2 Optical observations

The observations analysed here are taken from four sources: Carlsberg and Bordeaux Automatic Meridian Circles, and the Hipparcos and Tycho Catalogues.

2.1 Carlsberg Automatic Meridian Circle

The Carlsberg Automatic Meridian Circle (CAMC) is operated remotely via the Internet on the island of La Palma at the international observatory Roque de los Muchachos of the Instituto Astrofisica de Canarias. It is situated at a latitude of 28.7$^\circ$ north and an altitude of 2300m and is operated jointly by Copenhagen University Observatory, the Royal Greenwich Observatory and the Real Instituto y Observatorio de la Armada, San Fernando. The operating procedure is described in Helmer & Morrison (1985) and a description of the scanning-slit micrometer and photoelectric detector system can be found in Helmer et al. (1991). The positions of the Solar System objects are measured once nightly as they cross the prime meridian.

The observations discussed in this paper are published in an annual series of catalogues in the period 1985-1997, which have been compiled into one catalogue - Carlsberg Meridian Catalogue Number 10 (1998). The number of observations of Solar System objects in this composite catalogue are listed in Table 2. The accuracy of the positions is a function of epoch of observation and declination, as listed in Table  1. The best accuracy of $\pm 0\hbox{$.\!\!^{\prime\prime}$}12$ for a single observation is obtained in the zenith (Dec $\sim$ +30$^\circ$). The accuracy has improved with time as a consequence of improvements in instrumentation and processing of the raw data.

  

Table 1: Accuracy of individual Carlsberg observations

  

Table 2: Carlsberg observations of outer Solar System objects analysed in this paper

Before 1995-96 the positions were referred to a frame close to the FK5, as described by Morrison et al. (1990). This is referred to here as the Carlsberg reference frame. The Hipparcos Catalogue has been used to measure the systematic errors of the Carlsberg frame with respect to the ICRF. The observed positions have been corrected retrospectively for these systematic errors. Starting with 1995-96 the observations were made with respect to the Hipparcos Catalogue on the ICRF.

In the cases of Jupiter and Saturn, their bright satellites Ganymede, Callisto and Titan and Iapetus were observed, rather than the planets themselves, because of the difficulty of determining the barycentres from scans of their large, non-uniform disks. Lambert scattering was used in reducing the optical positions to the centre of figure (Lindgren 1977). The maximum correction for phase for the planets and satellites discussed in this paper is 50mas in the case of Ganymede. The choice of scattering law is not critical in this discussion since the consequential variation in position due to different models of the phase correction is always less than $\sim$10mas. Corrections for the asymmetry in the albedo of Iapetus are important, however. These have an estimated amplitude in orbital longitude of $\sim$30mas with the $\sim$80-day period of revolution of Iapetus which is tidally locked to Saturn. We return to this point in Sect. 3.

2.2 Bordeaux Automatic Meridian Circle

In 1995 the Bordeaux Automatic Meridian Circle was equipped with a CCD and this made possible the observation of Pluto relative to the Hipparcos/Tycho frame with an accuracy of 70mas for a single observation (Réquiemè et al. 1997). In the opposition of 1995, 13 observations were made, in 1996 six, and in 1997 13. The mean values and their standard errors were computed for these three oppositions.

2.3 Hipparcos/Tycho

During the Hipparcos mission from 1989 to 1993, 64 and 38 observations of Europa and Titan, respectively, were made by Hipparcos, and 13 and 16 of Callisto and Ganymede, respectively, by Tycho.

Whereas the satellites Callisto and Ganymede were scanned by Tycho using slits inclined at different angles, Europa and Titan were scanned uni-directionally by Hipparcos. The multi-directional scans by Tycho give directly two-dimensional coordinates (right ascension and declination) of Callisto and Ganymede for each observation. The uni-directional observations by Hipparcos, on the other hand, have to be combined in groups having preferably orthogonal directions of scan in order to solve for positions in right ascension and declination. The treatment of the Hipparcos data is given in some detail in Morrison et al. (1997). The three group solutions for Europa and four for Titan given in that paper are used here.