Modeling the partially coherent behavior of few-mode far-infrared grating spectrometers

Modeling ultra-low-noise far-infrared grating spectrometers has become crucial for the next generation of farinfrared space observatories. Conventional techniques are awkward to apply because of the partially coherent form of the incident spectral field, and the few-mode response of the optics and detectors. We present a modal technique for modeling the behavior of spectrometers that allows for the propagation and detection of partially coherent fields, and the inclusion of straylight radiated by warm internal surfaces.We illustrate the technique by modeling the behavior of the long wavelength band of the proposed SAFARI instrument on thewell-studied SPICA mission.