The balloon-borne Japan-United States Infrared Interferometry Experiment (JUStIInE) is a pathfinder for the first space-based far-IR interferometer. JUStIInE will mature the system-level technology readiness of spatio-spectral far-IR interferometry and demonstrate this technique with scientific observations. Operating at wavelengths from 30 to 90 µm, JUStIInE will provide unprecedented sub-arcsecond angular resolution and spectroscopic data. Our plan is to develop a cryogenic Michelson beam combiner and integrate it with an existing and tested telescope optical system and gondola from the Japanese Far-infrared Interferometric Telescope Experiment (FITE). With two JUStIInE balloon flights we plan to collect, calibrate, analyze, and publish scientific results based on the first far-IR spatio-spectral observations of young stellar objects, evolved stars, and the active galactic nucleus of NGC 1068. The NASA Astrophysics Roadmap envisages a future in which interferometry is applied across the electromagnetic spectrum, starting in the far-infrared. The Far-IR Probe recommended in the 2021 Decadal Survey presents an opportunity to take that important step. A Far-IR Probe mission based on this concept will enable us to understand terrestrial planet formation and spectroscopically study individual distant galaxies to understand the astrophysical processes that govern their evolution.
Recently, we have proposed a fourth-order coronagraph with inner working angles (IWA) of ∼ 1λ/D applicable with segmented telescopes, by deriving some complex-valued focal-plane mask patterns with the value between the interval [-1,1]. The mask pattern is implementable achromatically with a custom-patterned half-waveplate sandwiched between two linear polarizers orthogonal to each other. To enhance the system’s spectral bandwidth, we are now investigating the methods from various perspectives. One method to widen the system’s spectral bandwidth is to disperse point spread functions (PSF) incident to the focal-plane mask to the direction orthogonal to the mask pattern using a diffraction grating. Because the mask pattern is one-dimensional, we can optimize the mask pattern for each PSF dispersed by each wavelength (spectroscopic coronagraph). Another method focuses on the fact that the stellar leak due to a wide spectral bandwidth is flat at the Lyot stop and thus reducible with the successive use of the multiple coronagraph systems. Because the practical successive use of the multiple coronagraph systems requires a high off-axis throughput of the focal-plane mask, we derived new mask patterns by modifying the original pattern. This method can bring additional enhance of spatial resolution, although the current optimization limits the working angle to the separation angles of 0.7–1.4λ/D (super-resolution coronagraph or double coronagraph). Our fundamental simulation shows that both the methods can deliver a contrast of 10−10 at wavelengths of 650–750nm.
The atmospheric characterization of habitable candidates is one of the effective approaches for search for life out of the solar system. However, it is much hard by high planet-star flux contrast, 10-8 - 10-10 . A coronagraphic mask proposed by Itoh & Matsuo (2020) can suppress host stellar light but is imposed by a strict wavelength range limit of 0.3%. A spectroscopic coronagraph that combines the diffraction-limited coronagraph with a spectrograph is expected to achieve enlarges the effective bandwidth. On the other hand, a non-common path error, which is induced by the spectrograph, could limit the achievable contrast. We designed a high-accuracy spectrograph motivated for the spectroscopic coronagraph and measured its wavefront error. The common path error is 9.9 nm RMS, which is mostly caused by the alignment error between the convex grating and spherical mirror of the spectrograph. The achievable contrast of the spectroscopic coronagraph was also estimated from the non-common path error measurement. We found that the contrast of 10-8 could be achieved with a bandwidth of 5%, which is a promising result as the first step.
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