9 August 2016 First high-efficiency and high-resolution (R=80,000) NIR spectroscopy with high-blazed Echelle grating: WINERED HIRES modes
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WINERED is a PI-type 0.9 – 1.35 μm high-resolution spectrograph developed by the Laboratory of Infrared highresolution Spectrograph (LiH) of the Koyama Astronomical Observatory at Kyoto Sangyo University, Japan. The scope of WINERED is to realize a high-resolution near-infrared (NIR) spectrograph with both wide coverage and high sensitivity. WINERED provides three observational modes called as the Wide, Hires-Y and Hires-J modes. The Wide mode simultaneously covers the z, Y and J-bands in a single exposure with R ≡ λ/Δλ = 28,000 and was commissioned for the 1.3 m Araki Telescope of Koyama Astronomical Observatory in 2013. We have been building alternative observational modes “Hires-Y” and “Hires-J”, providing R = 80,000 spectra in the Y- and J-bands, respectively. There are two choices for realizing a compact spectrograph with a high spectral resolution of R ≧ 50,000: an immersion grating (IG) or a highblazed echelle grating (HBG). Investigating the availabilities of both optical devices, we selected an HBG solution for λ < 1.5 μm because can be realized with currently available technology in earlier time. The optical parameters of WINERED’s HBGs are as follows: groove pitch = 90.38 μm, blaze angle = 79.32 °, and apex angle = 88°, which are determined to minimize vignetting in the optical system as well as aberrations with the spectral resolution of R = 80,000. Custom HBGs were made by CANON Inc. Because of the size the size limitation in fabrication process, we decided to use a mosaicked grating consisting of two HBGs. The alignment tolerances of the two HBGs are very tight (< 0.5 arcsec for the parallelism between grooves of the two gratings and 1.5 arcsec for the flatness between the two grating surfaces). To enable these fine alignments, we designed a grating holder with an adjustment mechanism with sub-μm positional resolution. We adapted cordierite CO-220 as the material for the grating holder, thereby reducing the misalignment generated by thermal expansions/compression with extremely low coefficient of thermal expansion (CTE < 2.0 ×10−8 K-1 at 23 °C). As a result of the measurement of the two HBGs installed in the grating holder, we confirmed the parallelism of < 0.1 arcsec. Finally, we evaluated the total optical performances of the Hires modes with the HBGs. The widths of the monochromatic slitimages obtained with a Th-Ar lamp were measured to be 1.7 – 2.3 pixels, which agreed well with the designed values (1.6 – 2.6 pixels). These results should guarantee the spectral resolution (R = 78,000) estimated from the measurement of the linear dispersion [pix / μm]. Because there was an avoidable degradation in reducing the two-dimensional spectrum using HBGs with a large γ angle, the final spectral resolution of the reduced one-dimensional spectrum results in R = 68,000.
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Shogo Otsubo, Shogo Otsubo, Yuji Ikeda, Yuji Ikeda, Naoto Kobayashi, Naoto Kobayashi, Takashi Sukegawa, Takashi Sukegawa, Sohei Kondo, Sohei Kondo, Satoshi Hamano, Satoshi Hamano, Hiroaki Sameshima, Hiroaki Sameshima, Kei Fukue, Kei Fukue, Tomohiro Yoshikawa, Tomohiro Yoshikawa, Kenshi Nakanishi, Kenshi Nakanishi, Ayaka Watase, Ayaka Watase, Keiichi Takenaka, Keiichi Takenaka, Akira Asano, Akira Asano, Chikako Yasui, Chikako Yasui, Noriyuki Matsunaga, Noriyuki Matsunaga, Hideyo Kawakita, Hideyo Kawakita, } "First high-efficiency and high-resolution (R=80,000) NIR spectroscopy with high-blazed Echelle grating: WINERED HIRES modes", Proc. SPIE 9908, Ground-based and Airborne Instrumentation for Astronomy VI, 990879 (9 August 2016); doi: 10.1117/12.2233845; https://doi.org/10.1117/12.2233845

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