FIFI LS: a field-imaging far-infrared line spectrometer for SOFIA

We describe our design for an imaging far-IR spectrometer for NASA/DARA's SOFIA observatory. The design of the instrument is driven by the goal of maximizing observing efficiency. Since the sensitivity of well designed FIR instruments is limited by the thermal background of telescope and atmosphere, observing efficiency can only be increased by increasing the throughput of the spectrometer and the number of simultaneous data channels. Our instrument will feature two separate medium resolution grating spectrometers with common fore-optics feeding two large Ge:Ga arrays. The two Littrow spectrometers operate between 45-110 micrometers , and 110-210 micrometers , resp., in 1st and 2nd order. Multiplexing takes place both spectrally and spatially. An image slicer redistributes 5 by 5 pixel fields-of-view along the 1 by 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 flits are anamorphically 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 approximately 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 approximately 2 by 10(superscript -15)W/(root)Hz/pixel.