A Comparison Of Centering Algorithms In The Astrometry Of Cassini Imaging Science Subsystem Images And Anthe's Astrometric Reduction

The Cassini Imaging Science Subsystem(ISS)has recorded a large number of optical images,which provides important resource for natural satellite astrometry.Caviar is a standard tool for astrometry of ISS images,which uses two-dimensional Gaussian fitting method to measure the centre of point-like objects.Whether other commonly used centering algorithms can further improve the centering accuracy of objects in ISS images is still a problem worthy of further study.Anthe is a small inner Saturnian satellite,which is difficult to observe and we are short of data.ISS has taken a great number of Anthe images,and its reduction can fill in the gap of Anthe's position.Firstly,aiming at the accuracy of centering algorithms,we used 56 ISS images of star field to carry out the comparative experiment of the modified moment,two-dimensional Gaussian fitting and median method.The Gaussian fitting was performed by Caviar's existing two-dimensional Gaussian fitting from DAOPHOT.For the modified moment and median method,an appropriate scheme to automatically determine the stars' background and foreground area was proposed.Then three algorithms were used to measure the positions of many point-like stars in 56 ISS images.The positions of these stars in Gaia DR2 catalogue were also reduced to image coordinates.The observed-minus-calculated(O-C)residuals were calculated to evaluate the precision of each centering algorithm.Another method to evaluate centering algorithms' precision by excluding the error of calculated coordinates was also used as an auxiliary proof.The results showed that the modified moment method was the best,its standard deviation of residuals is 0.065 pixels in sample and 0.063 pixels in line.Compared with two-dimensional Gaussian fitting method currently used,the precision of the modified moment had been improved more than one time.Therefore,the modified moment should be used for the centering of point-like stars in ISS images.Based on the previous study,112 NAC measurable images of Anthe were measured by the modified moment,and the results were compared with those measured by two-dimensional Gaussian fitting.In this experiment,the centre positions of reference stars in ISS images were measured by the modified moment and two-dimensional Gaussian fitting respectively.Based on these results,the pointing correction of NAC images was carried out.Since Anthe was a point source in ISS images,the modified moment and two-dimensional Gaussian were used to measure Anthe's centre position then.Two sets of residuals were calculated by comparing Anthe's centre positions with SAT 393 catalogue.The results showed that the modified moment method still had the advantage of accuracy in the centering of point source satellites.The modified moment method was more suitable for the measurement of point-like satellites in ISS images than the existing two-dimensional Gaussian fitting method in Caviar.Finally,we measured all Anthe ISS images taken from 2008 to 2017.A total of112 Anthe NAC images and 70 WAC images were successfully reduced.The results showed that compared with SAT 393 catalogue,the mean values of all residuals of NAC images in the right ascension and declination directions were-0.849 km and0.022 km,and the standard deviations of residuals were 1.684 km and 0.917 km.The mean values of all residuals of WAC images in right ascension and declination were-3.472 km and-0.008 km,and the standard deviations of residuals were 4.177 km and4.017 km,respectively.According to the published data,the accuracy of these positions is the highest accuracy of Anthe's measurements so far,and the data also fill in the gap in Anthe's observations from 2008 to 2017.