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5.1 FIFI-LS Instrument Overview
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The Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) is an integral field, far infrared spectrometer. The instrument includes two independent grating spectrometers sharing one common field-of-view (FOV). Each spectrometer has a detector consisting of 400 pixels of Germanium Gallium-doped photoconductors. The short wavelength spectrometer (blue channel) operates at wavelengths between 50μm and 125 μm, while the long wavelength spectrometer (red channel) covers the range from 105 μm up to 200 μm. One of two dichroics has to be selected for an observation affecting the wavelength range of both channels in the overlap region.
The projection onto the sky of the 5 x 5-pixel FOVs of both channels is concentric (10'' offset), but the angular size of the FOVs differs. The red channel has a pixel size of 12" x 12" yielding a square 1' FOV , and the blue channel has a pixel size of 6" x 6", which yields a square 30" FOV.
The resolving power of both channels varies between 1000 and 2000 dependent on the observed wavelength. The higher values are reached towards the long wavelength ends of each spectrometer.
The detectors are cooled down to about 1.7K with super fluid helium. The spectrometers and all mirrors are cooled down to 4K with liquid helium. The exception is the entrance optics featuring a K-mirror (see Sect. 5.1.3) and an internal calibration source. These optical components are cooled to about 80K with liquid nitrogen.