In 1999 we developed a chronograph equipped with a variable-diameter occulting disk (Bourget et al. 2001) in order to make astrometric observations of small satellites near bright planets. Immediately after, as from 2000, a systematic program of observation of the Neptunian satellite Proteus begun. It was conducted at the Cassegrain-focus of the 1.6m Ritchey-Chretien reflector of the Laboratório Nacional de Astrofisica in Brazil. Such observations are particularly difficult and to this date there were only 12 ground-based published observations of this satellite. They were obtained at the 2.2m ESO telescope (Colas and Buil 1992) and at the 3.6m CFH telescope with adaptive optics (Roddier et al. 1997). Between 2000 and 2002, we obtained an ensemble of 63 images containing Neptune, Triton and Proteus, along 4 nights. The images fields cover about 4 by 4 arc minutes, so that several reference stars could be found for the astrometric calibration. During the observations the diameter of the chronograph occulting disk was changed depending on the seeing conditions. For all frames, the images were centered using an algorithm based on the adjustment of a point spread function and the background was removed employing a second order polynomial fit. For the astrometric calibration a 10 constants polynomial model was used. The reference stars were picked up from the USNO-A2.0 catalog, locally corrected by the Thyco 2 frame placed at the epoch of the observations. The results were compared against JPL positions for Proteus minus Triton. The observed minus calculated means and standard deviations are respectively -0".11 and 0".16 on the x direction and -0".03 and 0".10 on the y direction.
Bulletin of the Astronomical Society of Brazil
- Pub Date:
- August 2003