The Mercury's sodium atmosphere spectral imager (MSASI) on BepiColombo (BC) will address a range of fundamental scientific questions pertaining to Mercury's exosphere. The measurements will provide new information on regolith-exosphere-magnetosphere coupling as well as new understanding of the dynamics governing the exosphere bounded by the planetary surface, the solar wind and interplanetary space. MSASI is a high-dispersion visible spectrometer working in the spectral range around sodium D2 emission (589 nm). A tandem Fabry-Perot etalon is used to achieve a compact design. A one-degree-of-freedom scanning mirror is employed to obtain full-disk images of the planet. This paper presents an overview of the MSASI and the design of its spectral analyzer, which uses a Fabry-Perot interferometer. We conclude that: (1) The MSASI optical design is practical and can be implemented without new or critical technology developments. (2) The thermally tuned etalon design is based on concepts, designs and materials that have good space heritage. (3) The MSASI instrument achieves a high SNR (> 10) in the range of 2k-10 MRayleigh. (C) 2007 Elsevier Ltd. All rights reserved.