Astrometric reduction of Cassini ISS images of the Saturnian satellites Mimas and Enceladus^⋆

¹ IMCCE, Observatoire de Paris, UMR 8028 du CNRS, UPMC, Université de Lille 1, 77 av. Denfert-Rochereau, 75014 Paris, France
e-mail: tajeddine@imcce.fr
² Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London E1 4NS, UK
³ Laboratoire AIM, UMR 7158, Université Paris Diderot – CEA IRFU – CNRS, Centre de l’Orme les Merisiers, 91191 Gif-sur-Yvette Cedex, France

Received: 2 December 2012
Accepted: 6 February 2013

Abstract

Aims. We provide astrometric observations of two of Saturn’s main satellites, Mimas and Enceladus, using high resolution Cassini ISS Narrow Angle Camera images.

Methods. We developed a simplified astrometric reduction model for Cassini ISS images as an alternative to the one proposed by the Jet Propulsion Labratory (JPL). The particular advantage of the new model is that it is easily invertible, with only marginal loss in accuracy. We also describe our new limb detection and fitting technique.

Results. We provide a total of 1790 Cassini-centred astrometric observations of Mimas and Enceladus, in right ascension (α) and declination (δ) in the International Celestial Reference Frame (ICRF). Mean residuals compared to JPL ephemerides SAT317 and SAT351 of about one kilometre for Mimas and few hundreds of metres for Enceladus were obtained, in αcosδ and δ, with a standard deviation of a few kilometres for both satellites. A frequency analysis of the residuals revealed some periodic variability in the right ascension for Mimas. An additional analysis of Mimas’ mean longitude suggests that some short-period terms are missing in the TASS orbital model.