The Conjugate Etalon Spectral Imaging (CESI) concept enables the development of miniature instruments with high spectral resolution, suitable for LEO missions aboard CubeSat or nanosat buses, including constellation missions providing global coverage and characterization of dynamic phenomena. Small size, low power, and a simplified instrument architecture support missions for earth observation, atmospheric science, and planetary science. Unlike prior art hyperspectral and ultraspectral instruments that are much too large and complex for deployment on a nanosat, the CESI concept can be implemented in a small form factor using inexpensive components and requiring only a small optical aperture. CESI superimposes the interferogram from a conjugate Fabry-Perot etalon on the image of a scanned scene captured on a novel high-sensitivity low-noise SWIR focal plane. Using image processing, high resolution spectral characterization is performed independently for each point in the scene. The innovative focal plane and spectroscopic concepts have many promising scientific and commercial applications. The Scanned Etalon Methane Mapper (SEMM) is a CubeSat instrument that incorporates the CESI concept to perform global daytime mapping of atmospheric methane column density. Performance capabilities: ground resolution 100 m; concentration sensitivity 18 ppb; and global revisit ~ 60 days.