Characterization and Absolute Calibration of the Far Infrared Field Integral Line Spectrometer for SOFIA

TL;DR

This paper details the comprehensive characterization and flux calibration of the FIFI-LS instrument on SOFIA, including techniques for flat-fielding, spectral resolution, and calibration accuracy, with improvements after 2018.

Contribution

It provides the first definitive flux calibration and detailed response characterization of FIFI-LS, including response changes after filter modifications in 2018.

Findings

Flux calibration accuracy is better than 15%.

Linearity deviations affect flux estimates by less than 1%.

Response responses were characterized before and after filter change.

Abstract

We present the characterization and definitive flux calibration of the Far-Infrared Field Integral Line Spectrometer (FIFI-LS) instrument on-board SOFIA. The work is based on measurements made in the laboratory with an internal calibrator and on observations of planets, moons, and asteroids as absolute flux calibrators made during the entire lifetime of the instrument. We describe the techniques used to derive flat-fields, water vapor column estimates, detector linearity, spectral and spatial resolutions, and absolute flux calibration. Two sets of responses are presented, before and after the entrance filter window was changed in 2018 to improve the sensitivity at 52um, a wavelength range previously not covered by PACS on Herschel. The relative spectral response of each detector and the illumination pattern of the arrays of the FIFI-LS arrays are derived using the internal calibrator before each observational series. The linearity of the array response is estimated by considering observations of bright sources. We find that the deviation from linearity of the FIFI-LS arrays affects the flux estimations less than 1%. The flux calibration accuracy is estimated to be 15% or better across the entire wavelength range of the instrument. The limited availability of sky calibrators during each observational series is the major limiting factor of the flux calibration accuracy.