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2 The observations, measures and astrometric calibration

The observations were made at the Cassegrain focus of the 1.6 reflector of the Laboratório Nacional de Astrofísica, Brazil, where the scale at the focal plane is 13''/mm. For more details about the telescope we refer to Veiga et al. (1987). To take the 750 images, a CCD was used. It has an array with $770 \times 1152$ square pixels each one measuring $22\,\mu$m which corresponds to $0\hbox{$.\!\!^{\prime\prime}$}294$ on the sky. No filter was used and the exposure time varied from 3 to 10 seconds, depending on the meteorological conditions. To avoid the diffraction cross originated from the saturated image of Uranus, a mask with 8 circular apertures was placed between the secondary mirror support vanes.

The distribution of the observations during the four years are presented in Fig. 1. Every bar corresponds to an observational mission.

\includegraphics [width=8.8cm,clip]{graf1.eps}\end{figure} Figure 1: Distribution of the observations. Every bar corresponds to an observational mission

Table 2: Number of positions, mean and standard deviation, in arcseconds, of the observed minus calculated x and y of the Uranian satellites referred to Oberon. Paper I is for Veiga & Vieira Martins (1994) and Paper II is for Veiga & Vieira Martins (1995)

 & \multicolumn{2}{c}{\bf Paper I} & \multicolum...
 ...(0.031) & (0.020) & (0.056) & (0.052) & (0.037) & (0.056) \\ \hline\end{tabular}

The ASTROL reduction package (Colas & Serrau 1993) was used to find the center of the satellites and the field stars. For the determination of each center a small area was taken containing the image of a satellite or a star and a bi-dimensional Gaussian was used to fit the image. When necessary, a second degree polynomial was added. In some cases, the addition of the polynomial is essential since the sky background needs to be removed. This occurs for the images of Miranda which are contaminated by light from the planet. The centering error were $0\hbox{$.\!\!^{\prime\prime}$}030$ for Miranda and $0\hbox{$.\!\!^{\prime\prime}$}015$ for the other satellites and stars.

For the astrometric calibration, we used an adaptation of the secondary catalog method to the special conditions of the small CCD fields. This method, which is presented and tested in Vieira Martins et al. (1996) (see also Assafin et al. 1997a,b), consists in setting an astrometric catalogue for the stars on the CCD, using their images in the Digitized Sky Survey (DSS-I) and the positions of nearby stars from the Guide Star Catalog (GSC1.1) corrected by the PPM Catalog.

The CCD images were corrected with a 6 constants model in the astrometric calibration procedure. Therefore it is not necessary to add any correction for the refraction. Furthemore almost all images were taken at a zenithal distance smaller than $30^\circ$.

In Table 1 (accessible in electronic form) we list the observed positions of the four largest Uranian satellites relative to Oberon. The data are presented in the following form: the first line gives the year, month and day and decimal fraction of UTC days, corresponding to the mean instant of the observation. In the next lines we list the name of the satellite followed by $X= \Delta\alpha \cos\delta$ and $Y= \Delta\delta$,in arcseconds referred to Oberon. The reference system is placed on the equator and equinox of J2000.

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