Midcourse Space Experiment: Off-Axis Rejection Performance of the Infrared Sensor

The Spatial Infrared Imaging Telescope III (SPIRIT III) sensor on the Midcourse Space Experiment (MSX) satellite observed stray radiation from the lower atmosphere and terrestrial surface, nonrejected Earth radiance, at angles of approximately 2 to 14 deg from the optical axis in measurements of Earth limb radiance. Analysis indicates that direct scatter of terrestrial radiance from contaminants on the telescope primary mirror is the principal source of stray radiation and the bidirectional reflectance distribution function (BRDF) of the primary mirror for the 6.8-10.8-μm radiometer band is (1.11 ± 0.22) x 10 -2 persr at 1 deg off axis with an angular dependence of θ -1.71 ± 0.07 . Similarly, the BRDF values for the three other LWIR radiometer bands (11.1-13.2,13.5-15.9, and 18.2-25.1 μm) havebeendeterminedas(1.03 ± 0.05) × 10 -2 θ -1.66±0.10 , (2.21 ± 0.18) × 10 -2 θ -2.04±0.21 , and (2.59 ± 0.02) × 10 -2 θ -2.10±0.09 sr -1 , respectively. The BRDF values derived from the on-orbit data are significantly greater than prelaunch measurements, and the increase is attributed to particulate accumulation on the primary mirror during the prelaunch period, launch, and the on-orbit telescope aperture cover removal and ejection process.