  Spectrograph slit is conventionally used to enhance the spectral resolution
and manage how much light can be allowed to enter spectrograph. A narrower slit
provides a higher resolution but sacrifices efficiency of spectrograph and
results in a low signal to noise ratio (S/N) spectra product. We take
GuoShouJing telescope as an example and carry out a series of experiments to
study how its 2/3 slit mode affects the precision of stellar radial velocity
measurement and atmosphere parameters estimate. By transforming the resolution
and adding a Gaussian White Noise to the extremely high quality spectra from
the Sloan Digital Sky Survey, we generate synthetic stellar spectra of various
brightness with different S/Ns. Comparing the measurements on these noise added
spectra with the original high quality ones, we summarize the influences of the
2/3 slit mode on the measurement accuracy of stellar radial velocity and
atmospheric parameters.
