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Observations and results

The Astronomy Institute of Catania University and the Catania Astrophysical Observatory can boast of a long experience in photoelectric observations of mutual phenomena between minor bodies of the solar system. All the observations were carried out at Serra La Nave stellar station of Catania Astrophysical Observatory. Table 2 shows the instrumental characteristics of observation campaigns and the number of good registered events. Table 2 illustrates the relevant data of phenomena. The Planet, date and nature of the phenomena are listed in the first three columns, the observed duration and depths are listed in the following four columns. The greater amount of observed phenomena is constituted by mutual events between the Galilean satellites (Blanco and Catalano 1974, Aksnes et al. 1984, Blanco C. 1988, Arlot et al. 1992, Arlot et al 1995). The observations were collected during the last four campaigns over the years 1973, 1979, 1985 and 1991.

  
Table 1: Instrumental characteristics of observation campaigns and number of good registered events. All the observations were carried out at Serra La Nave stellar station of Catania Astrophysical Observatory.


  
Table 2: Relevant observed data of the phenomena. The first three columns report the Planet, data and nature of the phenomena, the following four the observed duration and depths

The observation results of 1973 mutual events have been much useful to define the parameters and the surface morphology of the satellites. The comparison between the albedo of the eclipsed area and the loss of light observed during Europa's occultation by Ganimede on Oct 22, infer the existence, on Europa, of polar caps covering about the of the satellite's surface (Blanco and Catalano 1974).
The 1979 apparition was a poor one, requiring that most observations be made through large air masses, thus resulting in uncertain sky corrections and approximate lightcurves. Since the Voyager missions have provided accurate radii for the Galilean satellites and described surface features and albedo variations, the observations of mutual events, were best used for astrometric purposes. The relative positions in right ascension,cos, and declination, of a satellite pair at midevent were tabulated (Aksnes et al. 1984).
In the following campaigns the astrometric purpose of the observation data and the improvement of the integration time up to measures of 0.1 sec, suggested the employement of the V filter only. The 1985 campaign was the most favorable because it occurred during the opposition of Jupiter with the Sun, thus the observations were easier and a total of 37 events was collected. 22 of these were of special quality with a resolution good enough to improve the satellites' orbits. An attempt to verify the longitudes discrepancy did not give meaningful results. The displacements both for the eclipses and for the occultations are without any preferential direction even for the same type of phenomenon. All the lightcurves that can be useful were enclosed in "A catalogue of the observations of the mutual phenomena of the Galilean satellites of Jupiter made in 1985 during the PHEMU85 campaign" (Arlot et al. 1992).
The 1991 phenomena were observed following the PHEMU91 campaign suggestions and their publications is in progress (Arlot et al. 1995). We observed 29 phenomena, 9 of which were of good quality, 4 sure and 1 not sure, relatively to the resolution needed to improve the satellites orbits.
The only observed campaign of mutual events between Saturn's satellites spanned from February to March 1980. To the intrinsic observation difficulties due to the proximity of the satellites to Saturn and the mutual distance between the satellites, the bad weather conditions that characterized the campaign must be added. 24 events were observed, however only one of them is of good quality and 4 may appear to be real eclipses and/or occultations (Aksnes et al. 1984). The phenomena were observed in the U, B, V colors with an integration time of 3 sec and a wait time of 1 sec.
Pluto and Charon remain the only planetary system of our solar system that has not been visited by a spacecraft yet, therefore, together with the stars' rare and sporadic occultations, the mutual events represented the best opportunity to define their surface morphological characteristics. From 1986 to 1990, on the ground of the "Pluto-Charon Mutual Eclipse Season Campaign" we observed 14 events. A global analysis of Pluto-Charon data shows no systematic relevant deviations between the predicted and observed times of the events and between the predicted and observed values of the light losses. The comparison between the behavior of B and V lightcurves shows that for the superior events the slope of the B light losses seems greater than that of the V light losses, while in the case of the inferior events there is a slight indication of a greater slope of V light losses. From an analysis of B-V plots it appears that during the transit of Charon in front of Pluto, the V depth clearly increases, while during the occultation of Charon by Pluto a slight increase in the B depth takes place in coincidence with the minimum light. These facts are in agreement with the different colors of the two bodies and confirm the redder color of Pluto with respect to a bluer (less red) color of Charon's surface (Blanco et al. 1993).




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