Realizing 3D spectral imaging in the far-infrared: FIFI LS

We present a progress report on the design and construction of the Field-Imaging Far-Infrared Line Spectrometer (FIFI LS) for the SOFIA airborne observatory. The design of the instrument is driven by the goal of maximizing observing efficiency, especially for observations of faint, extragalactic objects. Thus, FIFI LS utilizes an integral field technique that uses slicer mirrors to optically re-arrange the two-dimensional field into a single slit for a long slit spectrometer. Effectively, a 5×5 pixel spatial field of view is imaged to a 25×1 pixel slit and dispersed to a 25×16 pixel, two-dimensional detector array, providing diffraction-limited spatial and spectral multiplexing. In this manner, the instrument employs two parallel, medium resolution (R approximately 2000) grating spectrometers for simultaneous observations in two bands: a short wavelength band (42 to 110 μm) and a long wavelength band (110 to 210 μm). Overall, for each of the 25 spatial pixels, the instrument can cover a velocity range of approximately 1500 km/s around selected far-infrared lines with an estimated sensitivity of 2×10^-15 W Hz ^1/2 per pixel. This arrangement provides good spectral coverage with high responsivity.