring is resolved with a characteristic width of less than 0.26
arcsec (17 AU) and appears abruptly truncated at both the inner and outer
edges. The region of the disk-plane inward of ~60 AU appears to be relatively
free of scattering material. The integrated flux density of the part of the
disk that is visible (greater than 0.65 arcsec from the star) is found to be
7.5 +/- 0.5 mJy and 7.4 +/- 1.2 mJy at 1.1 and 1.6 microns, respectively.
Correcting for the unseen area of the ring yields total flux densities of 12.8
+/- 1.0 mJy and 12.5 +/- 2.0 mJy, respectively (Vega magnitudes = 12.92 /+-
0.08 and 12.35 +/-0.18). The NIR luminosity ratio is evaluated from these
results and ground-based photometry of the star. At these wavelengths
Ldisk(lambda)/L*(lambda) = 1.4 +/- 0.2E-3 and 2.4 +/- 0.5E-3, giving reasonable
agreement between the stellar flux scattered in the NIR and that which is
absorbed in the visible and re-radiated in the thermal infrared. The somewhat
red reflectance of the disk at these wavelengths implies mean particle sizes in
excess of several microns, larger than typical interstellar grains. The
confinement of material to a relatively narrow annular zone implies dynamical
constraints on the disk particles by one or more as yet unseen bodies.Comment: 14 pages, 1 figure for associated gif file see:
http://nicmosis.as.arizona.edu:8000/AAS99/FIGURE1_HR4796A_ApJL.gif . Accepted
13 January 1999, Astrophyical Journal Letter
BIGRE: a low cross-talk integral field unit tailored for extrasolar planets imaging spectroscopy
Integral field spectroscopy (IFS) represents a powerful technique for the
detection and characterization of extrasolar planets through high contrast
imaging, since it allows to obtain simultaneously a large number of
monochromatic images. These can be used to calibrate and then to reduce the
impact of speckles, once their chromatic dependence is taken into account. The
main concern in designing integral field spectrographs for high contrast
imaging is the impact of the diffraction effects and the non-common path
aberrations together with an efficient use of the detector pixels. We focus our
attention on integral field spectrographs based on lenslet-arrays, discussing
the main features of these designs: the conditions of appropriate spatial and
spectral sampling of the resulting spectrograph's slit functions and their
related cross-talk terms when the system works at the diffraction limit. We
present a new scheme for the integral field unit (IFU) based on a dual-lenslet
device (BIGRE), that solves some of the problems related to the classical TIGER
design when used for such applications. We show that BIGRE provides much lower
cross-talk signals than TIGER, allowing a more efficient use of the detector
pixels and a considerable saving of the overall cost of a lenslet-based
integral field spectrograph.Comment: 17 pages, 18 figures, accepted for publication in Ap
Deep imaging of ÎČ Pictoris at Lâ: asymmetries in the disc and constraints on planets
We demonstrate here the potential of VLT/NaCo at
Lâ (3.8ÎŒm) in Angular Differential Imaging (ADI). We detect
the ÎČ Pictoris disc above 5Ï between 0.4ââ and 3.8ââ, after
combining data from 7 epochs or 3.1 years. To avoid the
smearing of the planet due to its orbital motion within this
time span (green arrow), we subtracted the planet from the
7 datasets. We used a star subtraction technique based on
PCA (Soummer et al. 2012) and corrected for ADI biases by
iterating. We implemented a forward modeling approach to
constrain the dust distribution
SPHERE IRDIS and IFS astrometric strategy and calibration
We present the current results of the astrometric characterization of the VLT
planet finder SPHERE over 2 years of on-sky operations. We first describe the
criteria for the selection of the astrometric fields used for calibrating the
science data: binaries, multiple systems, and stellar clusters. The analysis
includes measurements of the pixel scale and the position angle with respect to
the North for both near-infrared subsystems, the camera IRDIS and the integral
field spectrometer IFS, as well as the distortion for the IRDIS camera. The
IRDIS distortion is shown to be dominated by an anamorphism of 0.60+/-0.02%
between the horizontal and vertical directions of the detector, i.e. 6 mas at
1". The anamorphism is produced by the cylindrical mirrors in the common path
structure hence common to all three SPHERE science subsystems (IRDIS, IFS, and
ZIMPOL), except for the relative orientation of their field of view. The
current estimates of the pixel scale and North angle for IRDIS are
12.255+/-0.009 milliarcseconds/pixel for H2 coronagraphic images and
-1.75+/-0.08 deg. Analyses of the IFS data indicate a pixel scale of
7.46+/-0.02 milliarcseconds/pixel and a North angle of -102.18+/-0.13 deg. We
finally discuss plans for providing astrometric calibration to the SPHERE users
outside the instrument consortium.Comment: 12 pages, 6 figures, 3 table
Review of small-angle coronagraphic techniques in the wake of ground-based second-generation adaptive optics systems
Small-angle coronagraphy is technically and scientifically appealing because
it enables the use of smaller telescopes, allows covering wider wavelength
ranges, and potentially increases the yield and completeness of circumstellar
environment - exoplanets and disks - detection and characterization campaigns.
However, opening up this new parameter space is challenging. Here we will
review the four posts of high contrast imaging and their intricate interactions
at very small angles (within the first 4 resolution elements from the star).
The four posts are: choice of coronagraph, optimized wavefront control,
observing strategy, and post-processing methods. After detailing each of the
four foundations, we will present the lessons learned from the 10+ years of
operations of zeroth and first-generation adaptive optics systems. We will then
tentatively show how informative the current integration of second-generation
adaptive optics system is, and which lessons can already be drawn from this
fresh experience. Then, we will review the current state of the art, by
presenting world record contrasts obtained in the framework of technological
demonstrations for space-based exoplanet imaging and characterization mission
concepts. Finally, we will conclude by emphasizing the importance of the
cross-breeding between techniques developed for both ground-based and
space-based projects, which is relevant for future high contrast imaging
instruments and facilities in space or on the ground.Comment: 21 pages, 7 figure
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