2 The instrumentation and observations

The two different telescopes used for our observation were all sponsored by the United Laboratory for Optical Astronomy of the Chinese Academy. In each of the years 1994, 1995 and 1996 we were allocated 5 days of telescope time to use a CCD camera for observations of Saturnian satellites at the Yunnan and another 8 days at the Sheshan Station. During the 3-year observing campaign, bad weather has prevented us from getting any observations at the Yunnan, so all of the 1994-1996 observations were made with the 1.56 m astrometric telescope. Specifications of the telescope and the chip used are given in Table 1.

  

Table 1: Specifications of the Sheshan telescope and chip

CCD suffers from drawbacks. The first problem is the overwhelming brightness of Saturn and its rings. In order to reduce the apparent brightness of the primary, the observations were carried out using an I-type filter with a central wavelength of 900 nm and full width at half maximum (FWHM) of 150 nm. The second is that the much smaller field of view of the CCD does not make it possible to include sufficient reference stars. In order to determine the orientation and scale of the CCD chip, $4 \sim 5$ images were taken of the wide double star 61 Cygni (HD 291091/2) on each night; in the same way bias and flat-field frames were taken during twilight at the beginning and each night of observation.

The exposure time of most of our CCD frames is $1 \sim$ 4 seconds depending on the weather and elevation above the horizon. Provided the usable number of satellites is ensured, an appropriate increase in exposure time can contribute to an improvement in quality of the satellite images. As the CCD produces digitized images of natural satellites, we can partially reduce the effect due to difference between satellite magnitudes by adjusting contrast.

A total of 165 CCD frames of the satellites of Saturn were obtained. A summary of the number of frames of the target satellites in each year is presented in the last column of Table 2. Because we were unable to observe the outermost satellite Iapetus in such a small field of view, it is not given in this table.

  

Table 2: Calibration parameters for each night of observation