82 research outputs found
The Bi-O-edge wavefront sensor: How Foucault-knife-edge variants can boost eXtreme Adaptive Optics
Direct detection of exoplanets around nearby stars requires advanced Adaptive
Optics (AO) systems. High order systems are needed to reach high Strehl Ratio
(SR) in near infrared and optical wavelengths on future Giant Segmented Mirror
Telescopes (GSMTs). Direct detection of faint exoplanets with the ESO ELT will
require some tens of thousand of correction modes. Resolution and sensitivity
of the wavefront sensor (WFS) are key requirements for this science case. We
present a new class of WFSs, the Bi-Orthogonal Foucault-knife-edge Sensors (or
Bi-O-edge), that is directly inspired by the Foucault knife edge test (Foucault
1859). The idea consists of using a beam-splitter producing two foci, each of
which is sensed by an edge with orthogonal direction to the other. We describe
two implementation concepts: The Bi-O-edge sensor can be realised with a sharp
edge and a tip-tilt modulation device (sharp Bi-O-edge) or with a smooth
gradual transmission over a grey edge (grey Bi-O-edge). A comparison between
the Bi-O-edge concepts and the 4-sided classical Pyramid Wavefront Sensor (PWS)
gives some important insights into the nature of the measurements.Our analysis
shows that the sensitivity gain of the Bi-O edge with respect to the PWS
depends on the system configuration. The gain is a function of the number of
control modes and the modulation angle. We found that for the sharp Bi-O-edge,
the gain in reduction of propagated photon noise variance approaches a
theoretical factor of 2 for a large number of control modes and small
modulation angle, meaning that the sharp Bi-O-edge only needs half of the
photons of the PWS to reach similar measurement accuracy.Comment: Accepted for publication in Astronomy and Astrophysic
Science with EPICS, the E-ELT planet finder
EPICS is the proposed planet finder for the European Extremely Large Telescope. EPICS is a high contrast imager based on a high performing extreme adaptive optics system, a diffraction suppression module, and two scientific instruments: an Integral Field Spectrograph (IFS) for the near infrared (0.95-1.65 μm), and a differential polarization imager (E-POL). Both these instruments should allow imaging and characterization of planets shining in reflected light, possibly down to Earth-size. A few high interesting science cases are presente
Comparison of coronagraphs for high contrast imaging in the context of Extremely Large Telescopes
We compare coronagraph concepts and investigate their behavior and
suitability for planet finder projects with Extremely Large Telescopes (ELTs,
30-42 meters class telescopes). For this task, we analyze the impact of major
error sources that occur in a coronagraphic telescope (central obscuration,
secondary support, low-order segment aberrations, segment reflectivity
variations, pointing errors) for phase, amplitude and interferometric type
coronagraphs. This analysis is performed at two different levels of the
detection process: under residual phase left uncorrected by an eXtreme Adaptive
Optics system (XAO) for a large range of Strehl ratio and after a general and
simple model of speckle calibration, assuming common phase aberrations between
the XAO and the coronagraph (static phase aberrations of the instrument) and
non-common phase aberrations downstream of the coronagraph (differential
aberrations provided by the calibration unit). We derive critical parameters
that each concept will have to cope with by order of importance. We evidence
three coronagraph categories as function of the accessible angular separation
and proposed optimal one in each case. Most of the time amplitude concepts
appear more favorable and specifically, the Apodized Pupil Lyot Coronagraph
gathers the adequate characteristics to be a baseline design for ELTs.Comment: 12 pages, 6 figures, Accepted for publication in A&
Tomographic reconstruction for Wide Field Adaptive Optics systems: Fourier domain analysis and fundamental limitations
Several Wide Field of view Adaptive Optics (WFAO) concepts like
Multi-Conjugate AO (MCAO), Multi-Object AO (MOAO) or Ground-Layer AO (GLAO) are
currently studied for the next generation of Extremely Large Telescopes (ELTs).
All these concepts will use atmospheric tomography to reconstruct the turbulent
phase volume. In this paper, we explore different reconstruction algorithms and
their fundamental limitations. We conduct this analysis in the Fourier domain.
This approach allows us to derive simple analytical formulations for the
different configurations, and brings a comprehensive view of WFAO limitations.
We then investigate model and statistical errors and their impact on the phase
reconstruction. Finally, we show some examples of different WFAO systems and
their expected performance on a 42m telescope case.Comment: 40 pages, 23 figures, accepted for publication in JOSA-
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
FFREE: a Fresnel-FRee Experiment for EPICS, the EELT planets imager
The purpose of FFREE - the new optical bench devoted to experiments on
high-contrast imaging at LAOG - consists in the validation of algorithms based
on off-line calibration techniques and adaptive optics (AO) respectively for
the wavefront measurement and its compensation. The aim is the rejection of the
static speckles pattern arising in a focal plane after a diffraction
suppression system (based on apodization or coronagraphy) by wavefront
pre-compensation. To this aim, FFREE has been optimized to minimize Fresnel
propagation over a large near infrared (NIR) bandwidth in a way allowing
efficient rejection up to the AO control radius, it stands then as a
demonstrator for the future implementation of the optics that will be common to
the scientific instrumentation installed on EPICS.Comment: 12 pages, 15 figures, Proceeding 7736120 of the SPIE Conference
"Adaptive Optics Systems II", monday 28 June 2010, San Diego, California, US
Accounting for anisoplanatic point spread function in deep wide-field adaptive optics images
In this paper we present the approach we have used to determine and account
for the anisoplanatic point spread function (PSF) in deep adaptive optics (AO)
images for the Survey of a Wide Area with NACO (SWAN) at the ESO VLT. The
survey comprises adaptive optics observations in the Ks band totaling ~ 30
arcmin^2, assembled from 42 discrete fields centered on different bright stars
suitable for AO guiding. We develop a parametric model of the PSF variations
across the field of view in order to build an accurate model PSF for every
galaxy detected in each of the fields. We show that this approach is
particularly convenient, as it uses only easily available data and makes no
uncertain assumptions about the stability of the isoplanatic angle during any
given night. The model was tested using simulated galaxy profiles to check its
performance in terms of recovering the correct morphological parameters; we
find that the results are reliable up to Ks ~ 20.5 (K_AB ~ 22.3) in a typical
SWAN field. Finally, the model obtained was used to derive the first results
from five SWAN fields, and to obtain the AO morphology of 55 galaxies brighter
than Ks = 20. These preliminary results demonstrate the unique power of AO
observations to derive the details of faint galaxy morphologies and to study
galaxy evolution.Comment: 12 pages, 12 figures. Accepted for publication in A&
SPICES: Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems
SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary
Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its
purpose is to image and characterize long-period extrasolar planets and
circumstellar disks in the visible (450 - 900 nm) at a spectral resolution of
about 40 using both spectroscopy and polarimetry. By 2020/22, present and
near-term instruments will have found several tens of planets that SPICES will
be able to observe and study in detail. Equipped with a 1.5 m telescope, SPICES
can preferentially access exoplanets located at several AUs (0.5-10 AU) from
nearby stars (25 pc) with masses ranging from a few Jupiter masses to Super
Earths (2 Earth radii, 10 M) as well as circumstellar
disks as faint as a few times the zodiacal light in the Solar System
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