2001A&A...367.1061S

This paper presents an analysis of astrometric reduction methods for the calibration of a CCD target. To compare these methods, we collected about 3000 recent CCD observations between 1990-1997. We discuss the comparison of the four main contemporary theories describing the eight major satellites of Saturn, used in recent CCD astrometric reduction. We show that these theories developed by Taylor & Shen (1988A&A...200..269T), Dourneau (1993A&A...267..292D), Harper & Taylor (1999, priv. comm.) and Duriez & Vienne (1997A&A...324..366D), give a rather good representation of the orbits of the eight main satellites, especially for satellites III-VI. In the CCD astrometric reduction, we point out a bias of the theories on the derived satellite positions, which can reach about 0.03''. Duriez and Vienne's TASS theory (1997A&A...324..366D), built with significantly higher consistency than the other three, generally leads to the lowest residuals for the observations analysed here. Due to its high quality, we recommend use of this theory for CCD reduction. Systematic errors affecting satellites' derived positions should be small, due to the quality of the TASS theory. This procedure might be an alternative to the multi-theory reduction method previously proposed by Qiao et al. (1999A&AS..137....1Q). Observations of satellites obtained from CCD reduction using TASS are expected to be significantly more accurate (0.015'') than observations reduced from any other theory (about 0.05'').