297 research outputs found
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&
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
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
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
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
Lyot-based Low Order Wavefront Sensor for Phase-mask Coronagraphs: Principle, Simulations and Laboratory Experiments
High performance coronagraphic imaging of faint structures around bright
stars at small angular separations requires fine control of tip, tilt and other
low order aberrations. When such errors occur upstream of a coronagraph, they
results in starlight leakage which reduces the dynamic range of the instrument.
This issue has been previously addressed for occulting coronagraphs by sensing
the starlight before or at the coronagraphic focal plane. One such solution,
the coronagraphic low order wave-front sensor (CLOWFS) uses a partially
reflective focal plane mask to measure pointing errors for Lyot-type
coronagraphs.
To deal with pointing errors in low inner working angle phase mask
coronagraphs which do not have a reflective focal plane mask, we have adapted
the CLOWFS technique. This new concept relies on starlight diffracted by the
focal plane phase mask being reflected by the Lyot stop towards a sensor which
reliably measures low order aberrations such as tip and tilt. This reflective
Lyot-based wavefront sensor is a linear reconstructor which provides high
sensitivity tip-tilt error measurements with phase mask coronagraphs.
Simulations show that the measurement accuracy of pointing errors with
realistic post adaptive optics residuals are approx. 10^-2 lambda/D per mode at
lambda = 1.6 micron for a four quadrant phase mask. In addition, we demonstrate
the open loop measurement pointing accuracy of 10^-2 lambda/D at 638 nm for a
four quadrant phase mask in the laboratory.Comment: 9 Pages, 11 Figures, to be published in PASP June 2014 issu
Lyot-based Low Order Wavefront Sensor: Implementation on the Subaru Coronagraphic Extreme Adaptive Optics System and its Laboratory Performance
High throughput, low inner working angle (IWA) phase masks coronagraphs are
essential to directly image and characterize (via spectroscopy) earth-like
planets. However, the performance of low-IWA coronagraphs is limited by
residual pointing errors and other low-order modes. The extent to which
wavefront aberrations upstream of the coronagraph are corrected and calibrated
drives coronagraphic performance. Addressing this issue is essential for
preventing coronagraphic leaks, thus we have developed a Lyot-based low order
wave front sensor (LLOWFS) to control the wavefront aberrations in a
coronagraph. The LLOWFS monitors the starlight rejected by the coronagraphic
mask using a reflective Lyot stop in the downstream pupil plane. The early
implementation of LLOWFS at LESIA, Observatoire de Paris demonstrated an open
loop measurement accuracy of 0.01 lambda/D for tip-tilt at 638 nm when used in
conjunction with a four quadrant phase mask (FQPM) in the laboratory. To
further demonstrate our concept, we have installed the reflective Lyot stops on
the Subaru Coronagraphic Extreme AO (SCExAO) system at the Subaru Telescope and
modified the system to support small IWA phase mask coronagraphs (< 1 lambda/D)
on-sky such as FQPM, eight octant phase mask, vector vortex coronagraph and the
phase induced amplitude apodization complex phase mask coronagraph with a goal
of obtaining milli arc-second pointing accuracy. Laboratory results have shown
the measurement of tip, tilt, focus, oblique and right astigmatism at 1.55 um
for the vector vortex coronagraph. Our initial on-sky result demonstrate the
closed loop accuracy of < 7 x 10-3 lambda/D at 1.6 um for tip, tilt and focus
aberrations with the vector vortex coronagraph.Comment: 9 pages, 9 Figures, Proc. of SPIE Astronomical Telescopes +
Instrumentation 201
On-sky demonstration of low-order wavefront sensing and control with focal plane phase mask coronagraphs
The ability to characterize exoplanets by spectroscopy of their atmospheres
requires direct imaging techniques to isolate planet signal from the bright
stellar glare. One of the limitations with the direct detection of exoplanets,
either with ground- or space-based coronagraphs, is pointing errors and other
low-order wavefront aberrations. The coronagraphic detection sensitivity at the
diffraction limit therefore depends on how well low-order aberrations upstream
of the focal plane mask are corrected. To prevent starlight leakage at the
inner working angle of a phase mask coronagraph, we have introduced a
Lyot-based low-order wavefront sensor (LLOWFS), which senses aberrations using
the rejected starlight diffracted at the Lyot plane. In this paper, we present
the implementation, testing and results of LLOWFS on the Subaru Coronagraphic
Extreme Adaptive Optics system (SCExAO) at the Subaru Telescope.
We have controlled thirty-five Zernike modes of a H-band vector vortex
coronagraph in the laboratory and ten Zernike modes on sky with an integrator
control law. We demonstrated a closed-loop pointing residual of 0.02 mas in the
laboratory and 0.15 mas on sky for data sampled using the minimal 2-second
exposure time of the science camera. We have also integrated the LLOWFS in the
visible high-order control loop of SCExAO, which in closed-loop operation has
validated the correction of the non-common path pointing errors between the
infrared science channel and the visible wavefront sensing channel with
pointing residual of 0.23 mas on sky.Comment: 12 pages, 15 figures, Accepted and scheduled for publication in
September 2015 issue of the PAS
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