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

Zhang, Qingfeng; Lainey, V.; Cooper, Nicholas J.; Vienne, A.; Peng, Q. Y.; Xiong, Yinquan

doi:10.1093/mnras/sty2187

Cited by 13 publications

(10 citation statements)

References 21 publications

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“…Similar to prior work 4 , we used more than a century of observations, starting in 1886 through the whole Cassini mission. New observations of the main moons 17,18 were added to supplement previous data 4 Prometheus, Pandora, Janus and Epimetheus is introduced by ephemerides. We checked that the chaos affecting the orbits of these moons, as well as a possible secular variation of Saturn's J 2 , did not affect our results (Methods).…”

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confidence: 99%

Resonance locking in giant planets indicated by the rapid orbital expansion of Titan

et al. 2020

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Tidal effects in planetary systems are the main driver in the orbital migration of natural satellites. They result from physical processes occurring deep inside celestial bodies, whose effects are rarely observable from surface imaging. For giant planet systems, the tidal migration rate is determined by poorly understood dissipative processes in the planet, and standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance 1 , implying little migration for outer moons such as Saturn's largest moon, Titan. Here, we use two independent measurements obtained with the Cassini spacecraft to measure Titans orbital expansion rate. We find Titan migrates away from Saturn at 11.3 ± 2.0 cm/year, corresponding to a tidal quality factor of Saturn of Q ' 100, and a migration timescale of roughly 10 Gyr. This rapid orbital expansion suggests Titan formed significantly closer to Saturn and has migrated outward to its current position. Our results for Titan and five other moons agree with the predictions of a resonance locking tidal theory 2 , sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturns moon system. The resonance locking mechanism could operate in other systems such as stellar binaries and exoplanet systems, and it may allow for tidal dissipation to occur at larger orbital separations than previously believed.Saturn is orbited by 62 moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Of crucial importance are tidal interactions between the moons and the planet. Each moon raises a tidal bulge in the planet, and because Saturn rotates faster than the moons orbit, frictional processes within the planet cause the tidal bulge to lead in front of each moon. Each moon's tidal bulge pulls the moon forward such that it gains angular momentum and migrates outward, similar to the tidal evolution of the Earth-Moon system. However, in giant planets such as Saturn, the dissipative processes that determine the bulge lag 2

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confidence: 99%

Resonance locking in giant planets indicated by the rapid orbital expansion of Titan

et al. 2020

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“…Both during, and after the end of the mission, these images have and continue to be an important resource for natural satellite astrometry. For example, Cooper et al (2006) reduced ISS images of inner Jovian satellites, while Cooper et al (2014) performed mutual-event astrometry of ISS images of the mid-sized Saturnian satellites and Tajeddine et al (2013Tajeddine et al ( , 2015 and Zhang et al (2018) reduced images of the main icy Saturnian satellites. A software package, Caviar 1 , dedicated to the astromet-E-mail: valery.lainey@obspm.fr (VL) † E-mail: n.cooper@qmul.ac.uk (NJC) ‡ E-mail: tpengqy@jnu.edu.cn (QYP) 1 Available under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (https://www.imcce.fr/recherche /equipes/pegase/caviar) ric reduction of Cassini ISS images has since been released to the community (Cooper et al 2018).…”

Section: Introductionmentioning

confidence: 99%

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

Zhang

Xiaomei

Yue-jin

et al. 2021

Monthly Notices of the Royal Astronomical Society

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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.

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“…The standard deviations of residuals were a few kilometers. Zhang et al (2017Zhang et al ( , 2018 have conducted astrometry of Enceladus using automatic method based on k-dimensional tree and Gaia DR1 star catalogue. Estimated standard deviations of residuals relative to JPL's SAT375 ephemeris were 2-4 kilometers.…”

Section: Introductionmentioning

confidence: 99%

“…At present, the three mismatching stars can only be removed with a manual method. To enhance the efficiency and decrease the disturbance, automatic methods should be developed (Zhang et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Automatic removal of false image stars in disk-resolved images of the Cassini Imaging Science Subsystem

Zhang,

Lu,

Zhou

et al. 2020

Preprint

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Taking a large amount of images, the Cassini Imaging Science Subsystem (ISS) has been routinely used in astrometry. In ISS images, disk-resolved objects often lead to false detection of stars that disturb the camera pointing correction. The aim of this study was to develop an automated processing method to remove the false image stars in disk-resolved objects in ISS images. The method included the following steps: extracting edges, segmenting boundary arcs, fitting circles and excluding false image stars. The proposed method was tested using 200 ISS images. Preliminary experimental results show that it can remove the false image stars in more than 95% of ISS images with disk-resolved objects in a fully automatic manner, i.e. outperforming the traditional circle detection based on Circular Hough Transform (CHT) by 17%. In addition, its speed is more than twice as fast as that of the CHT method. It is also more robust (no manual parameter tuning is needed) when compared with CHT. The proposed method was also applied to a set of ISS images of Rhea to eliminate the mismatch in pointing correction in automatic procedure. Experiment results showed that the precision of final astrometry results can be improve by roughly 2 times than that of automatic procedure without the method. It proved that the proposed method is helpful in the astrometry of ISS images in fully automatic manner.

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

Cited by 13 publications

References 21 publications

Resonance locking in giant planets indicated by the rapid orbital expansion of Titan

Resonance locking in giant planets indicated by the rapid orbital expansion of Titan

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

Automatic removal of false image stars in disk-resolved images of the Cassini Imaging Science Subsystem

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