3 Photoelectric observations of a mutual occultation of CCD photometry of mutual events of satellites at FAI AS RK

At FAI AS RK the observations were performed at two observatories. The Assy observatory is located at an altitude of 2750 m at east longitude $5^{\rm h} 11^{\rm m} 31^{\rm s}$, north latitude $43^{\circ} 13^{\prime} 20^{\prime\prime}$. The Almaty observatory (Kamenskoe Plato) has an altitude of 1450 m and is located at east longitude $5^{\rm h} 07^{\rm m} 49^{\rm s}$, north latitude $43^{\circ} 11^{\prime} 10^{\prime\prime}$.

At the Assy observatory the observations were made with a 1-meter Zeiss telescope and at the Almaty observatory, with a 0.6-meter Zeiss telescope. On both telescopes photometric observations were made using cameras with image-magnification resulting in focal lengths of 12381 and 21474 mm for Zeiss-600 and Zeiss-1000, respectively.

The images were projected onto a ST-6V CCD thermoelectrically cooled to $-20^{\circ}$C. The exposure times were always equal to 10 s and no filters were used. To avoid the scattered-light halo of the planet (which was not always possible), only a small CCD area containing the satellites to be observed was measured. The CCD was operated in the automatic image-file recording mode. The combined brightness of the two satellites was computed as the average over $11\times 11$ pixels square region using standard CCD OPS software for CCD operation and reduction of frames.

A total of 15 mutual events of Saturnian satellites were observed and photometric data were secured for two of them.

The results of the first observing set can be found in file a0914e32.pmr. Each line in this file corresponds to one measurement and consists of four numbers. The first three numbers (hours, minutes, and seconds) give the time of observation - the average exposure midtime in the UTC scale. The last, fourth number gives the combined brightness (flux) of the two satellites in arbitrary relative units.

Figure 3 shows the resulting light curve (the flux is in arbitrary units). Here are the characteristics of the first event:

This observing set was also analyzed in [12, (Emel'yanov et al. 1997]). The reduction procedure was similar to that employed in the previous case. We obtained the apparent angular coordinate differences for satellites at a certain instant of time within the observing period. The accuracy of the satellite coordinate difference turned out to be $0\hbox{$.\!\!^{\prime\prime}$}002$ and $0\hbox{$.\!\!^{\prime\prime}$} 008$ in the right ascension and declination, respectively. The values of the refined parameters are given in [12, (Emel'yanov et al. 1997]). In Fig. 4 we compare observations with the theoretical light curve computed after refining the parameters. The flux is normalized to the total light of the two satellites outside the event.

The second successful photometric observing set had the following characteristics:

The results of the second set can be found in file a1125e25.pmr. The format of measurements is similar to that used for the previous observing set. Figure 5 shows the resulting light curve and Fig. 6, the same light curve with a fit involving parameters based on the analysis performed in [12, (Emel'yanov et al. 1997]). The accuracy of the satellite coordinate difference turned out to be $0\hbox{$.\!\!^{\prime\prime}$}009$ and $0\hbox{$.\!\!^{\prime\prime}$}029$ in the right ascension and declination, respectively. The values of the parameters are given in [12, (Emel'yanov et al. 1997]).

  

Figure 5: Photometric observations of eclipse of Rhea by Enceladus performed from Almaty on November 25, 1995. The Rhea' flux, S, is given in relative units. The line is a preliminary ephemeris

  

Figure 6: The calculated (line) and observed (dots) brightness curves of Rhea for the eclipse of Rhea by Enceladus performed from Almaty on November 25, 1995. The calculated curve is based on adjusted event parameters. The observations have been corrected for scale, zero point and secular trend effects