Optical EngineeringSpectral resolution modeling with partial coherence for the Atmospheric Infrared Sounder instrument
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We generalize the spectral resolution model that was used for the Atmospheric Infrared Sounder (AIRS) instrument to account for the observed disparity between the predicted resolution and its measurement. The prelaunch-measured spectral resolution of the AIRS instrument was shown to be narrower or better than the prediction by 3% to roughly 14% with the narrowing increasing with wavelength across the AIRS infrared spectral band of 3.7 to 15.4 μm. The prediction was based on the common practice of using the slit as a secondary source, but with a tacit assumption of spatial incoherence for the slit illumination. We show that the narrowing of the spectral resolution is caused by partial coherence at the slit illumination, and is accounted for naturally by using the primary source and a system point spread function P, which includes the optical contributions of the preslit source optics or telescope in addition to that of the spectrometer. We model the disparity versus wavelength, and show a good comparison to that measured and reported in the literature.