The Four‐Quadrant Phase‐Mask Coronagraph. II. Simulations

Abstract

In the first paper in this series, we described the principle of a coronagraph utilizing a fourquadrant phase mask and the results of numerical simulations obtained in the perfect case. In this second paper, we performed additional numerical simulations to assess in more detail the performances and limitations of this coronagraph under real conditions. The effect of geometrical parameters such as shape and size of both the phase mask and the Lyot stop is studied. We also analyze the effect of low-and high-order aberrations generated, for instance, by the atmospheric turbulence. An important issue is the wavelength dependence of the phase mask. We show that the performance decreases rapidly as the spectral bandwidth is increased, and as a consequence, we discuss the manufacturing of achromatized masks using multiple thin films. An optical concept is proposed.

Speckle Symmetry with High‐Contrast Coronagraphs

“…The FQPM can take advantage of this welcoming effect with a centrosymmetrical subtraction and yield a higher S/N than others coronagraphs. On the contrary, the AIC cannot benefit from the speckle symmetry and gives a S/N lower than that of the FQPM (if the latter is made achromatic; see Riaud et al 2001) since the variance of the wave front is increased by a factor 2. However, a real telescope pupil is provided with a central obscuration, and in that case the AIC outperforms the Lyot as well as the FQPM since it can be fitted into any centrosymmetrical aperture.…”

“…As shown in Riaud et al (2001), the achievable detection threshold of the FQPM is now only 14 mag (the star is no longer totally attenuated). In such a case, and even with very low phase defects, the diffraction term is no longer negligible regarding the speckle term.…”

“…Nulling techniques using diluted aperture (Bracewell & MacPhie 1979) are excluded from this study since the impact of the speckle noise is a quite different issue. These designs are fully described in the aforementioned pa-pers, and their respective performance has already been studied with numerical simulations assuming current AO systems (Riaud et al 2001;Baudoz, Rabia, & Gay 2000a), but also on real stars in the case of the Achromatic Interfero-Coronagraph (Baudoz et al 2000b). Therefore, the goal of this paper is not to compare these designs with respect to their intrinsic performance but only to study their behavior at high Strehl ratio when the speckle noise is very faint yet still limiting the detection threshold in many cases.…”

“…This concept has proved more efficient than the former one since the residual intensity in the geometric pupil is strictly null (Rouan, Riaud, & Baudrand 2002) in perfect conditions (no phase aberrations and filled circular aperture). In the following sections the FQPM is assumed to be achromatic (an achromatic solution is proposed in Riaud et al 2001). A preliminary mask has been manufactured and is currently characterized on a laboratory optical bench.…”

An off-axis four-quadrant phase-mask coronagraph: concept and first results

“…In fact, at least one recent coronagraphic approach is ideally suited to an unvignetted, off-axis telescope configuration -the four-quadrant phase-mask (FQPM) coronagraph (Rouan it et al 2000, Boccaletti et al 2004), which introduces phase shifts of π radians in two diametrically opposed quadrants of the focal plane ( is no opaque starlight blocker in the FQPM approach (the star sits at the crosshairs of the phase plate), this approach is suited to observations very close to the optical axis (Riaud et al 2001). However, the performance of a FQPM coronagraph is degraded by the presence of an on-axis secondary mirror (Lloyd et al 2003).…”