The Caviar software package for the astrometric reduction of Cassini ISS images: description and examples

Abstract

Aims. Caviar is a software package designed for the astrometric measurement of natural satellite positions in images taken using the Imaging Science Subsystem (ISS) of the Cassini spacecraft. Aspects of the structure, functionality, and use of the software are described, and examples are provided. The integrity of the software is demonstrated by generating new measurements of the positions of selected major satellites of Saturn, 2013–2016, along with their observed minus computed (O−C) residuals relative to published ephemerides. Methods. Satellite positions were estimated by fitting a model to the imaged limbs of the target satellites. Corrections to the nominal spacecraft pointing were computed using background star positions based on the UCAC5 and Tycho2 star catalogues. UCAC5 is currently used in preference to Gaia-DR1 because of the availability of proper motion information in UCAC5. Results. The Caviar package is available for free download. A total of 256 new astrometric observations of the Saturnian moons Mimas (44), Tethys (58), Dione (55), Rhea (33), Iapetus (63), and Hyperion (3) have been made, in addition to opportunistic detections of Pandora (20), Enceladus (4), Janus (2), and Helene (5), giving an overall total of 287 new detections. Mean observed-minus-computed residuals for the main moons relative to the JPL SAT375 ephemeris were − 0.66 ± 1.30 pixels in the line direction and 0.05 ± 1.47 pixels in the sample direction. Mean residuals relative to the IMCCE NOE-6-2015-MAIN-coorb2 ephemeris were −0.34 ± 0.91 pixels in the line direction and 0.15 ± 1.65 pixels in the sample direction. The reduced astrometric data are provided in the form of satellite positions for each image. The reference star positions are included in order to allow reprocessing at some later date using improved star catalogues, such as later releases of Gaia, without the need to re-estimate the imaged star positions.

Interior properties of the inner saturnian moons from space astrometry data

“…Such work showed that the solar phase was an issue. Unfortunately, the authors did not (Cooper et al 2018). Clearly a more complex shape model is required to benefit fully from the high accuracy of ISS data.…”

First astrometric reduction of Cassini Imaging Science Subsystem images using an automatic procedure: application to Enceladus images 2013–2017

“…Table 2 provides a sample of the complete set of reduced observations. Following, for example Cooper et al (2018); Tajeddine et al (2013), the satellite positions in pixel coordinates and the camera pointing information in terms of the right ascension, declination (α c , δ c ) and twist angle θ are provided separately. This provides the user with the flexibility to be able to re-estimate the camera pointing correction at some later date, without having to re-measure the satellite positions, and vice versa for the satellite positions.…”

“…Methods for the astrometric reduction of Cassini ISS images using proprietary software have been described in Cooper et al (2006Cooper et al ( , 2008 and Tajeddine et al (2013). The development of these methods over the course of the Cassini mission culminated in the release to the community of the Caviar software package, developed jointly by QMUL (Queen Mary University of London) and IMCCE (Institut de mécanique céleste et de calcul des éphémérides) (Cooper et al 2018). In Caviar, the astrometric data reduction pipeline includes two main steps: a correction for errors in the nominal spacecraft camera pointing direction, followed by the astrometric measurement of the centre of the target planetary satellite.…”

“…At present, the Caviar package has no capability for the automatic correction of the camera pointing direction (Cooper et al 2018). Currently a 'bulk' manual alignment step is required in order to achieve an approximate match between the catalogue and imaged star positions before a final statistical fit is performed.…”

Astrometric Reduction of Cassini ISS Images of Enceladus in 2015 Based on Gaia DR1

“…During the past a few years, the Cassini ISS images have been routinely used to measure the astrometric positions of planetary satellites [1][2][3][4] . The soft package Caviar has been also implemented for the task [5] , which is convenient to reduce space images. Tajeddine et al [2] and Cooper et al [3] have reduced the ISS NAC images of Enceladus before 2014.…”