136 research outputs found
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
Independent confirmation of {\beta} Pictoris b imaging with NICI
Context. {\beta} Pictoris b is one of the most studied objects nowadays since
it was identified with VLT/NaCo as a bona-fide exoplanet with a mass of about 9
times that of Jupiter at an orbital separation of 8-9 AU. The link between the
planet and the dusty disk is unambiguously attested and this system provides an
opportunity to study the disk/planet interactions and to constrain formation
and evolutionary models of gas giant planets. Still, {\beta} Pictoris b had
never been confirmed with other telescopes so far. Aims. We aimed at an
independent confirmation using a different instrument. Methods. We retrieved
archive images from Gemini South obtained with the instrument NICI, which is
designed for high contrast imaging. The observations combine coronagraphy and
angular differential imaging and were obtained at three epochs in Nov. 2008,
Dec. 2009 and Dec. 2010. Results. We report the detection with NICI of the
planet {\beta} Pictoris b in Dec. 2010 images at a separation of 404 \pm 10 mas
and P A = 212.1 \pm 0.7{\deg} . It is the first time this planet is observed
with a telescope different than the VLT.Comment: Letter accepted for publication in Astronomy and Astrophysics on Feb.
21, 2013. 4 pages, 2 figure
Analytical model-based analysis of long-exposure images fromground-based telescopes
The search for Earth-like exoplanets requires high-contrast and high-angular
resolution instruments, which designs can be very complex: they need an
adaptive optics system to compensate for the effect of the atmospheric
turbulence on image quality and a coronagraph to reduce the starlight and
enable the companion imaging. During the instrument design phase and the error
budget process, studies of performance as a function of optical errors are
needed and require multiple end-to-end numerical simulations of wavefront
errors through the optical system.
In particular, the detailed analysis of long-exposure images enables to
evaluate the image quality (photon noise level, impact of optical aberrations
and of adaptive optics residuals, etc.). Nowadays simulating one long but
finite exposure image means drawing several thousands of random frozen phase
screens, simulating the image associated with each of them after propagation
through the imaging instrument, and averaging all the images. Such a process is
time consuming, demands a great deal of computer resources, and limits the
number of parametric optimization.
We propose an alternative and innovative method to directly express the
statistics of ground-based images for long but finite exposure times. It is
based on an analytical model, which only requires the statistical properties of
the atmospheric turbulence. Such a method can be applied to optimize the design
of future instruments such as SPHERE+ (VLT) or the planetary camera and
spectrograph (PCS - ELT) or any ground-based instrument.Comment: 7 pages, 0 figur
The Fast Atmospheric Self-Coherent Camera Technique: Laboratory Results and Future Directions
Direct detection and detailed characterization of exoplanets using extreme
adaptive optics (ExAO) is a key science goal of future extremely large
telescopes (ELTs). However, wavefront errors will limit the sensitivity of this
endeavor. Limitations for ground-based telescopes arise from both quasi-static
and residual AO-corrected atmospheric wavefront errors, the latter of which
generates short-lived aberrations that will average into a halo over a long
exposure. We have developed and tested the framework for a solution to both of
these problems using the self-coherent camera (SCC), to be applied to
ground-based telescopes, called the Fast Atmospheric SCC Technique (FAST). In
this paper we present updates of new and ongoing work for FAST, both in
numerical simulation and in the laboratory. We first present numerical
simulations that illustrate the scientific potential of FAST, including, with
current 10-m telescopes, the direct detection of exoplanets reflected light and
exo-Jupiters in thermal emission and, with future ELTs, the detection of
habitable exoplanets. In the laboratory, we present the first characterizations
of our proposed, and now fabricated, coronagraphic masks.Comment: submitted to Proceedings of Adaptive Optics for Extremely Large
Telescopes
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
SAXO+ upgrade : second stage AO system end-to-end numerical simulations
SAXO+ is a proposed upgrade to SAXO, the AO system of the SPHERE instrument
on the ESO Very Large Telescope. It will improve the capabilities of the
instrument for the detection and characterization of young giant planets. It
includes a second stage adaptive optics system composed of a dedicated
near-infrared wavefront sensor and a deformable mirror. This second stage will
remove the residual wavefront errors left by the current primary AO loop
(SAXO). This paper focuses on the numerical simulations of the second stage
(SAXO+) and concludes on the impact of the main AO parameters used to build the
design strategy. Using an end-to-end AO simulation tool (COMPASS), we
investigate the impact of several parameters on the performance of the AO
system. We measure the performance in minimizing the star residuals in the
coronagraphic image. The parameters that we study are : the second stage
frequency, the photon flux on each WFS, the first stage gain and the DM number
of actuators of the second stage. We show that the performance is improved by a
factor 10 with respect to the current AO system (SAXO). The optimal second
stage frequency is between 1 and 2 kHz under good observing conditions. In a
red star case, the best SAXO+ performance is achieved with a low first stage
gain of 0.05, which reduces the first stage rejection.Comment: 10 pages, 8 figures. Submitted to AO4ELT7 conference proceeding
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