Calibration Tests of a 25-GHz Mode-spacing Broadband Astro-comb on the Fiber-fed High Resolution Spectrograph (HRS) of the Chinese 2.16-m Telescope

A 25-GHz mode-spacing astro-comb with 470-720 nm wavelength coverage has been installed as the calibration source on the fiber-fed High Resolution Spectrograph (HRS) of the Chinese 2.16-m telescope at Xinglong Observatory. The calibration tests were carried out based on the single-channel system of HRS. The results have achieved a 2-8 times (for different orders) higher wavelength solution accuracy than the thorium argon (ThAr) lamp, and a short-term repeatability of 0.1 m s(-1), which is around the photon noise limit. It proved that the coupling system linking the astro-comb to HRS successfully suppressed the negative effects of laser speckles. The comb-line overlapping exists in the acquired spectrum of the astro-comb on HRS. We demonstrated that when determining the comb-line center, the comb-line overlapping leads to a systematic bias, which is caused by asymmetrical sampling, and meanwhile a larger uncertainty. The correction for the systematic bias is feasible by simulating the overlapping comb lines according to actual comb-line spacing and pixel phase and then calculating the difference of the fitted line center with the true line center. A higher accuracy of wavelength solution has been achieved after correction.