LAMOST is a 4m spectroscopic telescope recently operational at Xinglong, China. Several active optics are being used to remove optical aberration of the telescope, but large residual aberration exists since the active optics actuators on the telescope’s segmented mirrors cannot provide enough precision. We proposed a wave-front sensing system and the corresponding algorithm to measure this low frequency residual aberration. We developed a compact Shack-Hartmann wave-front sensor that can

use point source as well as extended structure images for wave-front sensing and can achieve good measurement accuracy. The wave-front sensing algorithm is realized by LabVIEW that is based on block-diagram programming and is suitable for rapid prototype development. Combined with deformable mirrors, the system will be able to provide a fine wave-front correction and therefore eventually remove the residual aberration for LAMOST. The wave-front sensor and the DMs will also be used for our high-contrast imaging coronagraph to remove speckle noise for the direct imaging of exoplanets.