Lightweight X-ray telescopes with high-angular resolution and a large effective area are essential for elucidating the physical mechanisms behind the evolution of super-massive black holes. To achieve such X-ray telescopes, we are developing X-ray mirrors using carbon fiber reinforced plastic (CFRP); CFRP is a lightweight but strong material that can be processed into a variety of shapes. However, CFRP has a finely textured surface, known as print-through, which is too rough to reflect X-rays effectively. To mitigate the print-through issue, we developed a method to form a thin amorphous nickel phosphide (NiP) layer on the surface of CFRP. The NiP surface was then finished using ultra-precision machining technology to achieve the required surface roughness for X-ray reflection. We fabricated a CFRP plane mirror, and an X-ray reflectivity measurement for the plane mirror using the ISAS X-ray beam line confirmed that the surface roughness was approximately 1.5 nm. Additionally, we made a Wolter-I type mirror and demonstrated that it could focus X-rays. However, the half-power width of the reflected X-ray image was 9.6 arcmin.
Conventional reflection echelle gratings of surface relief (SR) type have significantly different spectral characteristics for s- and p-polarizations. Another problem was the phenomenon of an anomaly, in which the efficiency of p-polarization of a specific diffraction order is greatly reduced due to the influence of surface plasmons. The reflector facet transmission (RFT) grating is a new echelle grating of transmission type in which the incident light from one side facet of the sawtoothshaped grating is reflected by the other side facet, and the diffracted light is exited from the flat back side of the grating. The RFT grating achieves a large angular dispersion, higher diffraction efficiency and wider bandwidth than SR echelle gratings. Furthermore, since the RFT grating can be placed near the collimator and camera (imaging) optical system, the astronomical instrument can be made smaller than the reflective echelle grating. We prototyped a RFT grating with hard resin by ultra-precision cutting using a diamond tool. As a result of measurement of the diffraction efficiency, it demonstrates that a RFT grating is highly efficient and has small scattering losses.
A grism (direct-vision diffraction grating) combines a diffraction grating and prism to make any order and wavelength go straight. By setting a grism into the collimating beam (necessary for an astronomical instrument to place pupil mask, filters, etc.) of an imaging instrument, it becomes possible to easily perform spectroscopic observations. The prism array is a device aimed at increasing the angular dispersion (resolving power) of a grism within the limited size of the collimating beam part of the instrument.
Theoretical calculations predict that high-resolution spectroscopy of H2O gas lines in the mid-infrared region is the most promising method to observationally identify the snow-line, which has been proposed as the critical factor separating gas giants from solid planets in the planetary formation process. This requires the spectroscopic observations from space with R = λ/Δλ ≥ 30, 000. For this purpose, we propose a mid-infrared (10-18 μm) high-resolution spectrometer to be onboard the GREX-PLUS (Galaxy Reionization EXplorer and PLanetary Universe Spectrometer) mission. We are developing "immersion grating” spectroscopy technology for high-resolution spectroscopy in space. We have chosen CdZnTe as a candidate for the optical material. We report the current status of the development of the CdZnTe immersion grating, including evaluation of its optical properties (absorption coefficient and refractive index) at cryogenic temperatures, development of an anti-reflection coating with a moth-eye structure for wide-wavelength coverage, and verification of machinability for grating production. We plan to make a prototype spectrometer to demonstrate the capability of the immersion grating with ground-based observations in the N-band (λ = 8–13 μm) and beyond.
Integral field spectroscopy (IFS) is an observational method for obtaining spatially resolved spectra over a specific field of view (FoV) in a single exposure. In recent years, near-infrared IFS has gained importance in observing objects with strong dust attenuation or at a high redshift. One limitation of existing near-infrared IFS instruments is their relatively small FoV, less than 100 arcsec2, compared with optical instruments. Therefore, we developed a near-infrared (0.9 to 2.5 μm) image-slicer type integral field unit (IFU) with a larger FoV of 13.5×10.4 arcsec2 by matching a slice width to a typical seeing size of 0.4 arcsec. The IFU has a compact optical design utilizing off-axis ellipsoidal mirrors to reduce aberrations. Complex optical elements were fabricated using an ultra-precision cutting machine to achieve root mean square surface roughness of less than 10 nm and a P-V shape error of less than 300 nm. The ultra-precision machining can also simplify the alignment procedures. The on-sky performance evaluation confirmed that the image quality and the throughput of the IFU were as designed. In conclusion, we successfully developed a compact IFU utilizing an ultra-precision cutting technique, almost fulfilling the requirements.
This conference presentation was prepared for the Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation V conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
A two-in-one optical element consisting of a planar pick-off mirror (16×14 mm2) and a slice mirror array (twentysix 0.52 mm-wide and 18 mm-long mirror facets) was developed as a component for a near-infrared integral field unit: SWIMS-IFU for a near-infrared imager and spectrograph SWIMS for TAO 6.5 m telescope. The two optical entities were formed on a single bulk of aluminum alloy using a 5-axis ultraprecision machine and a specialized pair of diamond cutting tools. The relative position and orientation of the two optical entities were controlled to satisfy the tight optical tolerance, by precise measurement of tool tip positions with respect to the axes of the machine tool. Regarding measurement, an X-ray CT scanner was introduced as a solution to a major problem in the evaluation of complex monolithic optics, where some facets cannot be measured due to geometric constraints. The diamond-machining and measurement methods proposed in this study will be useful also for more complex monolithic optics to be developed in the future.
MIMIZUKU is the first-generation mid-infrared instrument for the TAO 6.5-m telescope. It has three internal optical channels to cover a wide wavelength range from 2 to 38 µm. Of the three channels, the NIR channel is responsible for observations in the shortest wavelength range, shorter than 5.3 µm. The performance of the NIR channel is evaluated in the laboratory. Through the tests, we confirm the followings: 1) the detector (HAWAII 1RG with 5.3-µm cutoff) likely achieves ∼80% quantum efficiency; 2) imaging performance is sufficient to achieve seeing-limit spatial resolution; 3) system efficiencies in imaging mode are 2.4–31%; and 4) the system efficiencies in spectroscopic modes is 5–18%. These results suggest that the optical performance of the NIR channel is achieved as expected from characteristics of the optical components. However, calculations of the background levels and on-sky sensitivity based on these results suggest that neutral density (ND) filters are needed to avoid saturation in L ′ - and M′ -band observations and that the ND filters and the entrance window, made of chemical-vapor-deposition (CVD) diamond, significantly degrade the sensitivity in these bands. This means that the use of different window materials and improvements of the detector readout speed are required to achieve both near-infrared and long-wavelength mid-infrared (>30 µm) observations.
We are developing an image-slicer type integral field unit (IFU), SWIMS-IFU, for SWIMS (Simultaneous-color Wide-field Infrared Multi-object Spectrograph), a near-infrared instrument for TAO 6.5 m telescope. SWIMS- IFU divides a field-of-view of 16:006 12:008 into 26 slices with a width of 0:005, which is the largest FoV among near-infrared IFUs on 8 m class telescopes. It is also capable of obtaining entire near-infrared spectra from 0.9 to 2.5 m with R1000 with a single exposure. Because of limitations of space in SWIMS, SWIMS-IFU should fit in a volume of 170 x 220 x 60 mm3, which results in small and complicatedly aligned mirror facets. To reduce alignment procedures, we adopt an ultra-precision cutting technique to fabricate mirror arrays monolithically. We have completed one of the mirror arrays, the slit-mirror array which consists of 26 spherical mirror facets, and confirmed both their surface roughness and shape errors satisfy the requirements. We also have fabricated a prototype of the pupil-mirror array including some elliptical mirror facets and confirmed that the elliptical mirrors have enough surface qualities and produce better image quality than spherical ones by a pinhole imaging test.
We introduce novel gratings for next generation instruments of the TMT (Thirty Meter Telescope), the 8.2 m Subaru telescope, other ground-based and space-borne telescopes. The reflector facet transmission (RFT) grating which is a surface relief grating with sawtooth shaped grating lattice of an acute vertex angle, is developed for the WFOS of the TMT. The hybrid grism (direct vision grating) for the MOIRCSof the 8.2m Subaru Telescope is developed as a prototype of the RFT grating. The volume binary grating is developed for a high-dispersion echelle grism of the nuMOIRCS as the first light instrument of the ULTIMATE Subaru. We also developing a silicon grism for the MIMIZUKU of the 6.5m telescope of the University of Tokyo Atacama Observatory in Chile and a quasi-Bragg (QB) immersion grating.
We will introduce current status of development of a birefringence volume phase holographic (B-VPH) grating, volume binary (VB) grating and reflector facet transmission (RFT) grating developing as the novel dispersive optical element for astronomical instruments for the 8.2m Subaru Telescope, for next generation 30 m class huge ground-based telescopes and for next generation large space-bone telescopes. We will also introduce a hybrid grism developed for MOIRCS (Multi-Object InfraRed Camera and Spectrograph) of the Subaru Telescope and a quasi-Bragg (QB) immersion grating. Test fabrication of B-VPH gratings with a liquid crystal (LC) of UV curable and normal LCs or a resin of visible light curable are performed. We successfully fabricated VB gratings of silicon as a mold with ridges of a high aspect ratio by means of the cycle etching process, oxidation and removal of silicon oxide. The RFT grating which is a surface-relief (SR) transmission grating with sawtooth shaped ridges of an acute vertex angle. The hybrid grism, as a prototype of the RFT grating, combines a high-index prism and SR transmission grating with sawtooth shape ridges of an acute vertex angle. The mold of the SR grating for the hybrid grism on to a work of Ni-P alloy of non-electrolysic plating successfully fabricated by using our ultra-precision machine and a single-crystal diamond bite. The QB immersion grating was fabricated by a combination of an inclined QB grating, Littrow prism and surface reflection mirror.
As a transmission grating, a surface-relief (SR) grating with sawtooth shaped ridges and volume phase holographic (VPH) grating are widely used for instruments of astronomical observations. However the SR grating is difficult to achieve high diffraction efficiency at high angular dispersion, and the VPH grating has low diffraction efficiency in high diffraction orders. We propose novel gratings that solve these problems. We introduce the hybrid grism which combines a high refractive index prism with a replicated transmission grating, which has sawtooth shaped ridges of an acute apex angle. The birefringence VPH (B-VPH) grating which contains an anisotropic medium, such as a liquid crystal, achieves diffraction efficiency up to 100% at the first diffraction order for natural polarization and for circular polarization. The quasi-Bragg (QB) grating which consists of long rectangular mirrors aligned in parallel precisely, like a window blind, achieves diffraction efficiency of 60% or more in higher than the 4th diffraction order. The volume binary (VB) grating with narrow grooves also achieves diffraction efficiency of 60% or more in higher than the 6th diffraction order. The reflector facet transmission (RFT) grating which is a SR grating with sawtooth shaped ridges of an acute apex angle achieves diffraction efficiency up to 80% in higher than the 4th diffraction order.
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