7 Conclusions

During the apparitions of 1995-1997, we successfully used the 1.56 m astrometric reflector at the Sheshan station of Shanghai observatory to acquire a total of 864 observations of the major Uranian satellites. As far as we aware, apart from our observations, only Veiga & Vieira Martins (1999) have made observations during this period. These observations can be useful to researchers wishing to make further investigation of the analytical theories, especially GUST86.

Our calculations have shown that the observations of Uranian satellites using CCD are much more accurate than using traditional photographic astrometry. This was also proved in the case of Saturnian satellites, by the comparisons of Harper et al. (1997, 1999). A fit of the analytical theory GUST86 to the real observations has proven that GUST86 is a good analytical theory with good dynamical consistency. Our analysis has shown that GUST86 can provide a high-accuracy ephemeris that is the most appropriate choice for analyzing observations of the Uranian satellites.

As pointed out previously, the disadvantage of using the satellites themselves for determining calibration parameters comes mainly from the fact that systematic errors of satellite ephemerides affect their derived positions. In order to give an estimation of these errors, an ephemeris generated by numerical integration was also used for calibration and fitting. This has shown that the systematic errors in GUST86 are generally less than $0\hbox{$.\!\!^{\prime\prime}$ }01$.

The program of CCD astrometry of Saturnian and Uranian satellites using the 1.56 m astrometric reflector at the Sheshan station was inaugurated in 1994. Since then, a significant number of high-quality CCD observations has been obtained. We intend to continue this program in spite of our funding difficulties. Next year, a back-illuminated chip with the large size of $2048\times2048$ pixels will be installed on the telescope at Sheshan, giving a field of view of approximately $500\hbox{$^{\prime\prime}$ }\times500\hbox{$^{\prime\prime}$ }$. This will be large enough to include a sufficient number of reference stars as calibration. The use of the new CCD device would provide the possibility of developing new investigations. Significant improvement in the accuracy of satellite astrometry can be expected in the near future.

Acknowledgements

We would like to thank again Prof. B. C. Qian, Dr. J. Tao and the staff at the Sheshan Station of Shanghai Observatory for providing us many conveniences and help throughout our observing run. We are very grateful to Dr. Taylor and Dr. Jones for their valuable assistance and their programs. It is a pleasure also to thank the referee Dr. Vienne for his helpful comments on the original manuscript; we gained a good deal of enlightenment from discussion with him on refraction correction. This work was carried out with financial support from National Science Foundation of China (NSFC).