2 Instrumentation and observations

On the night of Jan. 4, 2002 (the opposition of Jupiter is on Jan. 1, 2002), 133 frames of CCD images in total were obtained with the 1-meter telescope at the Yunnan Observatory. While observing, Johnson B-type filter was used. Besides, the exposure times of 1, 2 and 3 second(s) were adopted alternately (numbers of frames of CCD images are 44, 46 and 43, respectively for the exposure time of 1, 2 and 3 second(s)). In theory, we should have used Johnson I-type filter for Jupiter, a methane atmosphere planet. In fact, we had tried to use Johnson I-type filter at the beginning of the observation. But the overwhelming brightness of Jupiter always caused over-saturated within less than 1 s of exposure time. In order to obtain good quality images of both Jupiter and its Galilean satellites, we choose the Johnson B-type filter to decrease the brightness of Jupiter. Besides, the use of B-type filter allowed us to increase exposure time so that relative positions between any two satellites as well as Jupiter and a satellite have quite good precision. Although the used telescope was not specially made for astrometry, some publications (for example, Tang et al. 2000; Tang et al. 2001; Tang et al. 2002 and Paper I) have shown that it allows us to obtain good information on astrometry. For the detailed specifications of the telescope and CCD chip the reader may refer to Paper I. Figure 1 is a typical CCD image for the Jupiter and its Galilean satellites. As we have seen, Europa is the nearest satellite to the Jupiter, and Callisto is the farthest one.