Almost all published positions of Nereid are obtained from photographic plates. The only exception are four observations by Shaefer & Shaefer (1988). Therefore we will analyze in detail our CCD observations.

There are many differences among the photographic and the CCD observations (see for example Jones et al. 1998), the most important beeing the number of positions achieved in a night. While there is, in general, only one photographic position in a night, the number of CCD positions is larger than ten. For the periapsis region, the large number of observations is important since they correspond to different positions in the orbit and so represent a key contribution to the orbit determination. On the other hand, for every night, all positions in the apoapsis region are almost the same and their contribution for the orbit fitting is less important.

Figure 9:
Correlation between the residuals of Triton-Neptune and
Nereid-Neptune. For Neptune it was taken the theoretical position given by
DE403 |

Considering the image types we observe (Fig. 8) that the bad residuals appear,
in general, for the images of the type 2 as expected. Regarding the number
of stars used for the reduction of each image, we verified that the residuals
that are far from the cluster corresponding to the observations in the same
night, appear in the images where a few numbers of stars (about five) was
used. This is the case of the four residuals of *x* and the 3 in *y* in 93 and
one in *y* in 94. The small cluster in *y* direction, corresponding to the first
mission in 95 is strongly correlated to the Triton residuals and so it is due
to a bad astometric calibration of these images of type 2.

Therefore, we conclude that the behavior of the residuals is due to the astrometric reduction process.

Copyright The European Southern Observatory (ESO)