FIFI LS: The optical design and diffraction analysis

We present the optical system of the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) for the SOFIA airborne observatory. The instrument is designed to allow diffraction limited integral field spectroscopy in the far infrared wavelength range 42 to 210 microns. Two parallel wavelength channels (42 - 110 microns and 110 - 210 microns) employ Littrow mounted diffraction gratings with anamorphic collimators. Mirror image slicers in each channel rearrange the 5 × 5 pixel field of view along the 1 × 25 entrance slit of the grating spectrograph. The spectral resolution varies in the range of R = 1400 - 6500, depending on observing wavelength. The optical components in the image slicer is comprised of several mirrors with physical dimensions on the order of a few tens of wavelength. Consequently diffraction effects are a serious concern in the design of the optical system. Substantial effort in modeling diffraction effects throughout the optical system and its impact upon the expected performance of the instrument have been made. The results of the scalar diffraction analysis carried out with a commercial software package has been confirmed by a full vectorial analysis, showing negligible dependence of the diffraction effects on the polarization properties of the electromagnetic field.