The mid-infrared spectrometer and camera transit spectrometer (MISC-T) is one of the three baseline instruments for Origins Space Telescope (Origins) and provides the capability to assess the habitability of nearby exoplanets and search for signs of life. MISC-T employs a densified pupil optical design, and HgCdTe and Si:As detector arrays. This optical design allows the instrument to be relatively insensitive to minor line-of-sight pointing drifts and telescope aberrations, and the detectors do not require a sub-Kelvin refrigerator. MISC-T has three science spectral channels that share the same field-of-view by means of beam splitters, and all channels are operated simultaneously to cover the full spectral range from 2.8 to 20 μm at once with exquisite stability and precision (<5 ppm between 2.8 to 11 μm, <20 ppm between 11 and 20 μm). A Lyot-coronagraph-based tip–tilt sensor located in the instrument fore-optics uses the light reflected by a field stop, which corresponds to 0.3% of the light from the target, to send fine pointing information to the field steering mirror in the Origins telescope. An additional MISC Wide Field Imager (WFI) is studied as an upscope option for the Origins. MISC-WFI offers a wide field imaging (3 ′ × 3 ′ ) and low-resolution spectroscopic capability with filters and grating-prisms (grisms) covering 5 to 28 μm. The imaging capability of the MISC-WFI will be used for general science objectives. The low-resolution spectroscopic capability in MISC-WFI with a resolving power R ( = λ / Δλ) of a few hundreds will be used to measure the mid-infrared dust features and ionic lines at z up to ∼1 in the Origins mission’s Rise of Metals and Black Hole Feedback programs. The MISC-WFI also serves as a focal plane pointing and guiding instrument for the observatory, including when the MISC-T channel is performing its exoplanet spectroscopy observations.
We report on the laboratory experiments of a densified pupil spectrograph designed for mid-infrared transit spectroscopy of exoplanets. We developed a testbed consisting of a blackbody infrared light source, a densified pupil spectrograph, and a prototype JWST Si:As Impurity Band Conduction (IBC) detector array to simulate observations of a planet’s host star. In order to thermally stabilize the measurement system, we installed all of the components in a large cryogenic dewar and controlled the temperatures of the thermal source and the Si:As IBC detector. The characteristics of the spectrum formed on the detector were consistent with the designed values. The photometric precision of the densified pupil spectrograph was 14 ppm on average over the whole observing wavelength range of 8.5 to 10.5 μm. The systematic noise component of the spectrograph hidden behind the transit spectrograph was 11 ppm.
Although segmented mirror technology is essential for realizing large telescope, it has potential problems, which should be solved, on the difficulties for aligning independently-supported mirrors and the leakage of thermal radiation from the mirror gaps. To overcome them, we proposed Alignment-Free Gapless Segmented Mirror (AFGSM). AFGSM is an assembled mirror consisting of several mirrors that are mechanically connected to each other with metal bolts. With the tightly fixed mirrors and no gap between mirrors, both the alignment work and the thermal radiation can be reduced. Nakahori et al. (2018) fabricated a 300mm flat AFGSM with 6 ribbed fan-shaped pieces. It uses cordierite CO720, developed by Kyocera. Cordierite is thought to be an alternative choice for developing segmented mirror because it has not only a low coefficient of thermal expansion (0 ± 0.02 ppm/K) but also various advantages on a very small aged deterioration in dimension, a high Young's modulus, and a high thermal conductivity compared to classical low expansion glasses such as ULE or Zerodur[1-6]. The first trial of AFGSM by Nakahori et al., however, could not pass environmental tests: the large surface deformation around the central hole (~ 7.12 um (PV)) and the misalignment between segmented mirrors (~ 0.5um) were occurred after a thermal cycle test and a vibration test. Therefore, we newly proposed an improved AFGSM that has intentionally-produced gaps (< 0.3 mm) to prevent the surface deformation due to the thermal stress generated by the variation of the ambient temperature or the stress release among segmented mirrors by the vibration. The thermal radiation from the small gaps are negligible compared to that of classical segmented mirrors. In this paper, we present the design of improved AFGSM and report results of the optical performances before and after environmental tests.
Reflective optical system free from chromatic aberration is essential for astronomical instruments, which usually require wider wavelength coverage. However, it cannot always be the optimum choice compared with refractive optical system in terms of cost-effectiveness because mirrors require high surface accuracy and also because non-co-axial systems force tough alignment work. This dilemma could be overcome by a monolithic reflective optical system made entirely of cordierite CO-720, a ceramic material by Kyocera, which is the first material that offers both high-precision 3D-shaping and surface polishing for optical quality. This material also possesses a very low coefficient of thermal expansion (CTE) enabling a genuine athermal system useful for various astronomical applications. This athermality could make a significant breakthrough especially for cryogenic infrared instruments since optical systems made of cordierite are expected to keep as-built performance throughout the cooling process, providing extremely high wavefront accuracy that has never been possible at cryogenic temperature with conventional optical systems made of glasses or metals. In this paper, we report the first cryogenic optical testing of a small cordierite-made imaging optical system that was simply assembled with mechanical accuracy at room temperature. We confirmed that the diffraction-limited optical performance is kept even down to ~80K as built in the room temperature.
WINERED is a highly sensitive near-infrared (NIR) high-resolution spectrograph. The spectral coverage is 0.90 to 1.35μm (z, Y, J-bands) and the spectral resolutions are R = 28,000 (WIDE-mode, covering an entire WINERED’s wavelength region with a single exposure) and R = 70,000 (HIRES-modes, covering either Y- or J-band with a single exposure). Owing to the high-throughput optics (> 0.5) and the very low noise of the system, WINERED has the potential to detect the faintest objects when attached to 10 m class telescopes as reported in the previous SPIE meeting. In the beginning of 2017, WINERED was relocated from the 1.3 m Araki telescope in Koyama Astronomical Observatory, Japan, to the ESO 3.58 m New Technology Telescope (NTT) in La Silla Observatory, Chile, and began its scientific observations. By March of 2008, 30 nights in total were allocated for observation with the WINERED at the NTT. To further improve observational efficiencies at the NTT, we upgraded and refined several units of WINERED. New slits were installed to realize a medium spectral resolution and the better correction of the distorted echellogram, the grating holder for the mosaicked high-blazed echelle gratings were modified, the ghost problems observed on the HIRES-Y mode were fixed, and the I/F mechanical parts were fabricated for easy and highlyreplicable attachment to the NTT. After verifying a few performances critical for the sensitivity of the new telescope, the background ambient radiation at the NTT, which determines the limiting magnitude because WINERED is a warm instrument with no cold stop, is very similar (~0.1 photons sec-1 pixel-1 at 290 K and ~0.04 photons sec-1 pixel1 at 280 K) to those measured at Kyoto. The stability in wavelength, which could degrade the signal-to-noise ratios (SNRs) by artificial spiky-noises generated in the subtraction and correction of telluric emission/absorption lines, is measured to be less than 0.2 pixels during an observational run, although these can be further reduced by the crosscorrelation method which are applied for spectra taken at different timings during reduction. WINERED routinely provides spectra of the SNR > 500 for bright stars, and realized the detection of those of SNR = 30 for faint objects of J = 16.4 mag (for WIDE mode) and J=15.0 (for HIRES mode) with the exposure time of 8 hours using the narrowest slit at the NTT (even without AO).