Complementary astrometry of Cassini Imaging Science Subsystem Images of Phoebe

Abstract: Phoebe is the only major satellite of Saturn with a retrograde orbit. The Cassini Imaging Science Subsystem (ISS) took a lot of Phoebe images between 2004 and 2017, but only a selection of them has been reduced. In this paper, we reduced the remaining ISS images of Phoebe. In the reduction, the Gaia EDR3 catalogue was used to provide the reference stars' positions, and the modified moment was used to measure the centre of image stars and Phoebe. Finally, a total of 834 ISS images of Phoebe have been reduced successfully. Compared with the JPL ephemeris SAT375, Phoebe's positions are consistent. The average residuals in the right ascension and declination are 0.08" and $-0.05$", and the standard deviations of the residuals are about 0.2". In terms of residuals in linear units, the means in the right ascension and declination are about 5 km and $-2$ km, respectively; The standard deviations are about 11 km. Compared with the JPL ephemeris SAT427 and IMCCE ephemeris PH20, our measurements show a strong bias and a large dispersion.