Advanced speckle sensing for internal coronagraphs

Abstract

A 4-8m diameter telescope carrying a coronagraph instrument is a leading candidate for an anticipated flagship mission to detect and characterize Earth-size exoplanets in the 2020s. 1 Many candidate coronagraph instruments have been proposed, and one is close to meeting some of the principal requirements for that mission. But the telescope and instrument will need exquisite stability and precise control of the incoming wavefront to enable detection of faint companions (10 -10 of the star) at an angular separation of 2-4 Airy radii. In particular, wavefront errors cause speckles in the image, and variations in those speckles can confound the exoplanet detection. This challenge is compounded by the background light from zodiacal dust around our Sun and the target star, which limits the speed with which we can estimate and correct the speckles. We are working on developing coherent speckle detection techniques that will allow rapid calibration of speckles on the science detector, allowing subtraction in post-processing or correction with deformable mirrors. The expected speed improvement allows a much quicker timeline for measurement & calibration, which reduces the required telescope stability requirement and eases both the flight system design and the challenge of ground testing. We will describe the experiments and summarize progress to date.

Methods and limitations of focal plane sensing, estimation, and control in high-contrast imaging

“…Thus, the controller is provided with an estimate of the electric field at the final image plane, rather than from a measurement taken by a separate wavefront sensor. The three principal methods used to sense the image plane electric field in high-contrast imaging are Gerchberg-Saxton based multiplane solutions, [19][20][21] differential fringe imaging, such as the self-coherent camera 22 or pinhole masks, 23,24 and DM probing. Multiplane solutions are typically used as an initialization step where the pupil field is flattened in an attempt to initialize control with the cleanest beam possible.…”