Astronomical navigation is a navigation method which obtains attitude information of the installation platform by detecting the relative positions of stars in space. It has the significant advantages of strong anti-interference ability,high measurement accuracy,and no accumulation of errors over time,and has been widely applied. As an important device in astronomical navigation,star sensor is used to measure the relative attitude information of carrier. It uses stars as reference sources to calculate the precise attitude information of carrier by comparing with the star maps. Unlike GPS navigation signal,which is prone to rejection and accumulates inertial navigation errors over time,star sensor has several advantages,such as high precision attitude measurement ability,autonomous working ability,stable passive measurement working mode,and anti-electromagnetic interference ability. It has become one of the most necessary devices for attitude and orbit control systems on platforms such as ships,aircrafts,and satellites. Star sensor navigation technology was first applied to satellite platforms outside the atmosphere,and the dark background in outer space has a relatively small impact on the recognition of dark star targets. In recent years,with the development of all-day star sensor optical technology,astronomical/inertial integrated navigation technology has been rapidly applied to airborne platforms and near-space aircraft platforms in the atmosphere. The working altitude in the near space of 20~100 km has attracted more and more attention,and under the daytime conditions in the atmosphere,strong sky background noise caused by atmospheric scattering in the near space leads to the submergence of dark star signals. Meanwhile,the platform working in near space has strict limitations on the volume,weight,working environment,and stray light suppression ability of the system,which puts high requirements on the design of optical system for the star sensor that works in the near space. The domestic all-day star sensor optical system mostly adopts transmissive structure and catadioptric structure. The catadioptric structure mostly adopts low expansion materials,such as indium steel,fused silica,microcrystalline glass,ULE,etc. to carry out athermal design,which leads to the problems that cannot meet the requirements of lightweight design,such as large mass and volume. With the development of materials science and optical precision manufacturing technology,aluminum alloy which is a representative of metal materials can serve as both the reflector material and the supporting structural material. It has many advantages such as low density,high thermal conductivity,low processing difficulty,and short processing cycle,and has been widely used in the optical system design both domestically and internationally. This article proposes a design scheme for the lightweight star sensor optical system,by using aluminum alloy as the primary,secondary mirror material,and load-bearing structural material,in response to the requirements for lightweight,wide temperature operation,and stray light suppression of near space all-day star sensor. By analyzing the focal temperature characteristics of coaxial dual reflector system and comparing several kinds of commonly used star sensor materials,a lightweight all-day star sensor optical system was designed using a microcrystalline aluminum alloy RSA-443 as the primary reflector,secondary reflector,and support structure. The star sensor optical system has a focal length of 450 mm,an effective entrance pupil diameter of 80 mm,a field of view of 2ω=3.6°,and a working spectrum range of 0.8~1.1 μm. Adopting a passive optical athermal technology to complete the athermal design,the system has good temperature adaptation in the temperature range of -20~55 ℃. Through the design of lens hood,the Point Source Transmittance (PST) of system beyond sunlight exclusion angle of 30° is better than 10^-6,which meets the requirement for high stray light suppression ratio. Compared with the design scheme of indium steel truss with fused silica or microcrystalline glass,which has low expansion coefficient,the overall mass of the system is reduced by 37%,reducing the load weight of the platform dramatically,improving the stability and durability of the system. It meets the system lightweight requirement while ensuring imaging quality,and provides effective support for the development of optical technology for all-day star sensor working in near space.