Fourth-order Coronagraph for High-contrast Imaging of Exoplanets with Off-axis Segmented Telescopes

Abstract

We propose a coronagraphic system with fourth-order null for off-axis segmented telescopes, which is sufficiently insensitive to the telescope pointing errors and finite angular diameter of the host star to enable high-contrast imaging of potentially habitable planets. The inner working angle of the coronagraphic system is close to 1λ/D, and there is no outer limit. The proposed coronagraphic system is made up of a new focal plane mask and an optimized Lyot stop with the second-order null. The new focal plane mask is an extension of the band-limited masks with a phase modulation. We construct a coronagraphic system with fourth-order null by placing two of the new coronagraph systems in succession to be orthogonal to each other. The proposed system is limited to narrow-band usage. The characteristics of the proposed coronagraph system are derived analytically, which includes: (1)the leak of stellar lights due to finite stellar diameter and pointing jitter of a telescope, and (2)the peak throughput.We achieve the performance simulations of this coronagraphic system based on these analytical expressions, considering a monochromatic light of 0.75µm and off-axis primary mirror with a diameter of 8.5m. Thanks to the wide working area of the mask, the result shows that terrestrial planets orbiting K and Gdwarfs can be detected under the condition that the telescope pointing jitter is less than 0.01λ/D ≈ 240as. The proposed coronagraphic system is promising for detection of potentially habitable planets with future space off-axis hexagonally segmented telescopes.

Spectroscopic fourth-order coronagraph for the characterization of terrestrial planets at small angular separations from host stars

“…In this section, we propose the spectroscopic fourth-order coronagraphy concept for achieving high-contrast over a wide observation bandwidth. This concept combines the coronagraph proposed by Itoh & Matsuo (2020) with two spectrographs. First, we present an overview of the concept and the new problems generated by applying the spectrographs.…”

“…However, it is technically difficult to manufacture such a complex amplitude mask, and the stellar light cannot be nulled over the wide observation bandwidth; this works only for the monochromatic light. The coronagraph proposed by Itoh & Matsuo (2020) also has the same characteristics because it applies the complex amplitude mask to the focal plane. Note, however, that the focal-plane mask modulates the complex amplitude along only one direction.…”

“…The complex amplitude is modulated along only one direction of the focal plane, α or β. Note that it is not necessary to separate the function of the entrance pupil into two functions of an independent variable; the entrance pupil applied by Itoh & Matsuo (2020) could not be written as P (x)P (y).…”

“…Note that, although Itoh & Matsuo (2020) considered off-axis segmented telescopes as the entrance pupil, this onedimensional complex amplitude mask could be applied to on-axis telescopes such as the LUVOIR-A concept and ELTs; we mask the shadows due to the secondary mirror and supports such that ξ i is constant along one direction of the entrance pupil. We present an example of the entrance pupil optimized for the LUVOIR-A telescope design in Section 3.3, and the throughput is more impacted by the mask than the off-axis telescopes, such as in the LUVOIR-B concept.…”

“…Based on this background, we propose an approach for achieving fourth-order null at the diffraction limit on the on-and off-axis segmented telescopes in a wide observation bandwidth: we develop the coronagraph proposed by Itoh & Matsuo (2020) that applies a complex amplitude mask on the focal plane instead of the combination of the pupil apodization with a focal-plane complex amplitude mask. Focusing on the fact that the coronagraphic mask modulates the complex amplitude along one direction of the focal plane, we apply a spectrograph for dispersing the white light along the direction perpendicular to the modulation one and introduce a new mask optimized for the spectrally-resolved Airy disk instead of the original complex amplitude mask.…”