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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. [ABSTRACT FROM AUTHOR]

We provide new astrometric observations of Enceladus from images taken using the Cassini Imaging Science Subsystem (ISS) Narrow-Angle Camera. Astrometric reduction was performed using a new fully automatic camera repointing technique built within the Caviar software package. The new automatic procedure is based on the k-d tree technique and has been developed as an alternative to the manual technique currently available in Caviar. We describe the new procedure and demonstrate its effectiveness compared with manual processing. Repointing corrections were computed using reference stars from the UCAC4 and Gaia DR1 star catalogues. We compare the observed-minus-computed (O−C) residuals of Enceladus based on UCAC4 with those based on Gaia DR1, relative to the JPL SAT375 ephemeris and the IMCCE NOE ephemeris, respectively. We reduced all available Cassini ISS images of Enceladus during the period 2013–2017, producing a total of 1109 Cassini -centred astrometric observations. These are provided in right ascension (α) and declination (δ) in the International Celestial Reference Frame (ICRF). Estimated mean O−C residuals relative to JPL's SAT375 ephemeris were 0.7 km in α  × cos(δ) and 1.8 km in δ, with standard deviations of 2.4 and 3.2 km, respectively. No improvement in the precision of the observations was obtained when using the GAIA DR1 star catalogue compared with UCAC4. [ABSTRACT FROM AUTHOR]

755 new CCD observations during the years 2014-2016 have been reduced to derive the precise positions of Triton, the first satellite of Neptune. The observations were made by the 1-m telescope at Yunnan Observatory over 15 nights. The positions of Triton are measured with respect to the stars in Gaia DR1 star catalogue. The theoretical position of Triton was retrieved from the Jet Propulsion Laboratory (JPL) Horizons System that includes the satellite ephemeris nep081xl, while the position of Neptune was obtained from the JPL ephemeris DE431mx. Our results show that the mean (O -- C)s (observed minus computed) are 0.042 and -0.006 arcsec in right ascension and declination, respectively. The dispersions of our observations are estimated at about 0.012 arcsec in each direction. [ABSTRACT FROM AUTHOR]

298 CCD observations during the year 2013 have been reduced to derive the precise positions of near-Earth asteroid (99942) Apophis. The observations were made by the 2.4-m telescope at Yunnan Observatory over 10 nights. The position and proper motion errors of the reference stars in the USNO CCD Astrograph Catalogue 4 (UCAC4) star catalogue are corrected by using the newest correction table provided by Farnocchia et al. The geometric distortion of the field of view is also derived from the unbiased star positions in UCAC4 and removed. The theoretical position of Apophis was retrieved from the Jet Propulsion Laboratory Horizons system. Our results show that the mean O-Cs (observed minus computed) are 0.016 and 0.034 arcsec in right ascension and declination, respectively. The dispersions of our observations are estimated at 0.041 and 0.045 arcsec in right ascension and declination, respectively. [ABSTRACT FROM AUTHOR]

346 new CCD observations during the years 2011-2014 have been reduced to derive the precise positions of Phoebe, the ninth satellite of Saturn. The observations were made by the 2.4 m telescope at Yunnan Observatory over nine nights. Due to the use of a focal-reducer on the telescope, its significant geometric distortion is solved for and removed for each CCD field of view. The positions of Phoebe are measured with respect to the stars in UCAC2 catalogue. The theoretical position of Phoebe was retrieved from the Institute de Méchanique Céleste et de Calcul des Éphémérides ephemeris which includes the latest theory PH12 by Desmars et al., while the position of Saturn was obtained from the Jet Propulsion Laboratory ephemeris DE431. Our results show that the mean O-Cs (observed minus computed) are -0.02 and -0.07 arcsec in right ascension and declination, respectively. The dispersions of our observations are estimated at about 0.04 arcsec in each direction. [ABSTRACT FROM AUTHOR]

This paper presents 210 positions of Phoebe, the ninth satellite of Saturn, observed with the 1-m telescope at the Yunnan Observatory during the years 2003–2005, using a CCD image-overlapping calibration method proposed recently by Peng et al. After the observed positions of Phoebe are compared with its theoretical positions computed by the new JPL ephemerides DE405 and SAT199, the mean residuals (observed minus computed) are 0.21 and −0.05 arcsec in right ascension and declination, respectively, with a standard deviation of 0.06 arcsec. [ABSTRACT FROM AUTHOR]

755 CCD observations during the years 2014-2016 have been reduced to derive the precise positions of Triton, the first satellite of Neptune. The observations were made by the 1 m telescope at Yunnan Observatory over 15 nights during the years 2014-2016. The theoretical position of Triton was retrieved from the Jet Propulsion Laboratory Horizons system. Our results show that when the newest Gaia catalogue (Gaia DR1) is referred to the mean O-Cs (observed minus computed) residuals are about 0.042 and -0.006 arcsec, the dispersions are 0.012 and 0.012 arcsec in right ascension and declination, respectively. The dispersions are improved very significantly when the Gaia DR1 is referred to. However, the agreement in right ascension is not so good as that in declination, the reason might come from the uncertainty of planet ephemeris. More observations are needed to confirm this. [ABSTRACT FROM PUBLISHER]