摘要：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 the 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 0.75 μm and an off-axis primary mirror with a diameter of 8.5 m.Thanks to the wide working area of the mask, the result shows that terrestrial planets orbiting K and G dwarfs can be detected under the condition that the telescope pointing jitter is less than 0.01λ/D ≈ 240 as.The proposed coronagraphic system is promising for the detection of potentially habitable planets with future space off-axis hexagonally segmented telescopes.
