Probing the Atmospheric Precipitable Water Vapor with SOFIA. V. Telluric Scaling: Correcting Telescope Elevation Effects in Non-chopped Observations by Modeling the Telluric Background Based on the Water Vapor Overburden

We present a new data reduction technique for unchopped spectroscopic data developed for the Far-Infrared Field Integral Line Spectrometer instrument on-board the Stratospheric Observatory for Infrared Astronomy (SOFIA) to reduce spectral artifacts caused by large nod offsets. These large offsets are required to quantify the background without astronomical signal when large regions in the sky are mapped, e.g., in the Large Magellanic Cloud where reference beam offsets up to 1° can be needed. The technique uses the precipitable water vapor overburden derived from re-analyses from the European Centre for Medium-Range Weather Forecasts to determine the fraction of flux measured in each spatial and spectral pixel contributed by the atmospheric background. With that it is then possible to scale the reference measurement in the off beam to the sky elevation of the on source measurement. These large maps on SOFIA typically use the “on-the-fly” observing mode. The mode is highly efficient, but each point in the final map is integrated with multiple scans. Therefore artifacts from background subtraction do not only generate point by point offsets but can also degenerate the signal to noise ratio in each of the spectra since the sky background is often not spectrally flat.