Abstract We introduce a method of subtracting geocoronal Hα emissions from the spectra of LAMOST medium-resolution spectral survey of Galactic nebulae (LAMOST-MRS-N). The flux ratios of the Hα sky line to the adjacent OH λ6554 single line do not show a pattern or gradient distribution in a plate. More interestingly, the ratio is well correlated to solar altitude, which is the angle of the Sun relative to the Earth’s horizon. It is found that the ratio decreases from 0.8 to 0.2 with the decreasing solar altitude from –17 to –73 degree. Based on this relation, which is described by a linear function, we can construct the Hα sky component and subtract it from the science spectrum. This method has been applied to the LAMOST-MRSN data, and the contamination level of the Hα sky to nebula is reduced from 40% to less than 10%. The new generated spectra will significantly improve the accuracy of the classifications and the measurements of physical parameters of Galactic nebulae.
Keywords techniques: spectroscopic — instrumentation: spectrographs — ISM: general
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