A correction method for the telluric absorptions and application to Lijiang Observatory

TL;DR

This paper introduces a novel method for correcting telluric absorption lines in astronomical spectra by using long slit spectroscopy with a comparison star, reducing the need for standard star observations and enabling long-term monitoring.

Contribution

The method combines long slit spectroscopy with slit rotation to simultaneously observe the scientific object and a comparison star, improving telluric correction accuracy in long-term spectral monitoring.

Findings

Telluric absorption varies seasonally with humidity, airmass, and seeing.

The correction method achieves 1% accuracy in telluric line correction.

Application to AGN spectra demonstrates effective long-term monitoring.

Abstract

Observing a telluric standard star for correcting the telluric absorption lines of spectrum will take a significant amount of precious telescope time, especially in the long-term spectral monitoring project. Beyond that, it's difficult to select a suitable telluric standard star near in both time and airmass to the scientific object. In this paper, we present a method of correcting the telluric absorption lines by combining the advantages of long slit spectroscopy. By rotating the slit, we 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. Using the long slit spectrograph of Lijiang 2.4-meter telescope, we designed a long-term spectroscopic observation strategy, and finished a four-year spectroscopic monitoring for a pair of objects (an active galactic nuclei and an non-varying comparison star). We applied this method to correct the telluric absorption lines of the long-term monitored spectra by Lijiang 2.4-meter telescope, and investigated the variation of the telluric absorptions at Lijiang Observatory. We found that the telluric absorption transparency is mainly modulated by the seasonal variability of the relative humidity, airmass and seeing. Using the scatter of the [O~III] $\lambda$5007 fluxes emitted from the narrow-line region of active galactic nuclei as an indicator, we found that the correction accuracy of the telluric absorption lines is 1%.