FIFI LS -- A Field-Imaging Far-Infrared Line Spectrometer for SOFIA

We describe a Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) for NASA/ DLR's Stratospheric Observatory For Infrared Astronmomy (SOFIA). The design of the instrument is driven by the goal of maximizing observing efficiency specifically for observations of faint, extragalactic objects. These observations, made possible by the unprecedented spatial resolution and sensitivity offered by SOFIA, will address key questions in modern astronomy: the formation of stars and the energy balance of the interstellar medium on galactic scales, effects of metallicity, interaction and merging of galaxies, and the power sources behind AGNs and ultraluminous infrared galaxies. For optimum observing efficiency, our instrument will feature two separate medium resolution (R 1700) grating spectrometers with common foreoptics feeding two large Ge:Ga arrays (16 x 25 pixels). The two Littrow spectrometers operate between 42-110 μ m, and 110-210 μ m, resp., in 1st and 2nd order. Multiplexing takes place both spectrally and spatially. An image slicer redistributes 5 x 5 pixel fields-of-view ( diffraction-limited in each wave band) along the 1 x 25 pixel entrance slits of the spectrometers. Anamorphic collimator mirrors help keep the spectrometer compact in the cross-dispersion direction. The spectrally dispersed images of the slits are anamorphicly projected onto the detector arrays, to independently match spectral and spatial resolution to detector size. We will thus be able to instantaneously cover a velocity range of 1500 km/s around a selected FIR spectral line, for each of the 25 spatial pixels. For calibration and flatfielding we use blackbody calibrators internal to the instrument, at signal levels comparable to the thermal background of the telescope. An image rotator compensates field rotation during long integrations. Estimated sensitivity of the spectrometer is 2x 10^-15 W/√ {Hz}/pixel. FIFI LS on SOFIA will be more sensitive than the ISO Long-Wavelength Spectrometer and have much higher spatial resolution and mapping capabilities. As a future option, an extension of the instrument to the 25-42 μ m range is planned upon availability of the Si:Sb detector arrays developed for SIRTF.