Low resolution spectrograph for LAMOST

LAMOST is a special Schmidt telescope of 4 meters aperture. It will be located in Xinglong station at Beijing Astronomical Observatory, China. The start of science operation is expected in 2004. There are 4000 optical fibers on the telescope focal surface that will feed 16 Low Resolution Spectrographs (LRS) and one or more Medium Resolution Spectrograph (MRS) and one High Resolution Spectrograph (HRS). Here we present a description of the LRS specification and its optical design. LRS is a multi object fiber spectrograph that is optimized for galaxy red shift surveys. The output beam with f-ratio of 4 from fiber are collimated by a spherical mirror and then are split to red and blue band by a dichroic filter. We use reflective grating as dispersion elements. Spectra are focused onto 2048X2048 CCD using fast Schmidt camera with f-ratio of I. Spectral coverage per exposure is from 370nm to 900nm. The beam size is 200mm resulting resolution 1000 with 3.3 arcsecond fiber slit. The slit with 130mm length picks up 250 fibers, so we need 16 low-resolution spectrograph to observe spectra of 4000 celestial objects in single exposure.