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First high-efficiency and high-resolution (R=80,000) NIR spectroscopy with high-blazed Echelle grating: WINERED HIRES modes

Otsubo, Shogo; Ikeda, Yuji; Kobayashi, Naoto; Sukegawa, Takashi; Kondo, Sohei; Hamano, Satoshi; Sameshima, Hiroaki; Fukue, Kei; Yoshikawa, Tomohiro; Nakanishi, Kenshi; Watase, Ayaka; Takenaka, Keiichi; Asano, Akira; Yasui, Chikako; Matsunaga, Noriyuki; Kawakita, Hideyo
2016, SPIE
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WINERED is a PI-type 0.9 - 1.35 ƒÊm 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 ß ƒÉ/ƒ¢ƒÉ = 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 ƒÉ < 1.5 ƒÊm 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 ƒÊm, 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-ƒÊm 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 / ƒÊm]. Because there was an avoidable degradation in reducing the two-dimensional spectrum using HBGs with a large ƒÁ angle, the final spectral resolution of the reduced one-dimensional spectrum results in R = 68,000.