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Long-term spectroscopic monitoring campaigns on active galactic nuclei (AGNs)provide a wealth of information about its interior structure and kinematics.However, a number of the observations suffer from the contamination ofsecond-order spectra (SOS) which will introduce some undesirable uncertaintiesat the red side of the spectra. In this paper, we test the effect of SOS andpropose a method to correct it in the time domain spectroscopic data using thesimultaneously observed comparison stars. Based on the reverberation mapping(RM) data of NGC 5548 in 2019, one of the most intensively monitored AGNs bythe Lijiang 2.4 m telescope, we find that the scientific object, comparisonstar, and spectrophotometric standard star can jointly introduce up to similarto 30% SOS for Grism 14. This irregular but smooth SOS significantly affectsthe flux density and profile of the emission line, while having little effecton the light curve. After applying our method to each spectrum, we find thatthe SOS can be corrected effectively. The deviation between corrected andintrinsic spectra is similar to 2%, and the impact of SOS on time lag is veryminor. This method makes it possible to obtain the H alpha RM measurements fromarchival data provided that the spectral shape of the AGN under investigationdoes not have a large change.

Correcting the Contamination of Second-order Spectra: Improving H{\alpha} Measurements in Reverberation Mapping Campaigns