3 Observed positions and comparison with the theory

As mentioned above, the observations were compared with TASS1.7 for the eight large satellites. If the $\rm \vert O{-}C\vert$ for one observation was larger than 0 $\hbox{$.\!\!^{\prime\prime}$ }$5, it was rejected. For the Lagrangian satellites, the JPL calculated positions (Jacobson 2001) was used and if the $\rm \vert O{-}C\vert$ was larger than 2 $\hbox{$.\!\!^{\prime\prime}$ }$0 for Helene and larger than 5 $\hbox{$.\!\!^{\prime\prime}$ }$0 for Telesto and Calypso it was rejected. We chose the above limits since the residuals out of these intervals are evidently outliers.

The (O-C) statistics are presented in Table 3 and Fig. 2 shows the corresponding histograms. We can observe that the largest residuals appear for Mimas in the x direction, which is explained by the difficulty of taking measurements of the center of the satellite image within a region where light scattered by the rings is very significant. The mean values for the Lagrangian satellites are very large. Besides intrinsic errors brought about by the faintness of the images, this suggests that the theoretical positions of these satellites are not very good. For Iapetus and Hyperion, the means are also large. However the means for these satellites in (VTVAM) are smaller. Probably this is a consequence of the poor quality of our images of these relatively faint satellites.

The catalog of our astrometric data is available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/ A+A/400/1095. It is also available at ftp://ftp.bdl.fr/ pub/NSDC/saturn/raw_data/position/. It contains astrometric coordinates, in the J2000 system, of the observations analyzed in this section. Table 4 shows an extract. It contains satellite/satellite positions expressed in arcseconds, gathered by series with the corresponding scale (in pixels/arcsec) and orientation (in radians). The dates correspond to the mid-time of the exposure. The date is not light-time corrected. In this catalog the positions with large residuals are also presented. so we have 2183 positions of Saturnian satellites but only 2072 of these positions are in the intervals defined in the other tables above.

The format and the conventions of our catalog are almost the same presented in (VTVAM) and quite similar to the one by Strugnell and Taylor (1990). In a FORTRAN code, lines are read with the format: (i3, i5, i3, f11.7, f7.3, 2i4, i2, 1x, a2, a1, i2, i1, 2(1x, f13.7), 2i2, 2(ix, f7.3), 3i3, 2f10.3). The meaning of each parameter can be found in Strugnell & Taylor (1990) and in (VTVAM), however here some are slightly different.

• opp.: opposition number, 1 = 1874 to 122 = 1999.
• year, m, utc: year, month and utc date of the observation (like VTVAM).
• tt-utc: TT-UTC in seconds (like VTVAM).
• obs.: IAU observatory code from the Minor Planet Center.
• reference code catalog (we have chosen 303 for the present observations).
• t.: observation type.  ,  ,  , 3= position angle, separation .
• obj.: subject satellite, reference satellite. 0 for Saturn, 1 to 8 for S1 to S8, 12 for Helene, 13 for Telesto, 14 for Calypso, C for the center of the plate which is not a physical object.
• fl.: flags of presence of the first and second coordinates.
• obs1, obs2: first and second coordinates. It is 0 when the coordinate is not present, see fl.
• s: reference system. 0 = mean equator and equinox B1950, 1 = true equator and equinox of date of observation, 2 = mean equator and equinox of J2000.
• f: reference frame. 0 = topocentric, 1 = geocentric, 2 = heliocentric.
• o-c1, o-c2: residuals of observations in arcsecond computed with TASS1.7, it is 999.999 when the residuals are not computed and it is 888.888 when the residuals are greater than 100 in absolute value.
• r: the number of the satellite used as reference in the computation of O-C.
• -: 0 for no meaning and 1 when the frame was not used in the computation of the scale and the direction of the series (see se column).
• se: the number of the series, the scale and orientation are the same for all observations of one series.
• xpix, ypix: original coordinates in pixels.

We add at the end of our catalog some lines which indicate how the reductions have been done and which corrections have been applied to get the coordinates. In particular, we give the scale and the orientation of each series of observations. Note that the orientation refers to the true equator of the date because the calibration parameters are issued from the direct comparison between the observed frame and a "computed frame''. But, as explained in (VTVAM) (see also Vienne et al. 2001b), it is possible to adjust the scale and the orientation directly by comparing the observed astrometric coordinates and the computed ones.