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

Abstract

Direct imaging of exoplanets represents a challenge for astronomical instrumentation due to the high-contrast ratio and small angular separation between the host star and the faint planet. Multi-star systems pose additional challenges for coronagraphic instruments due to the diffraction and aberration leakage caused by companion stars. Consequently, many scientifically valuable multi-star systems are excluded from direct imaging target lists for exoplanet surveys and characterization missions. Multi-star wavefront control (MSWC) is a technique that uses a coronagraphic instrument's deformable mirror (DM) to create high-contrast regions in the focal plane in the presence of multiple stars. Our previous paper introduced the Super-Nyquist Wavefront Control (SNWC) technique that uses a diffraction grating to enable the DM to generate high-contrast regions beyond the nominal region correctable by the DM. These two techniques can be combined to generate high-contrast regions for multi-star systems at any angular separation. As a case study, a high-contrast wavefront control (WC) simulation that applies these techniques shows that the habitable region of the Alpha Centauri system can be imaged reaching at least 8 × 10 −9 mean contrast in 10% broadband light in one-sided dark holes from 1.6-5.5λ/D.

Prospects for exoplanet imaging in multi-star systems with starshades

“…Additionally, it is worth noting that a coronagraph with only a DM and a diffraction grating can also image multi-star system using MSWC. 14,15 A rendezvous starshade probe with WFIRST enables the possibility of direct imaging of multi-star systems such as Alpha Centauri at deep raw contrast at the 10 −10 level necessary for direct imaging of rocky exoplanet. A particularly interesting option is hybrid operation of the on-axis starshade mitigating the raw contrast restrictions due to the WFIRST pupil combined with SNWC to enable off-axis leakage removal at wide angular separations using the already available DM onboard the WFIRST CGI.…”

Future Exoplanet Research: Science Questions and How to Address Them

“…They present a challenge for direct imaging since presently high contrast imaging (coronagraphy or interferometric nulling) works only for a single source. There is nevertheless a recent improvement in coronagraphy which opens the hope to take images of planets in binary star systems (Sirbu et al 2017). This is of particular importance for the alpha Cen A,B system where there is still a hope that it contains a terrestrial planet (Guedes et al 2008).…”

Sparse wavefront control: A new approach to high-contrast imaging

“…• Sparse DM can be constructed on curved surfaces with power, reducing optics complexity • Enables a path towards actuable primaries for wavefront control and DP calibration • Expands Super-Nyquist Wavefront Control dark zones 5,6 • Small Strehl degradation as a result of creating a dark zone compared to a conventional DM • Possible implementation using light actuation, eliminating the need to wire thousands of high-voltage cables • Compact and solid state, very compact packaging possible, suitable for space applications • Higher resilience to contrast degradation by actuator stroke creep than a normal DM…”

“…For that purpose, it is not necessary to modulate the entire surface of the DM; instead it is sufficient to create features with spatial frequency components that match the location of the speckle to be nulled. The ability of a Deformable Mirror with a very narrow influence function to null speckles has been described in the literature 5,6 . The high-frequency nature of narrow influence function allows SWFC to correct speckles at large angular distances from the PSF as they diffract light beyond the conventional (Nyquist-limited) outer working angle of the DM.…”