Phase-shifting coronagraph

Abstract

With the recent commissioning of ground instruments such as SPHERE or GPI and future space observatories like WFIRST-AFTA, coronagraphy should probably become the most efficient tool for identifying and characterizing extrasolar planets in the forthcoming years. Coronagraphic instruments such as Phase mask coronagraphs (PMC) are usually based on a phase mask or plate located at the telescope focal plane, spreading the starlight outside the diameter of a Lyot stop that blocks it. In this communication is investigated the capability of a PMC to act as a phase-shifting wavefront sensor for better control of the achieved star extinction ratio in presence of the coronagraphic mask. We discuss the two main implementations of the phase-shifting process, either introducing phase-shifts in a pupil plane and sensing intensity variations in an image plane, or reciprocally. Conceptual optical designs are described in both cases. Numerical simulations allow for better understanding of the performance and limitations of both options, and optimizing their fundamental parameters. In particular, they demonstrate that the phase-shifting process is a bit more efficient when implemented into an image plane, and is compatible with the most popular phase masks currently employed, i.e. fourquadrants and vortex phase masks.

Wavefront Sensing by a Common-Path Interferometer for Wavefront Correction in Phase and Amplitude by a Liquid Crystal Spatial Light Modulator Aiming the Exoplanet Direct Imaging

“…To find an incident wavefront characteristic, we solve the inverse problem substantially easier. We found, however, much closer similarity of PSI with [25,26]. The interfero-coronagraph in Figure 2a has the 180 • scheme, the Figure 2b scheme has a variable rotational shift, and the Figure 2c scheme has a fixed angle of rotation shear.…”

“…To find an incident wavefront characteristic, we solve the inverse problem substantially easier. We found, however, much closer similarity of PSI with [25,26].…”

New concepts for calibrating non-common path aberrations in adaptive optics and coronagraph systems

“…So far, the most well-known PMCs probably are the Roddier and Roddier, four-quadrants, and vortex coronagraphs, whose diffractive properties were discussed extensively in Refs. [4] and [14]. They show that most of the starlight is redirected outside of the pupil area (as schematized by the orange beam in Figure 4-b), which entails the presence of a Lyot stop preventing starlight from reaching the science detector 1 .…”

“…Numerical simulations in Ref. [4] demonstrated that for efficient WFE reconstruction their apertures should be oversized by a factor of three at least. Such requirement may turn even more critical if a spectrograph has to be coupled with the coronagraph.…”

“…It may imply the development of new types of WFS [1] or modifications of the optical architecture of the instrument. Here the goal is to push the beam splitter closer to the science detector [2][3], and eventually behind a coronagraph phase mask [4]. Some of these concepts have already been partly validated on-sky [5].…”