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:
,
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
0/mm at the focal plane.
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
1152, 770
1200 and 1024
1024 square pixels respectively,
each pixel measuring 22, 22.5 and 24
,
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
to
,
depending on the
meteorological conditions.
Number | Date |
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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
in zenithal
distance, while for the other 3 missions the observed zenithal distances were
smaller than
.
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
01.
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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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