544 research outputs found
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&
Post-coronagraphic tip-tilt sensing for vortex phase masks: the QACITS technique
Small inner working angle coronagraphs, like the vortex phase mask, are
essential to exploit the full potential of ground-based telescopes in the
context of exoplanet detection and characterization. However, the drawback of
this attractive feature is a high sensitivity to pointing errors, which
degrades the performance of the coronagraph. We propose a tip-tilt retrieval
technique based on the analysis of the final coronagraphic image, hereafter
called Quadrant Analysis of Coronagraphic Images for Tip-tilt Sensing (QACITS).
Under the assumption of small phase aberrations, we show that the behaviour of
the vortex phase mask can be simply described from the entrance pupil to the
Lyot stop plane by Zernike polynomials. This convenient formalism is used to
establish the theoretical basis of the QACITS technique. Simulations have been
performed to demonstrate the validity and limits of the technique, including
the case of a centrally obstructed pupil. The QACITS technique principle is
further validated by experimental results in the case of an unobstructed
circular aperture. The typical configuration of the Keck telescope (24% central
obstruction) has been simulated with additional high order aberrations. In
these conditions, our simulations show that the QACITS technique is still
adapted to centrally obstructed pupils and performs tip-tilt retrieval with a
precision of {\lambda}/D when wavefront errors amount to
{\lambda}/14 rms and {\lambda}/D for {\lambda}/70 rms errors (with
{\lambda} the wavelength and D the pupil diameter). The implementation of the
QACITS technique is based on the analysis of the scientific image and does not
require any modification of the original setup. Current facilities equipped
with a vortex phase mask can thus directly benefit from this technique to
improve the contrast performance close to the axis.Comment: 12 pages, 15 figures, accepted for publication in A&
Theory and laboratory tests of the multi-stage phase mask coronagraph
A large number of coronagraphs have been proposed to overcome the ratio that
exists between the star and its planet. The planet finder of the Extremely
Large Telescope, which is called EPICS, will certainly need a more efficient
coronagraph than the ones that have been developed so far. We propose to use a
combination of chromatic Four Quadrant Phase Mask coronagraph to achromatize
the dephasing of the device while maintaining a high rejection performance.
After describing this multi-stage FQPM coronagraph, we show preliminary results
of a study on its capabilities in the framework of the EPICS instrument, the
planet finder of the European Extremely Large Telescope. Eventually, we present
laboratory tests of a rough prototype of a multi-stage four-quadrant phase
mask. On one hand, we deduce from our laboratory data that a detection at the
10^-10 level is feasible in monochromatic light. On the other hand, we show the
detection of a laboratory companion fainter than 10^-8 with a spectral
bandwidth larger than 20%.Comment: 9 pages, 9 figures, To appear in SPIE proceeding- conference 7015
held in Marseille in June 200
High-contrast imaging in polychromatic light with the self-coherent camera
Context. In the context of direct imaging of exoplanets, coronagraphs are
commonly proposed to reach the required very high contrast levels. However,
wavefront aberrations induce speckles in their focal plane and limit their
performance. Aims. An active correction of these wavefront aberrations using a
deformable mirror upstream of the coronagraph is mandatory. These aberrations
need to be calibrated and focal-plane wavefront-sensing techniques in the
science channel are being developed. One of these, the self-coherent camera, of
which we present the latest laboratory results. Methods. We present here an
enhancement of the method: we directly minimized the complex amplitude of the
speckle field in the focal plane. Laboratory tests using a four-quadrant
phase-mask coronagraph and a 32x32 actuator deformable mirror were conducted in
monochromatic light and in polychromatic light for different bandwidths.
Results. We obtain contrast levels in the focal plane in monochromatic light
better than 3.10^-8 (RMS) in the 5 - 12 {\lambda}/D region for a correction of
both phase and amplitude aberrations. In narrow bands (10 nm) the contrast
level is 4.10^-8 (RMS) in the same region. Conclusions. The contrast level is
currently limited by the amplitude aberrations on the bench. We identified
several improvements that can be implemented to enhance the performance of our
optical bench in monochromatic as well as in polychromatic light.Comment: 4 pages, 3 figures, accepted in Astronomy & Astrophysics (02/2014
Expected Performance of a Self-Coherent Camera
Residual wavefront errors in optical elements limit the performance of
coronagraphs. To improve their efficiency, different types of devices have been
proposed to correct or calibrate these errors. In this paper, we study one of
these techniques proposed by Baudoz et al. 2006 and called Self-Coherent Camera
(SCC). The principle of this instrument is based on the lack of coherence
between the stellar light and the planet that is searched for. After recalling
the principle of the SCC, we simulate its performance under realistic
conditions and compare it with the performance of differential imaging.Comment: 6 pages, 4 figure
Self-coherent camera as a focal plane wavefront sensor: simulations
Direct detection of exoplanets requires high dynamic range imaging.
Coronagraphs could be the solution, but their performance in space is limited
by wavefront errors (manufacturing errors on optics, temperature variations,
etc.), which create quasi-static stellar speckles in the final image. Several
solutions have been suggested for tackling this speckle noise. Differential
imaging techniques substract a reference image to the coronagraphic residue in
a post-processing imaging. Other techniques attempt to actively correct
wavefront errors using a deformable mirror. In that case, wavefront aberrations
have to be measured in the science image to extremely high accuracy. We propose
the self-coherent camera sequentially used as a focal-plane wavefront sensor
for active correction and differential imaging. For both uses, stellar speckles
are spatially encoded in the science image so that differential aberrations are
strongly minimized. The encoding is based on the principle of light incoherence
between the hosting star and its environment. In this paper, we first discuss
one intrinsic limitation of deformable mirrors. Then, several parameters of the
self-coherent camera are studied in detail. We also propose an easy and robust
design to associate the self-coherent camera with a coronagraph that uses a
Lyot stop. Finally, we discuss the case of the association with a four-quadrant
phase mask and numerically demonstrate that such a device enables the detection
of Earth-like planets under realistic conditions. The parametric study of the
technique lets us believe it can be implemented quite easily in future
instruments dedicated to direct imaging of exoplanets.Comment: 15 pages, 14 figures, accepted in A&A (here is the final version
High-contrast imaging at small separation: impact of the optical configuration of two deformable mirrors on dark holes
The direct detection and characterization of exoplanets will be a major
scientific driver over the next decade, involving the development of very large
telescopes and requires high-contrast imaging close to the optical axis. Some
complex techniques have been developed to improve the performance at small
separations (coronagraphy, wavefront shaping, etc). In this paper, we study
some of the fundamental limitations of high contrast at the instrument design
level, for cases that use a combination of a coronagraph and two deformable
mirrors for wavefront shaping. In particular, we focus on small-separation
point-source imaging (around 1 /D). First, we analytically or
semi-analytically analysing the impact of several instrument design parameters:
actuator number, deformable mirror locations and optic aberrations (level and
frequency distribution). Second, we develop in-depth Monte Carlo simulation to
compare the performance of dark hole correction using a generic test-bed model
to test the Fresnel propagation of multiple randomly generated optics static
phase errors. We demonstrate that imaging at small separations requires large
setup and small dark hole size. The performance is sensitive to the optic
aberration amount and spatial frequencies distribution but shows a weak
dependence on actuator number or setup architecture when the dark hole is
sufficiently small (from 1 to 5 /D).Comment: 13 pages, 18 figure
Lyot-based Ultra-Fine Pointing Control System for Phase Mask Coronagraphs
High performance coronagraphic imaging at small inner working angle requires
efficient control of low order aberrations. The absence of accurate pointing
control at small separation not only degrades coronagraph starlight rejection
but also increases the risk of confusing planet's photons with starlight
leaking next to the coronagraph focal plane mask center. Addressing this issue
is essential for preventing coronagraphic leaks, and we have thus developed a
new concept, the Lyot-based pointing control system (LPCS), to control pointing
errors and other low order aberrations within a coronagraph. The LPCS uses
residual starlight reflected by the Lyot stop at the pupil plane. Our
simulation has demonstrated pointing errors measurement accuracy between 2-12
nm for tip-tilt at 1.6 micron with a four quadrant phase mask coronagraph.Comment: 7 pages, 5 figures, Proceedings of AO4ELTs3 conference, Paper 12667,
Florence, Italy, May 201
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