  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.
