We propose a new approach for high-contrast imaging at the diffraction limit using segmented telescopes in a modest observation bandwidth. This concept, named “spectroscopic fourth-order coronagraphy,” is based on a fourth-order coronagraph with a focal-plane mask that modulates the complex amplitude of the Airy disk along one direction. While coronagraphs applying the complex amplitude mask can achieve the theoretical limit performance for any arbitrary pupils, the focal-plane mask severely limits the bandwidth. Here, focusing on the fact that the focal-plane mask modulates the complex amplitude along one direction, we noticed that the mask can be optimized for each spectral element generated by a spectrograph. We combine the fourth-order coronagraph with two spectrographs to produce a stellar spectrum on the focal plane and reconstruct a white pupil on the Lyot stop. Based on the wave-front analysis of an optical design applying an Offner-type imaging spectrograph, we found that the achievable contrast of this concept is 10⁻¹⁰ at 1.2–1.5 times the diffraction limit over the wavelength range of 650–750 nm for the entrance pupil of the LUVOIR telescope. Thus, this coronagraph concept could bring new habitable planet candidates not only around G- and K-type stars beyond 20–30 pc but also around very nearby M-type stars. This approach potentially promotes the characterization of the atmospheres of nearby terrestrial planets with future on- and off-axis segmented large telescopes.

我们提出了一种在衍射极限处使用分段望远镜在适度的观察带宽下进行高对比度成像的新方法。这个名为“光谱四阶日冕仪”的概念基于带有焦平面掩模的四阶日冕仪，可沿一个方向调制艾里斑的复振幅。虽然应用复振幅掩模的日冕仪可以实现任何任意瞳孔的理论极限性能，但焦平面掩模严重限制了带宽。在这里，关注焦平面掩模沿一个方向调制复振幅这一事实，我们注意到掩模可以针对光谱仪生成的每个光谱元素进行优化。我们将四阶日冕仪与两个光谱仪相结合，在焦平面上产生恒星光谱，并在 Lyot 光阑重建白色瞳孔。基于应用 Offner 型成像光谱仪的光学设计的波前分析，我们发现该概念的可实现对比度为 10^-10在 LUVOIR 望远镜入射光瞳的 650-750 nm 波长范围内衍射极限的 1.2-1.5 倍。因此，这个日冕仪概念不仅可以带来新的宜居行星候选者，不仅可以围绕 G 型和 K 型恒星超过 20-30 pc，还可以围绕非常接近的 M 型恒星。这种方法有可能通过未来的同轴和离轴分段大型望远镜促进对附近类地行星大气的表征。