A comparison of solar and stellar coronagraphs that make use of external occulters

TL;DR

This paper compares solar and stellar coronagraphs with external occulters, highlighting geometric differences and the impact of design choices like petal number on the dark zone, with implications for exoplanet imaging.

Contribution

It introduces a simple shifted circular integral model for starshade diffraction and explains the reasons for internal coronagraph use in solar but not exoplanet applications.

Findings

Petal number influences the size of the dark zone.

Solar coronagraphs often include internal coronagraphs.

Starshades can be modeled with a shifted circular integral.

Abstract

Solar and stellar externally occulted coronagraphs share similar concepts, but are actually very different because of geometric characteristics. Solar occulters were first developed with a simple geometric model of diffraction perpendicular to the occulter edges. We apply this mere approach to starshades, and introduce a simple shifted circular integral of the occulter which allows to illustrate the influence of the number of petals on the extent of the deep central dark zone. We illustrate the reasons for the presence of an internal coronagraph in the solar case and its absence in the exoplanet case.