本文敘述福衛五號衛星遙測酬載 (RSI) 光學設計及性能評估分析，包括設計需求、設計結果、容差分析、 光學調變傳遞函數 (MTF) 及訊雜比 (SNR)。傳承福衛二號經驗，採用 Cassegrain 反射式望遠鏡設計，望 遠鏡包含主鏡 (M1)、次鏡 (M2) 與四透鏡組成之修正鏡組 (corrector)。主鏡與次鏡材質為零膨脹係數玻璃 Zerodur， 修正鏡組透鏡與波段濾波片 (fi lter) 使用熔融石英 (fused silica) 以符合抗輻射需求。望遠鏡焦長 3,600 mm，通光口徑為 450 mm。MTF 設計值趨近於繞射極限，SNR 符合 CMOS 感測器與 RSI 規格需 求。

This article describes the telescope optical design and performance analysis of the Remote Sensing Instrument (RSI) of Formosat-5. The design requirements, design results, tolerance analysis, evaluations of Modulation Transfer Function (MTF) and Signal to Noise Ratio (SNR) are included. Cassegrain telescope design is selected due to the existed experience. The telescope consists of a primary mirror, a secondary mirror and a four-lens corrector. Mirrors are made of Zerodur for very low CTE requirement. Fused silica is selected for corrector lens and band-pass fi lters to meet radiation resistance requirement in space. The telescope has the focal length of 3,600 mm, and the clear aperture of 450 mm. The analysis results show that the design MTF is diffraction limited, and SNR complies with the requirements of CMOS sensor and RSI.