A spectrometer optimized for measurement of narrow spectral features in the 10 pm atmospheric window has been constructed and used for astronomical observations. The instrument consists of a LN2 cooled, piezoelectrically scanned, Fabry-Perot interferometer and a LHe cooled grating monochromator and Si:As photoconductive detector. The Fabry-Perot has been used with a free spectral range of 3-10 cm-1 and a resolution of 0.07-0.3 cm-1. The grating, which isolates a single order of the Fabry-Perot, has a resolution of typically 2 cm-1 with an entrance aperture corresponding to 6" on a 3m telescope. The system sensitivity, which is photon shot noise limited, is 2x10-14WHz-1/2 , including all losses in the spec-trometer, telescope, and atmosphere, and due to chopping. The instrument can be used as a scanning spectrometer to measure line shapes and fluxes or as a monochromator to allow emission-line mapping of nebulae. Astronomical applications have included observations of the motion and distribution of the ionized gas in the Galactic center and first detections of H2 S(2) at 12.28 μm and H I Humphreys a at 12.37 μm.