Observatorio Nacional, Rio de Janeiro, Brazil
E-mail: rvm@on.br, cave@on.br
There are 15 years we started a systematic program of astrometric observations of faint satellites. We observed the Marthian satellite Deimos; The Jovian's Amalthea, Thebe, and the eight external satellites; Helene, Calypso, Telesco and Phoebe of Saturn; the satelites Miranda, Ariel, Umbriel, Titania and Oberon of Uranus; Triton and Nereid of Neptune. These observations were made at the Cassegrain focus (scale=13''/mm) of a 1.6m reflector in Brazil (latitude=-23°). From 1982 to 1988 the observations were made using photographic plates and since then CCD devices have been used.
To observe the faint satellites near their bright planets with the CCD, some particular techniques were used. For astrometric calibration of our frames, some methods were developed: the reference system was obtained using the apparent motion of the planet or by the construction of a secondary catalogue of very faint stars using ''The Digitized Sky Survey'' and ''The Guide Star Catalog'' corrected by modern astrometric catalogues.
For satellites which theoretical positions are well known like the Uranian satellites, our O-C are lesser than 0''.05. Otherwise, for those poorly determined like Amalthea and Thebe, the O-C may be greater than 0''.2.