Description:

Title: Integral Field Spectroscopy for Exoplanet Imaging

Speaker: Tyler Groff, Princeton University

Host: Mel Ulmer

Abstract: Direct Imaging of exoplanets using a coronagraph has become a major field of research both on the ground and in space. Key to the science of direct imaging is the spectroscopic capabilities of the instrument and our ability to fit spectra and understand the composition of the observed planets. Direct imaging instruments tend to use an integral field spectrograph (IFS), which encodes the spectrum into a two-dimensional image on the detector. This results in more efficient detection and characterization of targets, and the spectral information is critical to achieving detection limits below the speckle floor of the imager. The most mature application of these techniques is at more modest contrast ratios on ground-based telescopes, achieving approximately 5-6 orders of magnitude suppression. In space, where we are attempting to detect Earth-analogs, the contrast requirements are more severe and the application of spectra less understood. I will discuss this evolution in exoplanet spectroscopy in the context of the The Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) at the Subaru telescope and highlight how lessons learned are driving the development for the WFIRST integral field spectrograph, which is currently in Phase A of its design. CHARIS has been delivered to the observatory and now sits behind the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) and AO188 adaptive optics systems. CHARIS has a ‘high' and 'low' resolution operating modes. The high resolution mode is used to characterize targets in J, H, and K bands at ~R70. The ‘low-resolution’ prism is meant for discovery and spans J+H+K bands (1.15-2.37 microns) with a spectral resolution of ~R18. This discovery mode has already proven better than 15-sigma detections of HR8799c,d,e when combining ADI+SDI. Using SDI alone, planets c and d have been detected in a single 24 second image. The CHARIS team is optimizing instrument performance and refining ADI+SDI recombination to maximize our contrast detection limit. In addition to the new observing modes, CHARIS has demonstrated a design with high robustness to spectral crosstalk. CHARIS is in the final stages of commissioning, with the instrument open for science observations beginning February 2017. Here we review the science case, design, on-sky performance, and specific lessons learned for extremely high contrast imagers such as crosstalk optimization, wavefront correction using the IFS image, lenslet tolerancing, the required spectral resolution to fit exoplanet atmospheres, and the utility of the spectrum to both rule out false-positives and provide higher contrast detection limits.

Keywords: Physics, Astronomy, Astrophysics