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In this paper, we discuss the advantages of using EvWaCo to observe and characterize exoplanets with a space-based telescope. In the first section, we describe the system and present the current results obtained with the EvWaCo testbed. We also illustrate the capability of this coronagraph to detect the companion 30,000 times (respectively, 100,000 times) fainter than the central star at distances equal to 15 Airy radii (respectively, 30 Airy radii) from the PSF center in polychromatic and unpolarized light.
In the second section, we describe the design of the prototype dedicated to the on-sky tests of the instrument with the 2.4 m Thai National Telescope at horizon 2020. This prototype has been designed with the objective to reach a contrast equal to a few 10-4 at the inner working angle (IWA) equal to 3 λ/D from the star PSF center while observing through the atmosphere over the full photometric I-band. This prototype will include an adaptive optics specified to reach at λ ≈ 800 nm a Strehl ratio > 0.8 for magnitude m < 7.
In the third section, we show the theoretical performance of EvWaCo: a contrast comprised between a few 10-6 and 10-7 in the I-Band between 3 λ/D and 10 λ/D in the I-Band for an IWA equal to 3 λ/D with a Gaussian apodization in unpolarized light. We also show that similar contrasts performance are obtained in the V-, R-, bands, thus illustrating the EvWaCo quasi-achromaticity. Finally, we discuss the advantages and the limitation using the proposed concept for space-based observations and spectral characterization of exoplanets.
With a computational cost comparable to state-of-the-art but sub-optimal methods used in solar AO, our wavefront sensing algorithm estimates the local slopes and their covariances following a maximum likelihood registration method.
Themis AO system has a modest size but can be used to assert the benefits of maximum a posteriori (MAP) wavefront sensing and control,2, 3 of accounting of the covariances of the measure and of the temporal correlation of the turbulent wavefront.
The EvWaCo concept has been demonstrated and this instrument is achromatic over the I-band of the Johnson- Cousins photometric system in unpolarized light. We have measured over this photometric band an Inner Working Angle (IWA) equal to 6 λ/D and contrasts of a few 10-6 at distances greater than 10 Airy radii from the star Point Spread Function (PSF) center.
This paper describes the continuation of the project, from this setup of demonstration to the first prototype operating on the sky at horizon 2020. The objective is to show the capability of the full system to provide IWA and raw contrasts close to the state-of-art performance with the Thai National Telescope, by observing through an unobstructed elliptical pupil of major axis length equal to 1 m. The system will demonstrate over the full I-band an IWA close to 3 λ/D and raw contrasts equal to a few 10-4 at a distance equal to the IWA from the PSF.
Status of an extreme adaptive optics testbench using a self-referenced Mach-Zehnder wavefront sensor
The residual speckles1, 2 set the most serious limitation in the detection of exo-planets in high contrast coronographic images provided by instruments such as SPHERE3 at the VLT, GPI4, 5 at Gemini, or SCExAO6 at Subaru. A number of post-processing methods have been proposed to remove as much as possible of the residual speckles while preserving the signal from the planets. These methods exploit the fact that the speckles and the planetary signal have different temporal and spectral behaviors. Some methods like LOCI7 are based on angular differential imaging8 (ADI), spectral differential imaging9, 10 (SDI), or on a combination of ADI and SDI.11 Instead of working on image differences, we propose to tackle the exo-planet detection as an inverse problem where a model of the residual speckles is fit on the set of multi-spectral images and, possibly, multiple exposures. In order to reduce the number of degrees of freedom, we impose specific constraints on the spatio-spectral distribution of stellar speckles. These constraints are deduced from a multi-spectral Taylor series expansion of the diffraction pattern for an on-axis source which implies that the speckles are a combination of spatial modes with deterministic chromatic magnification and weighting. Using simulated data, the efficiency of speckle removal by fitting the proposed multi-spectral model is compared to the result of using an approximation based on the singular value decomposition of the rescaled images. We show how the difficult problem to fitting a bilinear model on the can be solved in practise. The results are promising for further developments including application to real data and joint planet detection in multi-variate data (multi-spectral and multiple exposures images).
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