KEYWORDS: Prisms, Calibration, Polarizers, Polarization, Spectroscopes, Spectral resolution, Equipment, Target detection, Space telescopes, Signal to noise ratio
The Nancy Grace Roman Space Telescope Coronagraph Instrument (CGI) will demonstrate spectroscopy of planets and polarization measurements of disks. The spectroscopy and polarization modes utilize Amici and Wollaston prism designs. The spectroscopy mode, designed and built and Goddard Space Flight Center (GSFC), has a resolution of R50 in 15% bands centered at 660nm and 730nm. The Wollaston design and optics are contributed by the Japanese Aerospace Exploration Agency, with final alignment and testing at GSFC. We present the requirements, ground-to-orbit calibration, and deployable slit operations. We also detail on the design, results from the as-built flight assemblies.
Various types of high-contrast imaging instruments have been proposed and developed for direct detection of exoplanets by suppressing nearby stellar light. Stellar speckles due to wavefront aberration can be suppressed by the appropriate wavefront control, called the dark hole control. However, the speckles, which fluctuate faster than the dark hole control due to atmospheric turbulence in ground-based telescopes or instrument deformation caused by temperature changes in space telescopes, cannot be suppressed by the control and remain in focal plane images. The Coherent Differential Imaging on Speckle Area Nulling (CDI-SAN) method was proposed to overcome such fast fluctuating speckles and detect exoplanetary light. We constructed an optical setup in a laboratory to demonstrate the CDI-SAN method. With the dark hole control and the CDI-SAN method, we achieved 10−8 level of contrasts.
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.
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.
GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) is a new mission concept for ISAS/JAXA’s strategic L-class mission program in the 2030s. With a 1.2 m aperture, a 50 K cryogenic space telescope will have a < 1, 400 arcmin2 wide-field camera with 6 bands in the 2–10 μm wavelength range and a high-dispersion spectrometer with a wavelength resolution of < 30, 000 in the 10–18 μm band. The cryogenic infrared mission concept of GREX-PLUS is based on SPICA, exploiting the technical resources so far studied and developed, such as an active cooling system. The high-dispersion spectrometer of GREX-PLUS is based on the high-dispersion channel of the SPICA Mid-Infrared Instrument (SMI). The wide-field camera of GREX-PLUS is also based on previous concept studies for the ISAS/JAXA’s WISH mission concept. GREX-PLUS is a concept proposal for a Japan-led mission but international collaborations are also welcome.
The mid/far infrared hosts a wealth of spectral information that allows direct determination of the physical state of matter in a large variety of astronomical objects, unhindered by foreground obscuration. Accessing this domain is essential for astronomers to much better grasp the fundamental physical processes underlying the evolution of many types of celestial objects, ranging from protoplanetary systems in our own milky way to 10-12 billion year old galaxies at the high noon of galaxy formation in our universe. The joint ESA/JAXA SPICA mission will give such access for the astronomical community at large, by providing an observatory with unprecedented mid- to far-infrared imaging, polarimetric and spectroscopic capabilities.
Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems.
SPICA is a mission concept aimed at taking the next step in mid- and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultra-sensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions.
The SPICA instrument complement offers spectral resolving power ranging from ~50 through 11000 in the 17-230 µm domain as well as ~28.000 spectroscopy between 12 and 18 µm. Additionally, SPICA will be capable of efficient 30-37 µm broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 µm range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of ~5x10-20 W/m2 (5σ/1hr) - at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.
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