Abstract
Earth based observations of Proteus are made highly difficult by the magnitude difference with Neptune, and the closeness to the planet. To overcome these difficulties, observation campaigns using a Hg-mask coronograph developed for astrometry are being pursued at the Laboratorio Nacional de Astrofisica, Brazil. The observations are made at visible light wavelengths with the 1.6 m reflector telescope. The large 4 � × 4 � field of view of the camera makes it possible to image a sufficient number of background stars that are used to carry out the astrometric analysis of these data. The first results of the campaigns are presented in this paper. They include 45 independent observations made over 2 nights in 2002. The positions are referred to the Hipparcos/Tycho2 frame. The mean offsets relative to the JPL ephemeris are −0. �� 083 (σ = 0. �� 148) in the x =∆ α cos δ direction, and −0. �� 051 (σ = 0. �� 115) in the y =∆ δ direction. Additional astrometric data need to be collected in order to investigate further the possible offset of Proteus with respect to the Voyager ephemerides. For the sake of completeness the early 2000 and 2001 campaigns, although less accurate, are also discussed.
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