A multipurpose fiber-fed VPHG spectrograph for LAMOST

A multipurpose fiber-fed double-beam Schmidt spectrograph using VPHG (volume phase holographic gratings) is under construction for LAMOST (The Large Sky Area Multi-Object Fiber Spectroscopic Telescope). There are 16 such spectrographs (hereafter referred to as LRSs) for the project. The spectrographs are designed with wavelength coverage from 370 to 900 nm, with spectral resolutions of 1000-10000, and with multi-object capability over a 5 degrees field of view. Each spectrograph will be accommodating 250 fibers of 320 microns diameter (corresponding 3.3 arcsecs). The 200 mm diameter collimated beam is split into two separate channels. The blue channel is optimized for 370nm-590nm, and the red channel for 570nm-900nm. The LRS can work in several varied resolution modes. The optical design and performance is described. The spectrograph is of simple design with moderate image quality and good throughput. Progress on the construction of LRS is reported as well.