Precise wavefront correction with an unbalanced nulling interferometer for exo-planet imaging coronagraphs

Abstract

Context. Coronagraphs of high dynamical range used for direct exo-planet detection (10 9 -10 10 contrast) on small angular separation (few λ/D units) usually require an input wavefront quality of approximately ten thousandths of a wavelength rms. Aims. We propose a novel method based on a pre-optics setup that behaves partly as a low-efficiency coronagraph, and partly as a high-sensitivity wavefront aberration compensator (phase and amplitude). The combination of the two effects results in a highly accurate corrected wavefront. Methods. First, an (intensity-) unbalanced nulling interferometer (UNI) performs a rejection of part of the wavefront electric field. Then, the input aberrations of the recombined output wavefront are magnified. Because of the unbalanced recombination scheme, aberrations can be free of phase singular points (zeros) and can therefore be compensated by a downstream phase and amplitude correction (PAC) adaptive optics system, using two deformable mirrors. Results. In the image plane, the central star's peak intensity and the noise level of its speckled halo are reduced by the UNI-PAC combination: the output-corrected wavefront aberrations can be interpreted as an improved compensation of the initial (eventually already corrected) incident wavefront aberrations. Conclusions. The important conclusion is that not all of the elements in the optical setup using UNI-PAC need to reach the λ/10 000 rms surface error quality.

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

“…They are part of the many methods proposed in the past few years, while other concepts rely on interferometry (Angel 1994;Nishikawa et al 2008;Galicher et al 2008Galicher et al , 2010, speckle-nulling technique (Trauger & Traub 2007), coronagraphic phase diversity (Sauvage et al 2010b), analysis of the light blocked by a coronagraph (Guyon et al 2009;Vogt et al 2011), or the Zernike phase contrast principle (Bloemhof & Wallace 2003;Wallace et al 2011;N'Diaye et al 2012). Several post-processing methods have also been developed to exploit the chromatic behavior of speckles (Marois et al 2000;Sparks & Ford 2002), using their rotation with respect to the sky (Marois et al 2006) or based on algorithms of optimal recombination of images (Lafrenière et al 2007).…”

Achromatic Eight-Octant Phase-Mask Coronagraph Using Photonic Crystal

“…The numerical simulations also suggest that extremely small phase aberration less than λ/10,000 rms will be required to achieve the contrast of 10 −10 for detecting extrasolar Earthlike planets. Thus, adaptive optics systems dedicated for highcontrast imagers will play an important role in the terrestrial planet finding missions (Bordé & Traub 2006;Give'on et al 2006;Nishikawa et al 2008). We note that differential imaging techniques can also be used for suppressing the speckle noise (Racine et al 1999;Baba & Murakami 2003;Marois et al 2006;Murakami et al 2006Murakami et al , 2007Zubko et al 2007).…”

Low-contrast pre-coronagraph for extra contrast of dark-hole

“…The low-contrast pre-coronagraph (LPC) is a new style of the unbalanced nulling interferometer (UNI) which was developed for precise wavefront control (Nishikawa et al 2008). The LPC is used in the four-stage coronagraph system: the first deformable mirror (DM), the LPC, the second DM, and the main coronagraph, to obtain an additional contrast to the main coronagraph.…”