573 research outputs found
Improved High Contrast Imaging with On-Axis Telescopes using a Multi-Stage Vortex Coronagraph
The vortex coronagraph is one of the most promising coronagraphs for high
contrast imaging because of its simplicity, small inner working angle, high
throughput, and clear off-axis discovery space. However, as with most
coronagraphs, centrally-obscured on-axis telescopes degrade contrast. Based on
the remarkable ability of vortex coronagraphs to move light between the
interior and exterior of pupils, we propose a method, based on multiple
vortices, that, without sacrificing throughput, reduces the residual light
leakage to (a/A)^n, with n >=4, and a and A being the radii of the central
obscuration and primary mirror, respectively. This method thus enables high
contrasts to be reached even with an on-axis telescope.Comment: 3 pages, 2 figure
Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes
Less than 3% of the known exoplanets were directly imaged for two main
reasons. They are angularly very close to their parent star, which is several
magnitudes brighter. Direct imaging of exoplanets thus requires a dedicated
instrumentation with large telescopes and accurate wavefront control devices
for high-angular resolution and coronagraphs for attenuating the stellar light.
Coronagraphs are usually chromatic and they cannot perform high-contrast
imaging over a wide spectral bandwidth. That chromaticity will be critical for
future instruments. Enlarging the coronagraph spectral range is a challenge for
future exoplanet imaging instruments on both space-based and ground-based
telescopes. We propose the multi-stage four-quadrant phase mask that associates
several monochromatic four-quadrant phase mask coronagraphs in series.
Monochromatic device performance has already been demonstrated and the
manufacturing procedures are well-under control since their development for
previous instruments on VLT and JWST. The multi-stage implementation simplicity
is thus appealing. We present the instrument principle and we describe the
laboratory performance for large spectral bandwidths and for both pupil shapes
for space- (off-axis telescope) and ground-based (E-ELT) telescopes. The
multi-stage four-quadrant phase mask reduces the stellar flux over a wide
spectral range (30%) and it is a very good candidate to be associated with a
spectrometer for future exoplanet imaging instruments in ground- and
space-based observatories.Comment: 7 pages, 11 figures, 4 tables, accepted in A&
Diversity among other worlds: characterization of exoplanets by direct detection
The physical characterization of exoplanets will require to take spectra at several orbital positions. For that purpose, a direct imaging capability is necessary. Direct imaging requires an efficient stellar suppression mechanism, associated with an ultrasmooth telescope. We show that before future large space missions interferometer, 4-8 m class coronograph, external occulter or Fresnel imager), direct imaging of giant planets and close-by super-Earth are at the cross-road of a high scientific interest and a reasonable feasibility. The scientific interest lies in the fact that super-Earths share common geophysical attributes with Earths. They already begin to be detected by radial velocity (RV)
and, together with giant planets, they have a larger area than Earths, making them detectable with a 1.5-2 m class
telescope in reflected light. We propose such a (space) telescope be a first step before large direct imaging missions
The Vector Vortex Coronagraph: Laboratory Results and First Light at Palomar Observatory
High-contrast coronagraphy will be needed to image and characterize faint
extra-solar planetary systems. Coronagraphy is a rapidly evolving field, and
many enhanced alternatives to the classical Lyot coronagraph have been proposed
in the past ten years. Here, we discuss the operation of the vector vortex
coronagraph, which is one of the most efficient possible coronagraphs. We first
present recent laboratory results, and then first light observations at the
Palomar observatory. Our near-infrared H-band (centered at ~ 1.65 microns) and
K-band (centered at ~ 2.2 microns) vector vortex devices demonstrated excellent
contrast results in the lab, down to ~ 1e-6 at an angular separation of 3 lb/d.
On sky, we detected a brown dwarf companion 3000 times fainter than its host
star (HR 7672) in the Ks band (centered at ~2.15 microns), at an angular
separation of ~ 2.5 lb/d. Current and next-generation high-contrast instruments
can directly benefit from the demonstrated capabilities of such a vector
vortex: simplicity, small inner working angle, high optical throughput (>90%),
and maximal off-axis discovery space
Imaging faint brown dwarf companions close to bright stars with a small, well-corrected telescope aperture
We have used our 1.6 m diameter off-axis well-corrected sub-aperture (WCS) on
the Palomar Hale telescope in concert with a small inner-working-angle (IWA)
phase-mask coronagraph to image the immediate environs of a small number of
nearby stars. Test cases included three stars (HD 130948, HD 49197 and HR7672)
with known brown dwarf companions at small separations, all of which were
detected. We also present the initial detection of a new object close to the
nearby young G0V star HD171488. Follow up observations are needed to determine
if this object is a bona fide companion, but its flux is consistent with the
flux of a young brown dwarf or low mass M star at the same distance as the
primary. Interestingly, at small angles our WCS coronagraph demonstrates a
limiting detectable contrast comparable to that of extant Lyot coronagraphs on
much larger telescopes corrected with current-generation AO systems. This
suggests that small apertures corrected to extreme adaptive optics (ExAO)
levels can be used to carry out initial surveys for close brown dwarf and
stellar companions, leaving followup observations for larger telescopes.Comment: accepted for publication in the Astrophysical Journa
Near-infrared scattered light properties of the HR 4796 A dust ring A measured scattering phase function from 13.6° to 166.6°
Context. HR 4796 A is surrounded by a debris disc, observed in scattered light as an inclined ring with a high surface brightness. Past observations have raised several questions. First, a strong brightness asymmetry detected in polarised reflected light has recently challenged our understanding of scattering by the dust particles in this system. Secondly, the morphology of the ring strongly suggests the presence of planets, although no planets have been detected to date.
Aims. We aim here at measuring with high accuracy the morphology and photometry of the ring in scattered light, in order to derive the phase function of the dust and constrain its near-infrared spectral properties. We also want to constrain the presence of planets and set improved constraints on the origin of the observed ring morphology.
Methods. We obtained high-angular resolution coronagraphic images of the circumstellar environment around HR 4796 A with VLT/SPHERE during the commissioning of the instrument in May 2014 and during guaranteed-time observations in February 2015. The observations reveal for the first time the entire ring of dust, including the semi-minor axis that was previously hidden either behind the coronagraphic spot or in the speckle noise.
Results. We determine empirically the scattering phase function of the dust in the H band from 13.6° to 166.6°. It shows a prominent peak of forward scattering, never detected before, for scattering angles below 30°. We analyse the reflectance spectra of the disc from the 0.95 μm to 1.6 μm, confirming the red colour of the dust, and derive detection limits on the presence of planetary mass objects.
Conclusions. We confirm which side of the disc is inclined towards the Earth. The analysis of the phase function, especially below 45°, suggests that the dust population is dominated by particles much larger than the observation wavelength, of about 20 μm. Compact Mie grains of this size are incompatible with the spectral energy distribution of the disc, however the observed rise in scattering efficiency beyond 50° points towards aggregates which could reconcile both observables. We do not detect companions orbiting the star, but our high-contrast observations provide the most stringent constraints yet on the presence of planets responsible for the morphology of the dust
Detecting Water In the atmosphere of HR 8799 c with L-band High Dispersion Spectroscopy Aided By Adaptive Optics
High dispersion spectroscopy of brown dwarfs and exoplanets enables exciting
science cases, e.g., mapping surface inhomogeneity and measuring spin rate.
Here, we present band observations of HR 8799 c using Keck NIRSPEC
(R=15,000) in adaptive optics (AO) mode (NIRSPAO). We search for molecular
species (HO and CH) in the atmosphere of HR 8799 c with a template
matching method, which involves cross correlation between reduced spectrum and
a template spectrum. We detect HO but not CH, which suggests
disequilibrium chemistry in the atmosphere of HR 8799 c, and this is consistent
with previous findings. We conduct planet signal injection simulations to
estimate the sensitivity of our AO-aided high dispersion spectroscopy
observations. We conclude that contrast can be reached in band.
The sensitivity is mainly limited by the accuracy of line list used in modeling
spectra and detector noise. The latter will be alleviated by the NIRSPEC
upgrade.Comment: 14 pages, 5 figures, 5 tables, accepted for publication on AJ,
references update
The Optimal Gravitational Lens Telescope
Given an observed gravitational lens mirage produced by a foreground
deflector (cf. galaxy, quasar, cluster,...), it is possible via numerical lens
inversion to retrieve the real source image, taking full advantage of the
magnifying power of the cosmic lens. This has been achieved in the past for
several remarkable gravitational lens systems. Instead, we propose here to
invert an observed multiply imaged source directly at the telescope using an
ad-hoc optical instrument which is described in the present paper. Compared to
the previous method, this should allow one to detect fainter source features as
well as to use such an optimal gravitational lens telescope to explore even
fainter objects located behind and near the lens. Laboratory and numerical
experiments illustrate this new approach
Resolving the delta Andromedae spectroscopic binary with direct imaging
We present a direct image of the innermost companion to the red giant delta
Andromedae using the Stellar Double Coronagraph at the Palomar Observatory. We
use a Markov chain Monte Carlo based algorithm to simultaneously reduce the
data and perform astrometry and photometry of the companion. We determine that
the companion is most likely a main-sequence K-type star and is certainly not
the previously hypothesized white dwarf.Comment: ApJ, accepted. 10 pages, 3 figure
Morphology of the very inclined debris disk around HD 32297
Direct imaging of circumstellar disks at high angular resolution is mandatory
to provide morphological information that bring constraints on their
properties, in particular the spatial distribution of dust. New techniques
combining observing strategy and data processing now allow very high contrast
imaging with 8-m class ground-based telescopes (10^-4 to 10^-5 at ~1") and
complement space telescopes while improving angular resolution at near infrared
wavelengths. We carried out a program at the VLT with NACO to image known
debris disks with higher angular resolution in the near IR than ever before in
order to study morphological properties and ultimately to detect signpost of
planets. The observing method makes use of advanced techniques: Adaptive
Optics, Coronagraphy and Differential Imaging, a combination designed to
directly image exoplanets with the upcoming generation of "planet finders" like
GPI (Gemini Planet Imager) and SPHERE (Spectro-Polarimetric High contrast
Exoplanet REsearch). Applied to extended objects like circumstellar disks, the
method is still successful but produces significant biases in terms of
photometry and morphology. We developed a new model-matching procedure to
correct for these biases and hence to bring constraints on the morphology of
debris disks. From our program, we present new images of the disk around the
star HD 32297 obtained in the H (1.6mic) and Ks (2.2mic) bands with an
unprecedented angular resolution (~65 mas). The images show an inclined thin
disk detected at separations larger than 0.5-0.6". The modeling stage confirms
a very high inclination (i=88{\deg}) and the presence of an inner cavity inside
r_0~110AU. We also found that the spine (line of maximum intensity along the
midplane) of the disk is curved and we attributed this feature to a large
anisotropic scattering factor (g~0.5, valid for an non-edge on disk). Abridged
...Comment: 12 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
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