Astrometric Reduction Of Cassini ISS Images Of Phoebe

Phoebe is a satellite orbiting Saturn,and it is the only retrograde satellite of Saturn’s major satellites.Cassini ISS observed Phoebe and took a lot of images during 2004-2017,the high-precision position data of Phoebe can be provided by these images.They can be used for improving dynamics models,orbit and ephemeris,and such as.However,most of the Phoebe images were unreduced.In addition,some images have bright and dark background stripes.In previous measurements,the researchers ignored these stripes and directly reduced the images.However,the effect of these stripes on the accuracy of astrometry has never been considered.This question is also investigated in this thesis.The whole of our work includes the following three parts:(1)Removal of stripe noise in the Cassini images of Phoebe.This thesis proposes a frequency-domain processing method based on Fourier transform,which removes the high-frequency components in the center vertical direction of the Fourier spectrum of the image,and then obtains an image without stripe in the spatial domain.The results show that the method can eliminate greatly the background stripes in the Cassini images of Phoebe.What’s more,the method keeps the stars in the image as much as possible.The quality of the image is improved after stripe removal.(2)Comparative experiments on the effect of stripe on astrometry.Firstly,This thesis measured 100 Cassini images of Phoebe with stripes to get Phoebe’s astrometric position,then removed the stripes in these images,and measured Phoebe’s position in the image without stripes.Finally,this thesis compares these positional data of Phoebe before and after stripe removal.The results show that in the Cassini images of Phoebe,the background stripe has no significant effect on the astrometric results.(3)Astrometry of unreduced Phoebe images.This thesis uses Caviar to perform astrometric reduction on the Phoebe images that have not been reduced.During the reduction,Gaia EDR3 catalogue has been used to provide the reference stars’ positions,the modified moment has been used to measure the centre of image stars and Phoebe,then,we convert the position of Phoebe to the International Celestial Reference Frame(ICRF)coordinates with origin of Cassini.Finally,834 Phoebe ISS images have been reduced successfully.Compared with JPL ephemerides SAT375,Phoebe’s positions have a good fitting.The average residuals in the right ascension and direction are 0.075’’ and-0.048’’,and the standard deviations of the residuals are within 0.2’’.In the view of the residual distance,the means in the right ascension and direction are about 5.289 km and-2.174 km,respectively;The standard deviations are 11.282 km and 10.916 km,respectively.These results achieve the best precision in the world so far.Overall,the three-part works have been finished.Firstly,we proposed a good stripe removal algorithm for the Cassini images of Phoebe,secondly,the comparative experiment show that the background stripe in the Phoebe images has no obvious effect on the astrometric accuracy.Thirdly,we provide some high-precision position data of Phoebe,which fills the gap of Phoebe position data in the world.