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2 The observations and measurements

All observations were made at the Cassegrain-focus of the 1.6 m Ritchey-Chretien reflector of the Laboratório Nacional de Astrofísica in Brazil (geographical longitude: $\rm 3^h 02^m 19^s$, latitude: -220 32' 04'' and altitude: 1872 m). The focal length of the Cassegrain combination is 15.8 m, which results in a plate scale of 13 $\hbox{$.\!\!^{\prime\prime}$ }$0/mm at the focal plane.


  \begin{figure}
\includegraphics[width=8.8cm,clip]{hist1.ps}\end{figure} Figure 1: Distribution of the observations. Every bar corresponds to an observational mission

Along the various missions, three CCD detectors were used: CCDs EEV P88231, EEV-05.20.0.202 and SITe SI003AB CCD. These are arrays with 770 $\times $ 1152, 770 $\times $ 1200 and 1024 $\times $ 1024 square pixels respectively, each pixel measuring 22, 22.5 and 24 $\mu$, which corresponds to 0.293, 0.294 and 0.312 arcsecs/pixel on the sky. No filter was used and the exposure time varied from $\rm 30^s$ to $\rm 180^s$, depending on the meteorological conditions.

   
Table 2: Normal place of Phoebe positions for each night
Number Date              $\overline{\alpha}$          $\overline{\delta}$        
Observ. Y M      D           h  m    s      0   '    ''   
03 1995 06 10.28839120 23 41 11.590 -04 11 53.39
02 1995 06 11.28233796 23 41 22.207 -04 11 00.23
09 1996 06 22.30696245 00 28 25.393 +00 32 47.85
05 1996 06 23.28531713 00 28 33.734 +00 33 24.34
02 1996 06 24.32598958 00 28 42.204 +00 34 00.79
10 1996 08 25.28588194 00 25 39.642 -00 02 14.67
08 1996 09 29.17321181 00 16 25.728 -01 04 58.67
09 1996 10 01.17351723 00 15 51.703 -01 08 36.20
05 1997 08 12.31695370 01 20 42.096 +05 41 49.55
03 1997 08 13.33337963 01 20 37.492 +05 41 03.17
01 1997 08 14.30817130 01 20 32.736 +05 40 16.02
03 1997 08 15.29278164 01 20 27.549 +05 39 26.70

The 60 observed positions of Phoebe were taken in 12 nights distributed over 5 observational missions in the years 1995-1997. In Fig. 1 the histogram of the observations in time is presented. Each bar corresponds to an observational mission. For the mission in 1995 and the first one in 1996, Phoebe was observed at about $40^{\rm o}$ in zenithal distance, while for the other 3 missions the observed zenithal distances were smaller than $20^{\rm o}$. Figure 2 presents the distribution of the observations on the orbit of Phoebe.

To find the centroids for the stars and for the satellite images the ASTROL reduction package (Colas & Serrau 1993) was used to fit a bidimensional Gaussian to the images. The centering error was typically 0 $\hbox{$.\!\!^{\prime\prime}$ }$01.


  \begin{figure}
\includegraphics[angle=-90,width=8.8cm]{graf1.ps}\end{figure} Figure 2: Positions of the observations on Phoebe elliptic orbit. The mean dates of the observation are: 1 (06/10/1995); 2 (06/23/1996); 3 (08/25/1996); 4 (09/30/1996) and 5 (08/13/1997)


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