Phase-Occultation Nulling Coronagraphy

The search for life via characterization of earth-like planets in the habitable zone is one of the key scientific objectives in Astronomy. We describe a new phase-occulting (PO) interferometric nulling coronagraphy (NC) approach. The PO-NC approach employs beamwalk and freeform optical surfaces internal to the interferometer cavity to introduce a radially dependent plate scale difference between each interferometer arm (optical path) that nulls the central star at high contrast while transmitting the off-axis field. The design is readily implemented on segmentedmirror telescope architectures, utilizing a single nulling interferometer to achieve high throughput, a small inner working angle (IWA), sixth-order or higher starlight suppression, and full offaxis discovery space, a combination of features that other coronagraph designs generally must trade. Unlike previous NC approaches, the PO-NC approach does not require pupil shearing; this increases throughput and renders it less sensitive to on-axis common-mode telescope errors, permitting relief of the observatory stability required to achieve contrast levels of � 10 −10 . Observatory operations are also simplified by removing the need for multiple telescope rolls and shears to construct a high contrast image. The design goals for a PO nuller are similar to other coronagraphs intended for direct detection of habitable zone (HZ) exoEarth signal: contrasts on the order of 10 −10 at an IWA of � 3λ/D over � 10% bandpass with a large (> 10 m) segmented aperture space-telescope operating in visible and near infrared bands. This work presents an