R. C. Qiao

CCD astrometric observations of Phoebe in 2003-2004

In 2003-2004, we obtained 115 new observations of Phoebe, the 9th Saturnian faint satellite (visual magnitude of about 16.5). We used a large CCD detector (2048 x 2048 pixels) mounted on the 1.56 m astrometric reflector at the Sheshan Station, near Shanghai. In our reduction, an up-to-date catalogue of stars, UCAC2 (Zacharias et al. 2004), was chosen to ensure a proper astrometric calibration. A comparison of our observations to three recently available, high quality ephemerides, including the JPL SAT185 by Jacobson (2004b), has shown that most of our observed positions of Phoebe present an accuracy of some tens of mas, which appears to be a very high level for such a faint satellite.

Re-determination of Phoebe's orbit

In order to improve the orbit of Phoebe, the ninth satellite of the Saturnian system, 101 new observations were made by our research team in 2003, using a CCD detector of large size (2048×2048 pixels) mounted on the 1.56 m astrometric reflector at the Sheshan Station of Shanghai Astronomical Observatory. We fitted a numerical integration of its orbit to all of the collected Earth-based astrometric observations from 1904 to 2003, including the newest precise data sets from Qiao & Tang and from Peng et al. A new set of initial conditions of Phoebe has been obtained, leading to an improved orbit of this satellite.

Astrometry of five major Uranian satellites in 1995-1997 ?

At the oppositions of 1995-1997, a total of 122 CCD frames were taken on the 1.56 m astrometric telescope at the Sheshan station, yielding 864 positions of the major Uranian satellites. The calibration of the images was carried out using a least-squares iterative program by fitting to the well-known orbits of the brighter moons of Uranus, based on the modern theory GUST86 and an ephemeris produced by numerical integration. The residuals lay between 0: 00 03 and 0: 00 05 for each of the inter-satellite positions, except for the innermost and faintest satellite Miranda, whose residuals exceeded 0: 08 due to the proximity of Uranus. No significant systematic errors were found when using satellites themselves for determining calibration parameters. The largest residual in the comparison between GUST86 and the numerical integration was about 0: 00 01.

CCD astrometric observations of faint satellites and update of their orbits

This paper reports on our observing campaign of faint satellites performed at the National Time Service Center and Sheshan station of SHAO from 1994 up to today. In the past few years due to benefit from using a large size CCD and the publication of the modern catalogues (UCAC2), a series of observations of faint satellites were obtained by us. Moreover the work of improving the orbit of Phoebe via numerical fit to the observations over a century is also presented.