Hitoshi Tokoro

Zinc sulfide and zinc selenide immersion gratings for astronomical high-resolution spectroscopy: evaluation of internal attenuation of bulk materials in the short near-infrared region

We measure the internal attenuation of bulk crystals of chemical vapor deposition zinc selenide (CVD-ZnS), chemical vapor deposition zinc sulfide (CVD-ZnSe), Si, and GaAs in the short near-infrared (sNIR) region to evaluate the possibility of astronomical immersion gratings with those high refractive index materials. We confirm that multispectral grade CVD-ZnS and CVD-ZnSe are best suited for the immersion gratings, with the smallest internal attenuation of att=0.01 to 0.03 cm−1 among the major candidates. The measured attenuation is roughly in proportion to −2, suggesting it is dominated by bulk scattering due to the polycrystalline grains rather than by absorption. The total transmittance in the immersion grating is estimated to be at least >80%, even for the spectral resolution of R=300,000. Two potential problems, the scattered light by the bulk material and the degradation of the spectral resolution due to the gradient illumination in the diffracted beam, are investigated and found to be negligible for usual astronomical applications. Since the remaining problem, the difficulty of cutting grooves on CVD-ZnS and CVD-ZnSe, has recently been overcome by the nanoprecision fly-cutting technique, ZnS and ZnSe immersion gratings for astronomy can be technically realized.

Development of immersion grating for mid-infrared high dispersion spectrograph for the 8.2m Subaru Telescope

The mid-infrared high dispersion spectrograph (IRHS; tentative name) with a resolving power of 200,000 at 10 μm is a candidate of the second-generation instrument for the 8.2m Subaru Telescope. A germanium immersion grating will be employed as a dispersing element for this instrument. Germanium immersion gratings for the prototype IRHS were successfully fabricated by using a nano precision 3D profile grinding/turning machine and ELID grinding method on diamond machining. As a result, the fabricated gratings observed to have grooves with ideal saw-tooth shape, smooth surface and acceptable wave front error of a diffraction beam at 10μm. In the present paper, we characterized the performance of the developed immersion gratings.

Mid-infrared high-resolution spectrograph for SPICA

We present a preliminary optical design and layout for the mid-infrared (4-18 μm) high-resolution spectrograph for SPICA, Japanese next-generation space IR observatory with 3.5 m telescope. MIR high-resolution spectroscopy is a powerful probe to study gas-phase molecules/atoms in a variety of astronomical objects. Space observation provides a great opportunity to study many molecular lines especially in between the atmospheric windows. SPICA gives us a chance to realize MIR high-resolution spectroscopy from space with the large telescope aperture. The major technical challenge is the size of the spectrograph, which tends to be too large for space. We hope to overcome this problem with a novel MIR immersion grating, which can make the instrument smaller by a factor of the refractive index of the grating material. We plan to fabricate a large pitch ZnSe (n = 2.4) immersion grating with the fly-cutting technique at LLNL (see Poster paper 7018-183 by Ikeda et al.1 and 7018-181 by Kuzmenko et al.2 in the proceedings of this conference). We show our preliminary spectrograph designs with a spectral resolution of ~30,000 in 4-8 μm (short mode) and 12-18 μm (long mode). The instrument size can be as small as 200 × 400 mm thanks to the MIR immersion gratings. With unprecedented spectral resolution in space, which is 10-times higher than ISO-SWS, the high-resolution spectrograph for SPICA (SPICA-HIRES) could be a unique instrument that can provide most sensitive and clear spectra of this kind.

Development of a mid-infrared high dispersion spectrograph (IRHS) for the Subaru telescope

A design of prototype Infra-Red High-dispersion Spectrograph (IRHS) is described. IRHS is a cryogenic echelle spectrometer for 8.2-m Subaru Telescope, which will operate at 8 to 13 μm with resolving power of 200,000. To achieve such a high dispersion and broad bandwidth, a Germanium immersion echelle grating was adopted. As a preliminary step, we started to develop the proto-type of IRHS (ProtoIRHS) with currently available Ge immersion grating (30x30x72 mm) and one 512x412 Si:As impurity band detector array, which will provide the maximum resolving power of 50,000 at 10 μm with slit width of 0.612 arcseconds (0.48 mm) and two-pixels sampling.

A design and trial production of the image slicer unit for the mid-infrared spectrograph

An image slicer is highly in demand for an integral field unit (IFU) spectrograph of the next generation infrared telescopes. This paper reports the results of the trial production of three key optical elements for a small format (number of slice; n=5) image slicer, i.e. monolithic slice mirrors, monolithic pupil mirrors and monolithic pseudo slit mirrors. We have demonstrated that sufficiently high processing accuracy and mirror surface accuracy for infrared observations are achieved for each optical element based on our super precision cutting techniques.

A trial production of the image slicer unit for next generation infrared instruments and the assembly of the evaluation system of the pseudo slit image quality

We have carried out the trial production of small format (n=5) image slicer aiming to obtain the technical verification of the Integral Field Unit (IFU) that can be equipped to the next generation infrared instruments such as TMT/MICHI and SPICA/SMI. Our goal is to achieve stable pseudo slit image with high efficiency. Here we report the results of the assembly of the image slicer unit and the non-cryogenic evaluation system of the pseudo slit image quality in the infrared.

A trial production of a large format image slicer unit for a possible future mid-infrared instrument on the TMT

We have carried out a trial production of the large-format (n=11) image slicer unit for a possible future mid-infrared instrument on the TMT aiming to verify its technical feasibility. The key elements in our trial production are the monolithic large-format slice mirrors and the monolithic large-format pupil mirrors. The results of our trial production of those key elements based on the ultra high-precision cutting techniques and the assembly of the large-format image slicer unit are presented in this paper.

Development of the mid-IR echelle high-dispersion spectrograph employing the germanium immersion grating

We have developed a germanium immersion grating mid-infrared cryogenic spectrograph (GIGMICS) designed for the Nasmyth focus stage of NAOJ Subaru 8.2 m telescope, which operates at N-band (8-13 μm) in wavelength with the R ~ 50,000. A single crystal germanium immersion echelle grating (30 × 30 × 72 mm) for collimated beam size of Φ28 mm was fabricated by utilizing ultra precision micro-grinding method coupled with the ELID (ELectrolytic In-process Dressing) technique (Ohmori, H. 1992)1. All optical components are arranged on the 800 mm diameter cold optical base plate (~30 K) of the cryostat. By the Si:As IBC (Impurity Band Conductor) focal plane array (FPA) detector (412 × 512 pixels, unit pixel size 30 μm) operated at 5 K simultaneously acquires ~13 % wavelength coverage for N-band. The instrument has been assembled and is now tested for the application to the gas-phase IR high-resolution spectroscopy.

Studies on evaluating and removing subsurface damage on the ground surface of CLEARCERAM-Z HS

We evaluated depth of subsurface damage on a ground surface of the ultra low expansion glass-ceramics CLEARCERAMR®-Z HS (CC-Z HS) by Ohara Inc., which is one of the candidates for material for segmented mirrors of the Thirty Meter Telescope. We made polishing spots of Magnetorheological Finishing on the ground surface of CC-Z HS and measured exposed subsurface damage features on the spot surface. We also studied on hydrofluoric acid etching of the CC-Z HS ground surface, which is expected to be an effective method to remove a subsurface damage layer compared with time-consuming polishing. We etched small ground surfaces of CC-Z HS and evaluated its uniformity.