A comparison of centring algorithms in the astrometry of Cassini imaging science subsystem images and Anthe’s astrometric reduction

Abstract

In the Caviar software package, a standard tool for astrometry of images from the Cassini Imaging Science Subsystem (ISS), Gaussian fitting is used to measure the centre of point-like objects, achieving a typical precision of about 0.2 pixels. In this work, we consider how alternative methods may improve on this. We compare three traditional centroiding methods: two-dimensional Gaussian fitting, median, and modified moment. Results using 56 selected images show that the centroiding precision of the modified moment method is significantly better than the other two methods, with standard deviations for all residuals in sample and line of 0.065 and 0.063 pixels, respectively, representing a factor of over 2 improvement compared to Gaussian fitting. Secondly, a comparison of observations using Cassini ISS images of Anthe is performed. Anthe results show a similar improvement. The modified moment method is then used to reduce all ISS images of Anthe during the period 2008-2017. The observed-minus-calculated residuals relative to the JPL SAT393 ephemeris are calculated. In terms of α×cos(δ) and δ in the Cassini-centred international celestial reference frame, mean values of all residuals are close to 0 km, and their standard deviations are less than 1 km for narrow angle camera images, and about 4 km for wide angle camera images.

Complementary astrometry of Cassini Imaging Science Subsystem images of Phoebe

“…So centring method is suitable. Zhang et al (2021) point out the modified moment is better than Gaussian fitting for the measurement of point-like objects due to the under-sampled feature of ISS NAC. Hence the modified moment method is used.…”

Astrometric Reduction of Saturnian Satellites with Cassini-ISS Images Degraded by Trailed Stars

“…After finding out all streak regions, an improved modified moment method is used to compute the centers of all streaks. Similar to the method described by Zhang et al (2021), we developed a modified moment method to locate the centers of streaks. It includes three secondary steps as follows.…”

“…Tajeddine et al (2013Tajeddine et al ( , 2015 and Cooper et al (2014) reduced a part of ISS images of several main Saturnian satellites. Zhang et al (2018Zhang et al ( , 2021Zhang et al ( , 2022 measured some ISS images of several Saturnian satellites. These data have been used in the studies of the dynamics of planetary systems (Desmars et al 2013;Cooper et al 2015;Lainey et al 2017Lainey et al , 2020.…”