44 research outputs found
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
Analysis of ground-based differential imager performance
In the context of extrasolar planet direct detection, we evaluated the
performance of differential imaging with ground-based telescopes. This study
was carried out in the framework of the VLT-Planet Finder project and is
further extended to the case of Extremely Large Telescopes. Our analysis is
providing critical specifications for future instruments mostly in terms of
phase aberrations but also regarding alignments of the instrument optics or
offset pointing on the coronagraph. It is found that Planet Finder projects on
8m class telescopes can be successful at detecting Extrasolar Giant Planets
providing phase aberrations, alignments and pointing are accurately controlled.
The situation is more pessimistic for the detection of terrestrial planets with
Extremely Large Telescopes for which phase aberrations must be lowered at a
very challenging level
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&
Optical performance of the JWST MIRI flight model: characterization of the point spread function at high-resolution
The Mid Infra Red Instrument (MIRI) is one of the four instruments onboard
the James Webb Space Telescope (JWST), providing imaging, coronagraphy and
spectroscopy over the 5-28 microns band. To verify the optical performance of
the instrument, extensive tests were performed at CEA on the flight model (FM)
of the Mid-InfraRed IMager (MIRIM) at cryogenic temperatures and in the
infrared. This paper reports on the point spread function (PSF) measurements at
5.6 microns, the shortest operating wavelength for imaging. At 5.6 microns the
PSF is not Nyquist-sampled, so we use am original technique that combines a
microscanning measurement strategy with a deconvolution algorithm to obtain an
over-resolved MIRIM PSF. The microscanning consists in a sub-pixel scan of a
point source on the focal plane. A data inversion method is used to reconstruct
PSF images that are over-resolved by a factor of 7 compared to the native
resolution of MIRI. We show that the FWHM of the high-resolution PSFs were
5-10% wider than that obtained with Zemax simulations. The main cause was
identified as an out-of-specification tilt of the M4 mirror. After correction,
two additional test campaigns were carried out, and we show that the shape of
the PSF is conform to expectations. The FWHM of the PSFs are 0.18-0.20 arcsec,
in agreement with simulations. 56.1-59.2% of the total encircled energy
(normalized to a 5 arcsec radius) is contained within the first dark Airy ring,
over the whole field of view. At longer wavelengths (7.7-25.5 microns), this
percentage is 57-68%. MIRIM is thus compliant with the optical quality
requirements. This characterization of the MIRIM PSF, as well as the
deconvolution method presented here, are of particular importance, not only for
the verification of the optical quality and the MIRI calibration, but also for
scientific applications.Comment: 13 pages, submitted to SPIE Proceedings vol. 7731, Space Telescopes
and Instrumentation 2010: Optical, Infrared, and Millimeter Wav
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of the MIRI Coronagraphs
The imaging channel on the Mid-Infrared Instrument (MIRI) is equipped with
four coronagraphs that provide high contrast imaging capabilities for studying
faint point sources and extended emission that would otherwise be overwhelmed
by a bright point-source in its vicinity. Such bright sources might include
stars that are orbited by exoplanets and circumstellar material, mass-loss
envelopes around post-main-sequence stars, the near-nuclear environments in
active galaxies, and the host galaxies of distant quasars. This paper describes
the coronagraphic observing modes of MIRI, as well as performance estimates
based on measurements of the MIRI flight model during cryo-vacuum testing. A
brief outline of coronagraphic operations is also provided. Finally, simulated
MIRI coronagraphic observations of a few astronomical targets are presented for
illustration
Fundamental limitations on Earth-like planet detection with Extremely Large Telescopes
We analyse the fundamental limitations for the detection of extraterrestrial
planets with Extremely Large Telescopes. For this task, a coronagraphic device
combined to a very high order wavefront correction system is required but not
sufficient to achieve the contrast level needed for detecting an
Earth-like planet. The stellar residuals left uncorrected by the wavefront
correction system need to be calibrated and subtracted. In this paper, we
consider a general model including the dynamic phase aberrations downstream the
wavefront correction system, the static phase aberrations of the instrument and
some differential aberrations provided by the calibration unit. A rather
optimistic case of a filled circular pupil and of a perfect coronagraph is
elsewhere assumed. As a result of the analytical study, the limitation mostly
comes from the static aberrations. Using numerical simulations we confirm this
result and evaluate the requirements in terms of phase aberrations to detect
Earth-like planets on Extremely Large Telescopes.Comment: 8 pages, 8 figures, accepted in A&
Spectral and polarimetric characterization of gazeous and telluric planets with SEE COAST
SEE COAST stands for Super Earth Explorer – Coronagraphic Off-Axis Space Telescope. The concept was initially proposed to ESA for Cosmic Vision. None of the direct detection exoplanet proposals were selected in 2007 and we are now pursuing our efforts to consolidate the astrophysical program and the technical developments for the next call for proposal. The prime objective of SEE COAST is to contribute to the understanding of the formation and evolution of planetary systems. Exploring the diversity of these objects is therefore the main driver to define the instrumentation. In the next decade the improvement of radial velocity instruments and obviously temporal coverage will provide us with a large numbers of long period giants as well as telluric planets, namely Super Earths. Obtaining the spectral and polarimetric signatures of these objects in the visible range to measure atmospheric parameters (molecular composition, clouds, soils, …) will be unique and with important scientific returns. A space mission complementary to near IR instruments like SPHERE, GPI, JWST and later ELTs for the full characterization of giants and Super Earths is a first secure step towards the longer term goal that is the characterization of telluric planets with mass and atmosphere comparable to that of the Earth. An overview of the astrophysical motivation and the trade-off that lead to a simple integrated concept of a space-based high contrast imaging instrument are given here
Detection of Neptune-size planetary candidates with CoRoT data. Comparison with the planet occurrence rate derived from Kepler
[Abridged] Context. The CoRoT space mission has been searching for transiting
planets since the end of December 2006. Aims. We aim to investigate the
capability of CoRoT to detect small-size transiting planets in short-period
orbits, and to compare the number of CoRoT planets with 2 \leq R_p \leq 4
Rearth with the occurrence rate of small-size planets provided by the
distribution of Kepler planetary candidates (Howard et al. 2012). Methods. We
performed a test that simulates transits of super-Earths and Neptunes in real
CoRoT light curves and searches for them blindly by using the LAM transit
detection pipeline. Results. The CoRoT detection rate of planets with radius
between 2 and 4 Rearth and orbital period P \leq 20 days is 59% (31%) around
stars brighter than r'=14.0 (15.5). By properly taking the CoRoT detection rate
for Neptune-size planets and the transit probability into account, we found
that according to the Kepler planet occurrence rate, CoRoT should have
discovered 12 \pm 2 Neptunes orbiting G and K dwarfs with P \leq 17 days in six
observational runs. This estimate must be compared with the validated Neptune
CoRoT-24b and five CoRoT planetary candidates in the considered range of
planetary radii. We thus found a disagreement with expectations from Kepler at
3 \sigma or 5 \sigma, assuming a blend fraction of 0% (six Neptunes) and 100%
(one Neptune) for these candidates. Conclusions. This underabundance of CoRoT
Neptunes with respect to Kepler may be due to several reasons. Regardless of
the origin of the disagreement, which needs to be investigated in more detail,
the noticeable deficiency of CoRoT Neptunes at short orbital periods seems to
indirectly support the general trend found in Kepler data, i.e. that the
frequency of small-size planets increases with increasing orbital periods and
decreasing planet radii.Comment: 10 pages, 7 figures. Accepted for publication in A&
The extreme physical properties of the CoRoT-7b super-Earth
International audience► Here, we discuss the extreme physical properties possible for the first characterized rocky super-Earth, CoRoT-7b ( = 1.58 , = 5.7 ). ► We make the working hypothesis that the planet is rocky with no volatiles in its atmosphere, and derive the physical properties that result. ► The dayside is very hot (2500 K at the sub-stellar point) while the nightside is very cold (∼ 50 K). The sub-stellar point is as hot as the tungsten filament of an incandescent bulb, resulting in the melting and distillation of silicate rocks and the formation of a lava ocean. ► These possible features of CoRoT-7b should be common to many small and hot planets, including Kepler-10b. They define a new class of objects that we propose to name ''Lava-ocean planets''
Transiting exoplanets from the CoRoT space mission - XIX. CoRoT-23b: a dense hot Jupiter on an eccentric orbit
We report the detection of CoRoT-23b, a hot Jupiter transiting in front of
its host star with a period of 3.6314 \pm 0.0001 days. This planet was
discovered thanks to photometric data secured with the CoRoT satellite,
combined with spectroscopic radial velocity (RV) measurements. A photometric
search for possible background eclipsing binaries conducted at CFHT and OGS
concluded with a very low risk of false positives. The usual techniques of
combining RV and transit data simultaneously were used to derive stellar and
planetary parameters. The planet has a mass of Mp = 2.8 \pm 0.3 MJup, a radius
of Rpl = 1.05 \pm 0.13 RJup, a density of \approx 3 g cm-3. RV data also
clearly reveal a non zero eccentricity of e = 0.16 \pm 0.02. The planet orbits
a mature G0 main sequence star of V =15.5 mag, with a mass M\star = 1.14 \pm
0.08 M\odot, a radius R\star = 1. 61 \pm 0.18 R\odot and quasi-solar
abundances. The age of the system is evaluated to be 7 Gyr, not far from the
transition to subgiant, in agreement with the rather large stellar radius. The
two features of a significant eccentricity of the orbit and of a fairly high
density are fairly uncommon for a hot Jupiter. The high density is, however,
consistent with a model of contraction of a planet at this mass, given the age
of the system. On the other hand, at such an age, circularization is expected
to be completed. In fact, we show that for this planetary mass and orbital
distance, any initial eccentricity should not totally vanish after 7 Gyr, as
long as the tidal quality factor Qp is more than a few 105, a value that is the
lower bound of the usually expected range. Even if Corot-23b features a density
and an eccentricity that are atypical of a hot Jupiter, it is thus not an
enigmatic object.Comment: Astronomy & Astrophysics in pres