Techniques for High-Contrast Imaging in Multi-Star Systems. I. Super-Nyquist Wavefront Control

Abstract

Extra-solar planets direct imaging is now a reality with the deployment and commissioning of the first generation of specialized ground-based instruments (GPI, SPHERE, P1640 and SCExAO). These systems allow of planets 10 7 times fainter than their host star. For space-based missions (EXCEDE, EXO-C, EXO-S, WFIRST), various teams have demonstrated laboratory contrasts reaching 10 −10 within a few diffraction limits from the star. However, all of these current and future systems are designed to detect faint planets around a single host star or unresolved multiples, while most non M-dwarf stars such as Alpha Centauri belong to multi-star systems. Direct imaging around binaries/multiple systems at a level of contrast allowing Earth-like planet detection is challenging because the region of interest is contaminated by the hosts star companion as well as the host Generally, the light leakage is caused by both diffraction and aberrations in the system. Moreover, the region of interest usually falls outside the correcting zone of the deformable mirror (DM) for the companion. Until now, it has been thought that removing the light of a companion star is too challenging, leading to the exclusion of binary systems from target lists of direct imaging coronographic missions.In this paper, we will show different new techniques for high-contrast imaging of planets around multi-star systems and detail the Super-Nyquist Wavefront Control (SNWC) method, which allows to control wavefront errors beyond nominal control region of the DM. Using the SNWC we reached contrasts around 5 × 10 −9 in a 10% bandwidth.

A method to directly image exoplanets in multi-star systems such as Alpha-Centauri

“…Thus, the elongation of the proxy star should help, rather than hinder, broadband control in that region. Indeed, as we already showed in the computer simulations in, 10 SNWC works fairly fell in broadband, at least under certain assumptions. Although we have not yet demonstrated broadband operation in general, the above arguments suggest that broadband challenges are entirely a function of stellocentric angle and not whether super-Nyquist or sub-Nyquist control is used.…”

“…In Thomas et al 2015, 10 we presented a technique called "Super-Nyquist Wavefront Control" (SNWC) which is a solution to the first challenge (limited field of view, or FOV, correctable by a given deformable mirror), and demonstrated computer simulations of SNWC. We also outlined proposed solutions to the other two challenges, which we call, respectively, Multi-Star Wavefront Control (MSWC), and Multi-Star Super-Nyquist Wavefront Control (MSSNWC).…”

Techniques for High-contrast Imaging in Multi-star Systems. II. Multi-star Wavefront Control

“…However, for stars with wider angular separations or a larger telescope aperture it is likely that the dark hole is located beyond the controllable Nyquist-limit for one of the stars. In this case, MSWC can be combined with the SNWC technique (Thomas et al 2015) that allows using higher quilting orders introduced by a diffraction grating to replicate the PSF and extend the controllable spatial frequencies to the Super-Nyquist regime (for example, creating a dark zone at 100 λ/D with a 32×32 DM). As part of the specific case study in this paper, we will demonstrate how SNWC can be combined with MSWC to gain complete coverage of the dynamically stable region in which potential circumstellar exoplanets could exist in the Alpha Centauri system.…”

“…To enable SNMSWC a mild diffraction grating must be introduced into the optical model (Thomas et al 2015). Here, we consider a grating already existing on many DMs, with phase arising from the DM printthrough and amplitude due to etch holes.…”

“…Section 4 describes the MSWC technique, and as an implementation example we extend the formalism of the widely-used single star electric field conjugation (EFC) algorithm to the multi-star case. In an earlier companion paper (Thomas et al 2015), the Super-Nyquist Wavefront Control (SNWC) technique was introduced. The SNWC tech-nique is briefly reviewed in Section 5 showing how enabling suppression of residual speckles beyond the nominal Nyquist controllable region is possible.…”