CCD observations of mutual events of Saturn's satellites made in 1995 at Bordeaux Observatory

Bordeaux Observatory

1.

Introduction.
In 1995, Bordeaux Observatory participated to the international campaign PHESAT95 of observation of mutual events of Saturn's satellites. Photometric observations were made by the recent CCD camera of Bordeaux Observatory. The higher quality light curves so-obtained were then compared to the predictions.

2.

The CCD camera.
The CCD camera was designed in 1991 at Bordeaux Observatory in order to participate to the previous campaign PHEMU91 of observation of mutual events relative to the galilean satellites of Jupiter. A detailed description of this camera is given by Le Campion et al.(1992). The TH7863 chip was used in preference to the smaller TH7852.

3.

Observations.
The observations were made with the 62cm Cassegrain telescope of Bordeaux Observatory. As its focal lenght is about 9.6m, the corresponding size of a pixel is then 0.6". No use of any color filter was made as Saturn's satellites are very faint. A reference satellite, not concerned by the mutual event, was generally simultaneously observed. The time acquisition of images was about 10 seconds. Dark counts and flat field were made each night of observation. Only a dozen of events were observed, out of the forty possible ones. This is consequent to the weather conditions and to the difficulties of observing so faint satellites close to a so bright planet.

4.

Light curves.
The images of satellites were enhanced by removing the near sky background to the raw data. This process was performed by the ASTROL program designed by F. Colas (1991). Flux ratio of the images including the occulted or eclipsed satellite over the images of the reference satellite was computed and visualized in light curves versus time. The successive points so obtained were then fitted by using either a Bspline or a polynomial of degree three. Its derivation led to the observed time of the event maximum. A total of seven high quality curves were obtained.

5.

Comparison to theories.
Prediction of times and intensity drops were made for each event by various authors (Arlot and Thuillot, 1993, Aksnes and Dourneau, 1994). The observed minus computed (O-C) residuals relative to event maxima generally were lower than 30 seconds and, for the best curves, lower than 10 seconds.

6.

Conclusion.
The above comparison to theories shows the high quality of mutual events observations as well as the quality of the analytical theories of motion of these satellites (Dourneau, 1993).