Focal Plane Wavefront Estimation Using an Integral Field Spectrograph

As wavefront control techniques continue to improve, coronagraphic imaging of exoplanets is becoming a promising technique for both ground and space observatories. The inherent sensitivity of a coronagraph to perturbations in the electric field means that residual quasi-static speckles appear in the image plane, constraining the relative brightness (or contrast) and angular separation of an exoplanet relative to its parent star. On the ground, the quasi-static speckles are residual aberrations that the non-common path wavefront sensors cannot correct. In the case of a space observatory we expect these perturbations to largely be from temperature fluctuations and vibration within the spacecraft. In either case we require some method to remove this energy, improving our detection limit for the exoplanet. Post-processing techniques have been applied to data taken from both ground and space based telescopes to subtract these speckles with a high degree of success even when not using a coronagraph, but they can skew our spectral measurements and become less effective as the angular separation decreases. Therefore, we seek wavefront estimation and control techniques based on the focal plane electric field to correct for these quasi-static speckles. The relative timescale of quasi-static speckle evolution compared to the time required for estimation and control will ultimately determine the residual intensity in our search area, bounding our lowest achievable contrast. We wish to spectrally characterize these targets so that we might learn about its composition. We investigate combining these two concepts using an integral field spectrograph (IFS). An IFS images each wavelength simultaneously, providing spectral diversity information in a single image. The spectral data provides a chromatic snapshot in time for estimating the electric field, potentially lowering the residual energy of the quasi-static speckles.