Spectral and Spatial Characterization and Calibration of FIFI-LS - The Field Imaging Spectrometer on SOFIA

The field-imaging far-infrared line spectrometer (FIFI-LS) is a science instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA). FIFI-LS allows simultaneous observations in two spectral channels. The "blue" channel is sensitive from 51 um to 125 um and the "red" channel from 115 um to 203 um. The instantaneous spectral coverage is 1000–3000 km/s in the blue and 800–2500 km/s in the red channel with a spectral resolution between 150 km/s and 600 km/s. Each spectral channel observes a field of five by five spatial pixels on the sky. The pixel size in the blue channel is 6.14 by 6.25 square arc seconds and it is 12.2 by 12.5 square arc seconds in the red channel. FIFI-LS has been operating on SOFIA since 2014. It is available to the astronomical community as a facility science instrument. We present the results of the spectral and spatial characterization of the instrument based on laboratory measurements. This includes the measured spectral resolution and examples of the line spread function in the spectral domain. In the spatial domain, a model of the instrument's point spread function (PSF) and the description of a second pass ghost are presented. We also provide an overview of the procedures used to measure the instrument's ¯eld of view geometry and spectral calibration. The spectral calibration yields an accuracy of 15–60 km/s depending on wavelength.