We present infrared spectrometer design options offered through a wave propagation analysis throughout the optical system that would not be known otherwise. A recent inclusion of wave propagation into the atmospheric infrared sounder (AIRS) design model to account for an unanticipated measured spectral line narrowing with wavelength has shown that a wider slit option could have been used on the spectrometer to improve energy throughput. The underlying slit image width that narrowed could have been traded for a wider entrance slit increasing the instrument light sensitivity up to 15%, restoring the original un-narrowed slit width baseline, and recovering the spectral sampling requirement of two detectors per line profile. Of the 11 slits on the AIRS spectrometer, each slit width could have been spectrally tailored to become wider as the dispersed wavelength band increased, leading to a more optimal sounder configuration. We will show the newer suggested AIRS slit array below for future sounder design consideration after first reviewing past results that substantiate the wave propagation model being used.
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.
The baseline multiaperture echelle spectrometer for the Atmospheric IR Sounder (AIRS) is described in terms of design and applications. The functional requirements for the optical design are set forth including the 1-K measurement goal, the 3.4-15.4 spectral bandpass, and the full global coverage twice daily. The multiaperture spectrometer is compared to the cross-dispersed spectrometer, and the multiaperture model is found to permit specific adjustments to the signal-to-noise ratio. The optical design of the spectrometer is described in terms of the focal-plane constraints, the multiaperture pupil-imaging relay, the spectrometer collimator, and the grating format and efficiency. The multiaperture design is found to have a good spectral-response function, and a 1.2 percent signal change is noted for a 95-percent unpolarized scene. The AIRS instrument is illustrated in its deployment configuration and is concluded to be capable of fulfilling the performance requirements.
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