The combined coronagraph and wavefront control system is characterized by the contrast, inner working angle, and stability achieved. Contrast is the degree to which the instrument can suppress scattered and diffracted starlight in order to reveal a faint companion. Inner Working Angle (IWA) is the smallest angle on the sky at which it can reach its designed contrast. This angle is typically only a few times larger than the theoretical diffraction limit of the telescope. The resulting residual stellar halo must also be stable over the time scale of an observation, so that the halo can be subtracted to reveal an exoplanet or disk.
In principle, with a well-characterized and stable point spread function, various subtraction methods that have been developed and used on both ground and space images can be employed to average the background photon noise and extract faint planets that are below the raw contrast level. The recent history of planet imaging shows that recovering planets with up to factors of 10 fainter contrast than the background is regularly accomplished, both on large ground telescopes and from the Hubble Space Telescope. We thus characterize the coronagraph instrument by its detection limit, that is, the limiting magnitude of a recoverable planet relative to the star from the combination of the coronagraph and wavefront control and data processing.
Only a handful of exoplanets have been imaged to date from the ground and with the Hubble Space Telescope. We have obtained spectra on an even smaller number. All of them are gas giants more massive than Jupiter that reside at great distances (> 10 AU) from their parent stars. Additionally, observational biases have restricted those detections to very young planets — typically less than a hundred million years — that shine brightly at infrared wavelengths through their interior heat. This warm phase constitutes only a small fraction of a planet’s lifetime immediately following its formation. Therefore, the far more numerous population of mature, evolved planets has eluded the current generation of high-contrast imaging instruments. WFIRST provides the first opportunity to observe and characterize planets physically resembling those in our Solar System, spanning ages up to several billion years. The coronagraph on WFIRST will operate in visible wavelengths at flux ratios down to a few parts per billion and an inner working angle of less than 0.2 arcseconds. It will be capable of imaging a dozen known radial velocity planets in reflected starlight at orbital separations ranging from 2 to 10 AU.
Spectroscopic observations of directly imaged exoplanets are challenging, partly due to the extremely low irradiances of planets observed in reflected starlight. Nevertheless, because exoplanet characterization is the chief scientific motivation for advancing high-contrast imaging technology, the WFIRST Coronagraph Instrument is designed to include a modest spectroscopy capability. The prism spectroscopy mode on CGI will disperse the point spread function of an individual exoplanet over the wavelength range 675—785 nm with a spectral resolving power of l/Dl³50. For Jupiter-sized planets with flux ratios in the detection range of CGI, this spectroscopy mode will measure the shape and depth of the methane absorption feature at 720 nm. In conjunction with atmosphere modeling, such observations can place rudimentary constraints on methane gas abundances and cloud properties.
The coronagraph will also be sensitive to debris disks with a few times the solar system's level of dust in the habitable zones of nearby (~10 pc) sun-like stars. The high sensitivity and spatial resolution (0.05 arcsec is 0.5 AU at 10 pc) of WFIRST CGI images will map the structure of these disks, revealing asymmetries, belts, and potentially gaps due to unseen planets. CGI will make the most sensitive measurements yet of the amount of dust in or near the habitable zones of nearby stars. This is important for assessing the difficulty of imaging Earth-like planets with future missions as well as for understanding nearby planetary systems.