The Self-Coherent Camera is dedicated to the direct detection of exoplanets. This instrument can be used as a focal plane wavefront sensor to measure static aberrations that induce speckles on the detector, which prevents the detection of faint companions. The Self-Coherent Camera creates a reference beam in the Lyot stop pupil plane in order to spatially modulate the speckle pattern with Fizeau fringes. We can then estimate for wavefront aberrations upstream of the coronagraphic mask and correct for them using a deformable mirror. Currently, the Self-Coherent Camera is combined with a deformable mirror located in the pupil plane upstream of a Four-Quadrant Phase Mask Coronagraph. In this paper, we present the formalism that explains how the Self-Coherent Camera encodes speckles and how we estimate the wavefront aberrations directly from the science image. We present numerical simulation results on speckle suppression in the focal plane. Then, we give experimental results on wavefront correction on our optical bench using a 32x32 actuators deformable mirror. We show that we can improve the contrast in the focal plane by a factor of more than 100 in the PSF wings up to 12/λD.
Direct imaging and low-resolution spectroscopy of extrasolar planets are exciting but challenging scientific applications of coronagraphy. While the angular separation is well within the reach of actual telescope in the
near IR or visible, the planet-star contrast (from 10−6 to 10−10) requires wavefront quality and stability hard to reach even with a well-polished space telescope. Several solutions have been proposed to tackle the speckle noise introduced by the residual optical defects. While some concepts rely only on active wavefront correction
using deformable mirror, other techniques are based on post-processing and subtract a reference image recorded
sometimes simultaneously with the science image. One interesting solution is to choose a concept that allows
both active correction and post-processing of high contrast coronagraphic images. This is the case of the Self
Coherent Camera (SCC), which has been proposed for the project of space coronagraph SPICES and for the
ground-based planet finder EPICS studied for the European Extremely Large Telescope. After recalling the SCC
principle, we present both monochromatic and modest bandwidth (2%) experimental results of Dark Hole in the
focal plane using a SCC. Example of a post-processing result with SCC is also given to emphasize the interest
of combining it with active correction.
Direct imaging of exoplanets requires the detection of very faint objects orbiting close to very bright stars. In this context, the SPICES mission was proposed to the European Space Agency for planet characterization at visible wavelength. SPICES is a 1.5m space telescope which uses a coronagraph to strongly attenuate the central source. However, small optical aberrations, which appear even in space telescopes, dramatically decrease coronagraph performance. To reduce these aberrations, we want to estimate, directly on the coronagraphic image, the electric field, and, with the help of a deformable mirror, correct the wavefront upstream of the coronagraph. We propose an instrument, the Self-Coherent Camera (SCC) for this purpose. By adding a small "reference hole" into the Lyot stop, located after the coronagraph, we can produce interferences in the focal plane, using the coherence of the stellar light. We developed algorithms to decode the information contained in these Fizeau fringes and retrieve an estimation of the field in the focal plane. After briefly recalling the SCC principle, we will present the results of a study, based on both experiment and numerical simulation, analyzing the impact of the size of the reference hole.
In the framework of Extremely Large Telescope (ELT), several instruments are considered for the characterization of
extrasolar planets. Since the performance of such an instrument is limited by wavefront errors, the use of extreme
Adaptive Optic (AO) systems is mandatory. Studies for future planet finder instruments such as SPHERE/VLT or
GPI/GEMINI show that one limitation of the performance for a planet finder is the differential aberrations that are not
measured by the wavefront sensor, which is physically separated from the common optics by a beam splitter. These
defects create a field of residual speckles in the focal plane that need to be calibrated to separate the planet signal from
the speckle noise [1].
We propose here to simultaneously estimate these aberrations and also detect the planet directly from the final science
image. To do so, we propose to couple the foreseen extreme high speed AO of an ELT planet Finder with a low speed
Self-Coherent Camera (SCC [2-5]). The SCC which is based on the principle of light coherence can both estimate the
wavefront errors and reduce speckle noise by calibration. After recalling the principle of the SCC, we present simulation
results of the SCC performance in the context of EPICS.
Extreme adaptive optics and coronagraphy are mandatory for direct imaging of exoplanets. Quasi-static aberrations
limit the instrument performance producing speckle noise in the focal plane. We propose a Self-Coherent
Camera (SCC) to both control a deformable mirror that actively compensates wavefront error, and calibrate the
speckle noise. We create a reference beam to spatially modulate the coronagraphic speckle pattern with Fizeau
fringes. In a first step, we are able to extract wavefront aberrations from the science image and correct for them
using a deformable mirror. In a second step, we apply a post-processing algorithm to discriminate the companion
image from the residual speckle field.
To validate the instrumental concept, we developed a high contrast imaging bench in visible light. We associated
a SCC to a four quadrant phase mask coronagraph and a deformable mirror (DM) with a high number
of actuators (32x32 Boston Michromachines MEMS). We will present this bench and show first experimental
results of focal plane wavefront sensing and high contrast imaging. The measurements are compared to numerical
simulations.
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