Terrestrial planet detection approaches: externally occulted hybrid coronagraphs

Abstract

Externally occulted coronagraphs have garnered widespread attention as a potentially viable approach to starlight suppression to enable direct detection and characterization of exo-solar terrestrial planets. Externally occulted coronagraphs consist of a large mask (occulter) in front of the telescope as compared to an internal coronagraph which performs all suppression within the telescope system using combinations of pupil and/or focal plane masks. The advantages of external over internal are that the (i) inner working angle (IWA) is nearly independent of wavelength and (ii) diffracted light is suppressed prior to the telescope; allowing science over a wide spectral band with a conventional telescope with little or no wavefront control. For an internal coronagraph the IWA generally increases with wavelength and scattered/diffracted light levy exquisite tolerances on wavefront, amplitude and polarization errors. An external coronagraph comes with the added complexity and expense of requiring two spacecraft, flying in formation, at separation distances of tens of thousands of kilometers. Re-targeting requires flying one or both spacecraft and aligning them to the target star and hence added fuel and time as well as closed-loop control between them. One approach may be to construct a smaller occulter possibly at closer distances and use it in series with a simple internal coronagraph, i.e. a hybrid, approach. This may simplify requirements on the external occulter but requires more precise tolerances on the telescope system. The question remains as to whether an acceptable balance between the two approaches exists. Herein we look at one approach to designing a hybrid occulter system.