Study On The Pointing Correction Technique In Astrometry Of Cassini ISS Images

Images taken by Cassini ISS(Imaging Science Subsystem)are important resources for astrometry of Saturn's satellites.Researchers have got high precision position data of Saturn's satellites from ISS images.These data can promote the research of several areas,e.g.celestial mechanics and planetary physics.Therefore,the high precision astrometry of ISS images is of great value.Camera pointing correction is one of the most important procedure in high precision astrometry of ISS images.But there are two problems in the procedure of camera pointing correction.One is that,false image stars were often detected in the disk-resolved object in an ISS image,which would cause adverse effect in the precision of pointing correction.The other problem is that,all reference stars are streaks(trailed objects)in a part of ISS images.Their center positions cannot be obtained accurately,so the pointing correction cannot be done for these images.Furthermore,they are not available for astrometry.For the first problem,this thesis proposes an edge detection based algorithm,which includes four steps: extracting edges,dividing edge arcs,circular fitting of edge arcs and excluding false image stars in the fitting circles.The method can automatically remove false image stars in diskresolved objects in the Cassini ISS image.The result shows that it can remove the false image stars in more than 95% of ISS images with disk-resolved objects.After comparing with the frequentlyused Circular Hough Transform(CHT)based algorithm,this algorithm surpassed the CHT based algorithm in automation,success rate of detection,and processing speed.The proposed algorithm was also applied in a set of ISS images of Rhea to eliminate the mismatch in pointing correction in automatic procedure.Experiment result showed that the precision of final astrometry results can be improve after using the algorithm.For the second problem,template-based method was proposed to measure the streaks' center positions.In the method,a semi-manual operation is applied in an easy-used GUI.These measured streaks' centroids are used for pointing correction.Furthermore,the astrometry of the kind of images has been done.Finally,a set of images of Mimas with streaked stars were used to test the method,the preliminary astrometry result shows the method is available for astrometry of ISS images with streaked reference stars.Solving the above two problems can improve the accuracy and efficiency of astrometry,and expand the number of images that can be used for astrometry,making good use of precious data from Cassini ISS.