An Investigation of Spectral-Line Profiles from the Wavelength- Scanning with a Narrow-Band Hα Lyot Filter on the YNAO New Vacuum Solar Telescope

The 1m New Vacuum Solar Telescope (NVST) at the Fuxian-Lake Solar Observation Station of the YNAO is a new-generation ground-based solar research facility of China. One instrument on the NVST is a multi-channel high-resolution imaging system, which has been in operation since October 2010. The observation wavelength range of the system includes the Hα, TiO band, G band, Ca II 854.2nm, and He I 1083nm. Although only the channels of the Hα, TiO band, and G band have been used, the observations impressively demonstrate the high-resolution capability of the NVST. The channels for the TiO band and G band both use broad-band filters with full widths of 1nm. In contrast, the Hα channel uses a narrow-band filter with a full width of 0.025nm (corresponding to a spectral FWHM of ~11km/s). The wavelength center of the channel can be adjusted within the range 656.281 ± 0.4nm. Since profiles observed with narrow-band filters are severely blurred by Doppler broadening, some spectral-line information is needed to extract useful physical results from these. Profiles from wavelength scanning can provide the needed line information, which makes it important to achieve accurate wavelength scanning. A new Hα Lyot filter was installed on the NVST in April 2013. In this paper, we investigate the performances of this filter system by examining spectral-line profiles from the wavelength-scanning with it. We use the observations of the central parts of the solar disk with the multi-channel imaging system to derive Hα line profiles. Our investigation focuses on the following aspects: deviations between the filter central band and the centers (e.g. absorption peaks) of the line profiles from the scanning, the symmetries of the profiles from the scanning, influences of a front broad-band filter, and the stability of the temperature of the system. We have found the following results. (1) The center of a line profile from the scanning is 0.013nm away from the filter central band ("0nm"). The deviation can be corrected through increasing the working temperature of the system by about 0.3℃. (2) After the correction, the deviation is reduced to less than 0.004nm by keeping the asymmetry of a profile below 10%. (3) The profile intensities at "0nm" are higher than the expected values by 6% to 8%. (4) The front broad-band filter has appreciable influences on the shapes of the profiles from the scanning only in the wavelength range 656.281-0.15nm to 656.281-0.4nm. (5) The working temperature of the filter system is stable, with the monthly standard deviation at about 0.0017℃.