Dr. LU Kai-Xing from Yunnan Observatories of Chinese Academy of Sciences with collaborator, presented a new method for correcting the telluric absorptions from the spectrum, they found that the correction accuracy of this method to the telluric absorption is 1%.

This research, published online in Research in Astronomy and Astrophysics, offers a better choice than the traditional and complicated method of correcting telluric absorptions, and is helpful for understanding the telluric absorption features of the observation site.

Telluric absorption is a kind of extinction, which is the selective absorption of electromagnetic radiation by various components in the telluric atmosphere. Ground-based optical and near-infrared spectra are contaminated by the variable absorptions of the telluric atmosphere. The telluric absorptions from the red side of the optical spectrum to the near infrared are mainly caused by water vapor and oxygen.

The traditional method of correcting telluric absorption is to observe a telluric standard star near in both time and airmass to the scientific object, then using the telluric spectrum created from standard star to correct the telluric absorptions by dividing the object’s spectrum during the data reduction. In a long-term spectral monitoring study, this method will take a significant amount of precious telescope time to observe the telluric standard star. In addition, it is almost impossible to obtain the spectrum of scientific object and telluric standard star under the same observing conditions, such as weather, seeing and telescope pointing state.

Researchers presented a method of correcting the telluric absorption lines based on the long slit spectroscopy of the 2.4-m telescope of Lijiang observatory. The telescope is located at the Lijiang Observatory and administered by Yunnan Observatories, Chinese Academy of Sciences. By rotating the slit, the research team observed the scientific object and a nearby comparison star in one exposure, so that the spectra of both objects should have the same telluric transmission spectrum. The telluric transmission spectrum was constructed by dividing the observed spectrum of comparison star by its stellar template, and was used to correct the telluric absorption lines of the scientific object.

Dr. LU selected an active galactic nuclei (AGN) with redshift 0.3889 to put its [O III] λ5007 (O3) emission line drop into telluric absorption bands caused by water, and developed a long-term spectroscopic monitoring. Because the fluxes of O3 forbidden line emitted from the narrow-line region of AGN are stable in the timescale of years, the research team measured the light curves of O3 fluxes before and after correcting the telluric absorptions, then used the scatter of O3 fluxes to estimate the correction accuracy of the telluric absorptions. The research found that the correction accuracy of telluric absorptions from the observed spectrum is 1%.

Using uncorrected O3 fluxes, and a series of the telluric transmission spectra constructed from the template matching of comparison stars, the research investigated the telluric absorption features of the Lijiang Observatory. ‘We found that the telluric absorption transparency is mainly modulated by the seasonal variability of the relative humidity, airmass and seeing.’ Said Dr. LU. ‘This research provides a kind of worthy observation strategy and corresponding method for correcting the telluric absorptions.’ Dr. LU Added.

Contact:

LU Kaixing,  lukx@ynao.ac.cn

Yunnan Observatories, CAS