The characterization and calibration of the OSIRIS infrared imager

Abstract

OSIRIS, a Canadian built instrument on-board the Swedish-led remote sensing satellite, Odin, consists in part of three single lens imagers that measure near infrared light from atmospheric scattering and emission. A full calibration of the imaging system is required to remove all instrument dependent effects that modify the observations. This work presents the characterization and calibration of the OSIRIS imaging system in an attempt to produce observations that are instrument independent measurements of the atmospheric brightness. The required product is the number of photons per second emitted, or scattered, from the atmosphere that are within the sampling wavelength range and incident on the detector area in the instrument field of view. A major portion of the present work involves understanding the dark current production mechanisms and the development of a technique to characterize the dark current and manufacturer imposed electronic offsets. It is demonstrated that with a current set of dark calibration images, the developed algorithm effectively removes the dark current and electronic offsets over a wide operating temperature range. The relative calibration of pixels is presented in terms of the electronic gain, or flat field response, and the angular look direction. It is apparent that a change in the relative pixel gain occurred between pre-flight calibration and the first in-flight images. However, it is shown that with a recalculation of the flat field response using in-flight images, an acceptable gain calibration is obtained. The angular look direction of the pixels is determined from the results of two separate in-flight experiments. The characterization and removal of the stray light signal is shown to be effective. Finally, the absolute calibration of the instrument is presented. While several issues remain to be addressed, the comparison with a simple atmospheric brightness model provides a first order verification of the results.