CCD Positions Determined in the International Celestial Reference Frame for the Outer Planets and Many of Their Satellites in 1995-1999

摘要

This paper presents 1155 accurate equatorial positions for the outer planets Uranus, Neptune, and Pluto and 17 satellites of Jupiter–Neptune. Additional positions for Jupiter, Saturn, Uranus, and Neptune can be determined from the planetocentric motions of their satellites given in this paper. All the positions were determined in the International Celestial Reference Frame (ICRF), the IAU standard for all future astrometric reductions, from CCD observations taken with the Flagstaff Astrometric Scanning Transit Telescope (FASTT) and reduced differentially using the ACT reference stars. The methods used to determine these positions are fully described. Accuracies of ±008 to ±025 were obtained in each coordinate, depending on the signal-to-noise ratio observed for each object. In many cases, planets and satellites were imaged in the same CCD field of view, thereby giving excellent relative astrometry (±004) for well-exposed images. Moreover, 424 older FASTT positions determined in 1995–1997 for Uranus, Neptune, and Pluto were converted to the ICRF and are given also in this paper. When FASTT positions are compared with modern Jet Propulsion Laboratory (JPL) ephemerides for the planets and satellites considered herein, there is generally good agreement (less than 005) between observation and theory. For the planets, FASTT observations and DE405 ephemerides all agree to better than 005 and, in most cases, less than 003 when mean differences are formed. There are some exceptions for the satellites. Namely, the ephemerides for the outer satellites of Jupiter and Saturn considered in this paper (Himalia, Elara, Pasiphae, and Phoebe) and the Uranian satellites Titania and Oberon probably need improvement as indicated from the FASTT observational data. The former show systematic trends when (FASTT - JPL ephemeris) coordinate differences are plotted against either coordinate position or orbital phase, and the latter show a possible offset between the right ascension of Uranus and those of the two satellites. Future FASTT observations of these planets and satellites are planned in the coming years.