Astrometric reduction of Cassini ISS images of Helene

Abstract

Helene is a small irregular Saturnian satellite. It is difficult to observe Helene by ground-based telescopes because of its weak lightness (18.5 mag) and proximity to Saturns rings. The number of historical observations of the satellite is small, but a large number of images of Helene were obtained between 2004 and 2017 using the Imaging Science Subsystem (ISS) in Cassini Spacecraft. In this paper, astrometry reduction was performed based on these observations. Specifically, we first corrected the initial pointing of the Narrow Angle Camera in ISS using the referencing Gaia DR2, and then measured the Helenes center position in image coordinates using the two-dimensional Gaussian fitting method. Finally the position was transformed to the Right Ascension ($\alpha$) and Declination ($\delta$) in International Celestial Reference Frame (ICRF) centered at Cassini. The result was compared with JPL SAT393 ephemerides, and the position residual was calculated. Totally 890 images were effectively measured. The results showed that compared with SAT393, the mean values of all residuals in $\alpha$$*$cos$(\delta)$ and $\delta$ directions were $0.23$ and $-0.63$ km, respectively, and the standard deviations were $3.48$ and $2.23$ km, respectively. This precision was much higher than that of ground observations, indicating the advantage of space-based observations.