Abstract: The Optical Assembly (OA) for the Multispectral Thermal Imger (MTI) program has been fabricated, assembled, and successfully tested for its performance. It represents a major milestone achieved towards completion of this earth observing EO imaging sensor that is to be operated in low earth orbit. Along with its wide field of view, 1.82 degrees along-track and 1.38 degrees cross-track, and comprehensive on-board calibration system, the pushbroom imaging sensor employs a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 to 10.7 $mu@m. The OA has an off-axis three-mirror anastigmatic telescope with a 36-cm unobscured clear aperture. The two key performance criteria, 80 percent enpixeled energy in the visible and radiometric stability of 1 percent 1 $sigma in the visibleear-IR and short wavelength IR, of 1.45 percent 1 $sigma in the medium wavelength IR, and of 0.53 percent 1 $sigma long wavelength IR, as well as its low weight and volume constraint drive the overall design configuration of the OA and fabrication requirements. !4
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