Installation/Adjustment of a 6m Near-Infrared Spectrograph for the 1m New Vacuum Solar Telescope in the Fuxian-Lake Solar-Observation Station and Preliminary Observation Results of Solar Spectra around 1.56μm Using the Spectrograph

This paper first presents the design, layout, and installation of a near-infrared grating spectrograph installed on the 1m new vacuum solar telescope in the Fuxian-Lake Solar-Observation Station of the Yunnan Observatories. It records spectra in the wavelength range of 1.0μm-2.0μm. The focal length of its collimating mirror is 6m and its spectral resolution can reach about 6×10^-6μm/pixel. Using this spectrograph and a previously built near-infrared detector (with an InGaAs detector of 512×640 pixels and a pixel size of 25μm) we can obtain solar spectra with a wavelength coverage of 0.004μm. One purpose of making this horizontal-type spectrograph is to evaluate main designs to be used in the vertical-type 9m near-infrared spectrograph in future. The designs include those of the Echelle grating, infrared detector, and infrared front-filter. With the spectrograph we can also accumulate experience of observing near-infrared solar spectra, particularly, in setting the slit width, exposure time, and wavelength calibration. Using the spectrograph we successfully observed a sunspot region in a wavelength range around 1.565μm. This range covers two commonly studied spectral lines (Fe I 1.5648μm and Fe I 1.5653μm), which are sensitive to solar magnetic-field strengths. We reduced the data using codes designed for the 1m telescope. The wavelength calibration is based on the standard solar spectrum. We calculated the magnetic-field strength (instead of the magnetic-field flux) from the line split rather straightforwardly. These preliminary results demonstrate the effectiveness of this 6m spectrograph in achieving its scientific goals.