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<sup>-1</sup> pixel<sup>-1</sup> at 290 K and ~0.04 photons sec<sup>-1</sup> pixel<sup>1 </sup>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).