  The Beijing Faint Object Spectrograph and Camera (BFOSC) is one of the most
important instruments of the 2.16-m telescope of the Xinglong Observatory.
Every year there are ~ 20 SCI-papers published based on the observational data
of this telescope. In this work, we have systemically measured the total
efficiency of the BFOSC of the 2.16-m reflector, based on the observations of
two ESO flux standard stars. We have obtained the total efficiencies of the
BFOSC instrument of different grisms with various slit widths in almost all
ranges, and analysed the factors which effect the efficiency of telescope and
spectrograph. For the astronomical observers, the result will be useful for
them to select a suitable slit width, depending on their scientific goals and
weather conditions during the observation; For the technicians, the result will
help them systemically find out the real efficiency of telescope and
spectrograph, and further to improve the total efficiency and observing
capacity of the telescope technically.
