# Sky Subtraction for LAMOST

## TL;DR

This paper presents a specialized sky subtraction method for LAMOST spectra, utilizing instrument calibration and PCA to significantly reduce sky line residuals, improving data quality in multi-fiber spectroscopic surveys.

## Contribution

The paper introduces a tailored sky subtraction technique for LAMOST that integrates calibration and PCA, achieving lower residuals than previous methods.

## Key findings

- Sky subtraction residuals are about 3% for sky lines and continuum.
- Wavelength calibration accuracy is approximately 4.5 km/s.
- Residuals can reach as low as 1.5% in dark nights.

## Abstract

Sky subtraction is the key technique in data reduction of multi-fiber spectra. Knowledge of the related instrument character is necessary to determine the method adopted in sky subtraction. In this study, we described the sky subtraction method designed for LAMOST(Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. The method has been intergrated into LAMOST 2D Pipeline v2.6 and applied to data of LAMOST DR3 and later. For LAMOST, sky emission line calibration is used to alleviate the position-dependent (thus time-dependent) ~4% fiber throughput uncertainty and the small wavelength instability (0.1\AA ) during observation. PCA (Principal Component Analysis) sky subtraction further reduces 25% of the sky line residual of the OH lines in the red part of the LAMOST spectra after the mater sky spectrum, which is derived from a B-spline fit of 20 sky fibers in each spectrograph, is adjusted by sky emission line and subtracted from each fiber. Further analysis shows that our wavelength calibration accuracy is about 4.5km/s, and the average sky subtraction residuals are about 3% for sky emission lines and 3% for continuum region. The relative sky subtraction residuals vary with the moon light background brightness, could reach as low as 1.5% for the sky emission line regions in the dark night. Tests on the F stars of both similar sky emission line strength and similar object continuum intensity show that the sky emission line residual of LAMOST is smaller than those of SDSS survey.