本文报道一台以Cassegrain聚焦系统工作的天文外差光谱仪。工作波长是10μm,光谱分辨率λ/⊿在6×10~5～6×10~6。噪声等效功率接近理论极限1.88×10~(-20)W/Hz。阐述了在红外波段在外差接收条件下使噪声等效功率最佳化的方法,以及信噪比远小于工时信号检测的手段。文中还介绍了整个接收系统的光、电结构和一些实验数据。 This article reports an astronomical heterodyne spectrometer operating in the Cassegrain focusing system. Working wavelength is 10μm, spectral resolution λ / ⊿ at 6 × 10 ~ 5 ~ 6 × 10 ~ 6. The noise equivalent power is close to the theoretical limit of 1.88 × 10 ~ (-20) W / Hz. The method of optimizing the noise equivalent power in the infrared band under heterodyne reception is described, and the means of signal-to-noise ratio is far less than the detection of working hours signal. The article also introduced the entire receiving system of light, electricity structure and some experimental data.