112 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
Active compensation of aperture discontinuities for WFIRST-AFTA: analytical and numerical comparison of propagation methods and preliminary results with a WFIRST-AFTA-like pupil
The new frontier in the quest for the highest contrast levels in the focal
plane of a coronagraph is now the correction of the large diffractive artifacts
effects introduced at the science camera by apertures of increasing complexity.
The coronagraph for the WFIRST/AFTA mission will be the first of such
instruments in space with a two Deformable Mirrors wavefront control system.
Regardless of the control algorithm for these multi Deformable Mirrors, they
will have to rely on quick and accurate simulation of the propagation effects
introduced by the out-of-pupil surface. In the first part of this paper, we
present the analytical description of the different approximations to simulate
these propagation effects. In Annex A, we prove analytically that, in the
special case of surfaces inducing a converging beam, the Fresnel method yields
high fidelity for simulations of these effects. We provide numerical
simulations showing this effect. In the second part, we use these tools in the
framework of the Active Compensation of Aperture Discontinuities technique
(ACAD) applied to pupil geometries similar to WFIRST-AFTA. We present these
simulations in the context of the optical layout of the High-contrast imager
for Complex Aperture Telescopes, which will test ACAD on a optical bench. The
results of this analysis show that using the ACAD method, an apodized pupil
lyot coronagraph and the performance of our current deformable mirrors, we are
able to obtain, in numerically simulations, a dark hole with an AFTA-like
pupil. Our numerical simulation shows that we can obtain contrast better than
in monochromatic light and better than 3.e-8 with 10% bandwidth
between 5 and 14 lambda/D.Comment: 16 pages, 5 figures, Accepted for publication (Oct. 23, 2015) in
Journal of Astronomical Telescopes, Instruments, and Systems, special
WFIRST-AFTA coronagrap
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