On the radial velocity calibrations in the LAMOST Medium-Resolution
  Spectroscopic Survey of Nebulae
  Accurate radial velocity determinations of optical emission lines (i.e.
[NII]${\lambda}{\lambda}$6548,6584, H${\alpha}$, and
[SII]${\lambda}{\lambda}$6717,6731) are very important for investigating the
kinematics and dynamics properties of nebulae. The second stage survey program
of Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) has
started a sub-survey of nebulae (MRS-N) which will spectroscopically observe
the optical emission lines of a large sample of nebulae near the Galactic
plane. Until now, 15 MRS-N plates have been observed from 2017 September to
2019 June. Based on fitting the sky emission lines in the red band spectra of
MRS-N, we investigate the precision of wavelength calibration and find there
are systematic deviations of radial velocities (RVs) from $\sim$0.2 to 4 km/s
for different plates. Especially for the plates obtained in 2018 March, the
systematic deviations of RVs can be as large as $\sim$4 km/s, which then go
down to $\sim$0.2-0.5 km/s at the end of 2018 and January 2019. A RVs
calibration function is proposed for these MRS-N plates, which can
simultaneously and successfully calibration the systematic deviations and
improve the precision of RVs.
