Complementary astrometry of Cassini Imaging Science Subsystem Images of Phoebe

TL;DR

This study reprocessed 834 Cassini ISS images of Phoebe using Gaia EDR3 data, achieving precise astrometric positions that are consistent with JPL ephemeris SAT375, but revealing biases with other ephemerides.

Contribution

The paper provides a comprehensive reduction of previously unprocessed Cassini images of Phoebe, improving astrometric accuracy using Gaia data and comparing results with existing ephemerides.

Findings

Positions are consistent with JPL ephemeris SAT375.

Residuals are approximately 0.2" or 11 km in linear units.

Strong biases and dispersion observed when compared with other ephemerides.

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.