COMPUTER MODELING OF THE OSIRIS INFRARED IMAGER

Abstract

The OSIRIS Optical Spectrometer and Infrared Imager was developed at the Institute of Space and Atmospheric Studies (University of Saskatchewan) for the Odin satellite that will make measurements of anthropogenic effects on the Earth's atmosphere, such as Antarctic ozone depletion, global warming etc. The OSIRIS spectrometer will make limb observations of the Earth's atmosphere and measure both the scattered sunlight and the airglow spectra of the middle and upper levels of the atmosphere. The total flux of light scattered by Earth's surface and entering the OSIRIS IR imager can be 3-4 orders of magnitude greater than the flux from the airglow. Thus the IR imager must have good stray light rejection in order to prevent unwanted radiation from off-axis sources interfering, or limiting, the orbital measurements. In this thesis the problem of the OSIRIS stray light performance evaluation has been addressed. The potential impact of the interfering radiation on the orbital measurements has been determined with a computer simulation of the optical performance of the OSIRIS IR imager in the ASAP optical ray tracing software. The ASAP software is an optical modeling program that is specifically designed for the stray light analysis of optical instrumentation. A model of the OSIRIS IR imager has been created in the ASAP ray tracing software program. The image deterioration, in-field and out-of field stray light has been analyzed by tracing rays through the model. The Point-Spread Function and Normalized Detector Irradiance functions were obtained and the critical objects and effects that dominate the image deterioration and stray light were identified. The ASAP IR imager model was also used for a vignetting and tolerancing analysis. Those objects responsible for the vignetting were identified and the sensitivity of the channel response function with respect to the positioning of IR imager optical and mechanical components determined. The simulated optical performance and the measured calibration and characterization data are shown to be in good agreement. The relative error in the IR imager orbital measurements that is due to the image deterioration and stray light is estimated. It is shown that the total measurement error of the IR imager orbital measurements is less than 3% for the orbit-solar angle equal to 0° and so should not affect the accuracy of the orbital observations and the tomographic data analysis.