The achromatic chessboard, a new concept of phase shifter for Nulling Interferometry - I. theory

Direct detection of a planet around a star in the mid-IR, requires a nulling interferometer featuring an achromatic phase shift of pi on broad range. A new concept for designing such an achromatic phase shifter is presented here. The major interest of this solution is that it allows a simple design, with essentially one device per beam. The heart of the system consists in two cellular mirrors where each cell has a thickness introducing for the central wavelength, a phase shift of (2k+1)pi or of 2k pi on the fraction of the wave it reflects. Each mirror is put in one of the collimated beams of the interferometer. Because of the odd/even distribution, when recombining the two beams, a destructive interference is produced on axis for the central wavelength . If the distribution of cells thickness follows a rather simple law, based on the Pascal's triangle, then the nulling is also efficient for a wavelength not too far from the central wavelength. For instance, with two mirrors of 64x64 cells, one reaches a nulling of 1.e-6 on more than one complete octave. This could satisfy the specifications of space mission as Darwin. We also show the way to distribute the cells in the plane of the pupil for the optimum isolation of the planet image from the residual. We present the nulling performances of those various configurations.Comment: Accepted in Astronomy and Astrophysic

Bruits et Signaux.

1 - Eléments de théorie des probabilités2 - Statistique des variables aléatoires3 - AnnexesCe cours porte sur sur la statistique des variables aléatoires (351 pages)

Fundamental limitations of high contrast imaging set by small sample statistics

In this paper, we review the impact of small sample statistics on detection thresholds and corresponding confidence levels (CLs) in high contrast imaging at small angles. When looking close to the star, the number of resolution elements decreases rapidly towards small angles. This reduction of the number of degrees of freedom dramatically affects CLs and false alarm probabilities. Naively using the same ideal hypothesis and methods as for larger separations, which are well understood and commonly assume Gaussian noise, can yield up to one order of magnitude error in contrast estimations at fixed CL. The statistical penalty exponentially increases towards very small inner working angles. Even at 5-10 resolution elements from the star, false alarm probabilities can be significantly higher than expected. Here we present a rigorous statistical analysis which ensures robustness of the CL, but also imposes a substantial limitation on corresponding achievable detection limits (thus contrast) at small angles. This unavoidable fundamental statistical effect has a significant impact on current coronagraphic and future high contrast imagers. Finally, the paper concludes with practical recommendations to account for small number statistics when computing the sensitivity to companions at small angles and when exploiting the results of direct imaging planet surveys.Comment: 12 pages, 10 figures, accepted to Ap

Nulling Interferometry: A New Concept of Achromatic Phase Shifter Using Cellular Mirrors

International audienc

Nulling Interferometry: A New Concept of Achromatic Phase Shifter Using Cellular Mirrors

International audienc

Nulling Interferometry: A New Concept of Achromatic Phase Shifter Using Cellular Mirrors

International audienc

Study of an achromatic phase shifter "chessboards" in a nulling interferometer for the future characterization of exoplanets

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF