The MMX Infrared Spectrometer (MIRS) is an imaging spectrometer aboard the MMX (Martian Moons eXploration) JAXA mission. MIRS was constructed at LESIA in collaboration with five other French laboratories, with the collaboration and financial support of CNES, and in close partnership with JAXA. MIRS will remotely provide near-infrared spectral maps of Phobos and Deimos, containing spectral features used to analyze surface composition. MIRS will also study the Martian atmosphere, focusing on spatial and temporal changes in clouds, dust, and water vapor. A span of three and a half years separates the initial optical design studies for this instrument and the ongoing final tests conducted under vacuum on MIRS PFM (Proto-Flight Model) at LESIA facilities. Many design and development choices were influenced by this limited lead time. This paper will describe the key stages in the optical architecture's evolution during the instrument's development, guided by a design-to-effectiveness philosophy. Following an overview of the mission, its objectives, and the optical requirements of the instrument, we will expose the trade-off between a dioptric and a catoptric design, resulting in a catadioptric design including four freeform mirrors, a plane-ruled grating, two lens assemblies, and a linear variable filter. Each optical subassembly will be detailed, presenting its primary as-built characteristics. The alignment tolerances and procedures will also be briefly explained. Lastly, the measured optical performances of MIRS PFM will be presented and compared to the theoretical expectations.
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