113 research outputs found
Apodized phase mask coronagraphs for arbitrary apertures
Phase masks coronagraphs can be seen as linear systems that spatially
redistribute, in the pupil plane, the energy collected by the telescope. Most
of the on-axis light must ideally be rejected outside the aperture to be
blocked with a Lyot stop, while almost all off-axis light must go through it.
The unobstructed circular apertures of off-axis telescopes make this possible
but all major telescopes are however on-axis and the performance of these
coronagraphs is dramatically reduced by the central obstruction. Their
performance can be restored by using an additional optimally designed apodizer
that changes the amplitude in the first pupil plane so that the on-axis light
is rejected outside the obstructed aperture of the telescope. The numerical
optimization model is built by maximizing the apodizer's transmission while
setting constraints on the extremum values of the electric field that the Lyot
stop does not block. The coronagraphic image is compared to what a non-apodized
phase mask coronagraph provides and an analysis is made of the trade-offs that
exist between the apodizer transmission and the Lyot stop properties. The
existence of a solution and the mask transmission depend on the aperture and
the Lyot stop geometries, and on the constraints that are set on the on-axis
attenuation. The system throughput is a concave function of the Lyot stop
transmission. In the case of a VLT-like aperture, apodizers with a transmission
of 0.16 to 0.92 associated with a four-quadrant phase mask provide contrast as
low as a few 1e-10 at 1 lambda/D from the star. The system's maximum throughput
is 0.64, for an apodizer with an 0.88 transmission and a Lyot stop with a 0.69
transmission. Optimizing apodizers for a vortex phase mask requires computation
times much longer than in the previous case, and no result is presented for
this mask.Comment: 16 page
Apodized phase mask coronagraphs for arbitrary apertures. II. Comprehensive review of solutions for the vortex coronagraph
With a clear circular aperture, the vortex coronagraph perfectly cancels an
on-axis point source and offers a 0.9 or 1.75 lambda/D inner working angle for
topological charge 2 or 4, respectively. Current and near-future large
telescopes are on-axis, however, and the diffraction effects of the central
obscuration, and the secondary supports are strong enough to prevent the
detection of companions 1e-3 - 1e-5 as bright as, or fainter than, their host
star. Recent advances show that a ring apodizer can restore the performance of
this coronagraph by compensating for the diffraction effects of a circular
central obscuration in a 1D modeling of the pupil. We extend this work and
optimize apodizers for arbitrary apertures in 2D in order to tackle the
diffraction effects of the spiders and other noncircular artefacts in the
pupil. We use a numerical optimization scheme to compute hybrid coronagraph
designs that combine the advantages of the vortex coronagraph (small in IWA)
and of shaped pupils coronagraphs (robustness to central obscuration and pupil
asymmetric structures). We maximize the apodizer transmission, while
constraints are set on the extremum values of the electric field that is
computed in chosen regions of the Lyot plane through closed form expressions.
Optimal apodizers are computed for topological charges 2 and 4 vortex
coronagraphs and for telescope apertures with 10-30% central obscurations and
0-1% thick spiders. We characterize the impacts of the obscuration ratio and
the thickness of the spiders on the throughput and the IWA for the two
topological charges.Comment: 23 pages, 12 figures, 2 table
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
Ring-apodized vortex coronagraphs for obscured telescopes. I. Transmissive ring apodizers
The vortex coronagraph (VC) is a new generation small inner working angle
(IWA) coronagraph currently offered on various 8-meter class ground-based
telescopes. On these observing platforms, the current level of performance is
not limited by the intrinsic properties of actual vortex devices, but by
wavefront control residuals and incoherent background (e.g. thermal emission of
the sky) or the light diffracted by the imprint of the secondary mirror and
support structures on the telescope pupil. In the particular case of unfriendly
apertures (mainly large central obscuration) when very high contrast is needed
(e.g. direct imaging of older exoplanets with extremely large telescopes or
space- based coronagraphs), a simple VC, as most coronagraphs, can not deliver
its nominal performance because of the contamination due to the diffraction
from the obscured part of the pupil. Here we propose a novel yet simple concept
that circumvents this problem. We combine a vortex phase mask in the image
plane of a high-contrast instrument with a single pupil-based amplitude ring
apodizer, tailor designed to exploit the unique convolution properties of the
VC at the Lyot-stop plane. We show that such a ring-apodized vortex coronagraph
(RAVC) restores the perfect attenuation property of the VC regardless of the
size of the central obscuration, and for any (even) topological charge of the
vortex. More importantly the RAVC maintains the IWA and conserves a fairly high
throughput, which are signature properties of the VC.Comment: 10 pages, 6 figure
Apodized pupil Lyot coronagraphs for arbitrary apertures. V. Hybrid Shaped Pupil designs for imaging Earth-like planets with future space observatories
We introduce a new class of solutions for Apodized Pupil Lyot Coronagraphs
(APLC) with segmented aperture telescopes to remove broadband diffracted light
from a star with a contrast level of . These new coronagraphs provide
a key advance to enabling direct imaging and spectroscopy of Earth twins with
future large space missions. Building on shaped pupil (SP) apodization
optimizations, our approach enables two-dimensional optimizations of the system
to address any aperture features such as central obstruction, support
structures or segment gaps. We illustrate the technique with a design that
could reach contrast level at 34\,mas for a 12\,m segmented telescope
over a 10\% bandpass centered at a wavelength 500\,nm. These
designs can be optimized specifically for the presence of a resolved star, and
in our example, for stellar angular size up to 1.1\,mas. This would allow
probing the vicinity of Sun-like stars located beyond 4.4\,pc, therefore fully
retiring this concern. If the fraction of stars with Earth-like planets is
\eta_{\Earth}=0.1, with 18\% throughput, assuming a perfect, stable wavefront
and considering photon noise only, 12.5 exo-Earth candidates could be detected
around nearby stars with this design and a 12\,m space telescope during a
five-year mission with two years dedicated to exo-Earth detection (one total
year of exposure time and another year of overheads). Our new hybrid APLC/SP
solutions represent the first numerical solution of a coronagraph based on
existing mask technologies and compatible with segmented apertures, and that
can provide contrast compatible with detecting and studying Earth-like planets
around nearby stars. They represent an important step forward towards enabling
these science goals with future large space missions.Comment: 9 pages, 6 figures, ApJ accepted on 01/04/201
Shaped Pupil Lyot Coronagraphs: High-Contrast Solutions for Restricted Focal Planes
Coronagraphs of the apodized pupil and shaped pupil varieties use the
Fraunhofer diffraction properties of amplitude masks to create regions of high
contrast in the vicinity of a target star. Here we present a hybrid coronagraph
architecture in which a binary, hard-edged shaped pupil mask replaces the gray,
smooth apodizer of the apodized pupil Lyot coronagraph (APLC). For any contrast
and bandwidth goal in this configuration, as long as the prescribed region of
contrast is restricted to a finite area in the image, a shaped pupil is the
apodizer with the highest transmission. We relate the starlight cancellation
mechanism to that of the conventional APLC. We introduce a new class of
solutions in which the amplitude profile of the Lyot stop, instead of being
fixed as a padded replica of the telescope aperture, is jointly optimized with
the apodizer. Finally, we describe shaped pupil Lyot coronagraph (SPLC) designs
for the baseline architecture of the Wide-Field Infrared Survey
Telescope-Astrophysics Focused Telescope Assets (WFIRST-AFTA) coronagraph.
These SPLCs help to enable two scientific objectives of the WFIRST-AFTA
mission: (1) broadband spectroscopy to characterize exoplanet atmospheres in
reflected starlight and (2) debris disk imaging.Comment: 41 pages, 15 figures; published in the JATIS special section on
WFIRST-AFTA coronagraph
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