5 Results and analyses

If the scale and orientation of CCD chip are adopted, we can calculate conveniently the mean (O-C)s (Observed minus Computed) and standard errors of the observations in form of inter-satellite or Jupiter-satellite $\Delta\alpha$cos$\delta$ and $\Delta\delta$measures (see Vienne et al. 2001 and Paper I for detailed reference). In our implement, Ganymede has been chosen as the reference satellite since its near-center position in the field of view. Table 2 shows the mean (O-C)s and their standard errors in right ascension and declination of Io, Europa, Callisto and Jupiter. As shown in Table 2, the mean (O-C)s for each satellite and Jupiter are quite small when JUP204 is referred. Moreover, a very good agreement among the mean (O-C)s for CCD images obtained with different exposure time can be seen when the same ephemeris is referred. But slightly different standard errors when different exposure time is used mean that a longer exposure time produces better precision. We also note that almost all mean (O-C)s from G5 are greater than those from JUP204. The results suggest that JUP204 might be superior to G5 though one night observations cannot draw this conclusion. However, the best precision is found for Io among the four satellites, especially in right ascension and when G5 is referred. It reveals a fact that a short separation between two satellites allows us to obtain very good positional precision. We believe that a short separation CCD observation would be very profitable to two Galilean satellites before and after their mutual phenomena-occultation. Better precision for Io when G5 is referred might be explained as a coincidence. In more detail, the variation of its light center might coincide with the variation of its prediction position of G5. In addition, the differences in mean (O-C)s for all satellites and Jupiter are usually small when two sets of calibration parameters are adopted respectively. The biggest difference is 23 mas for Europa in declination. This is understandable because of a quite great separation between this satellite and Ganymede. It suggests that a stricter calibration for the CCD field of view deserves to be investigated. At last, the appreciable mean (O-C)s even when the newest ephemeride JUP204 is used could be explained mainly as the difference in each satellite's positions between center-of-light relative to center-of-figure, since this difference may reach "the order of 75-100 km'' (i.e., 25-33 mas for each satellite) according to Lieske (1998).