4 Accuracy of our observations

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 7: O-C residuals of our CCD observations as function of the true anomaly

  

Figure 8: O-C residuals of our CCD observations as function of time

  

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

The variation of our positions in the orbit can be observed in Fig. 7 where the residuals are presented as function of the true anomaly. In Fig. 8, these same residuals are presented as function of the time. We can observe that the means are different of zero for every night with small scatter for some nights. In order to analyze the source of these errors we plotted the residuals of our measures of Triton-Neptune by Nereid-Neptune for x and y, which is presented in Fig. 9. For y, we observe that there is a good correlation between these two residuals pointing out that the astrometric reduction process is responsible for the errors in this direction. However for x direction, there is no clear correlation but we verify that the scatter is more important for the positions of Triton. Probably, this is due to the bad centering of the saturated and contaminated images of Triton in the direction of CCD columns which is the charges scattering direction.

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.