Effect Of The Geometric Distortion On Phoebe’s Positional Measurement

According to the recent publication[1], the ephemeris of Phoebe is not veryaccurate for three reasons. The main reason is the astrometry of Phoebe. Because ofits faintness (about magnitude16), Phoebe is not easy to observe and was especiallydifficult to observe in the past. Another reason is that the dynamical models ofPhoebe’s motion are usually not complete and take into account only a fewperturbations. The last reason is the dependence on Saturn’s position. In this paper;some experiments have been done in order to analyze the effect of the GeometricDistortion (called GD hereafter) on the positional measurement of Phoebe, the ninthsatellite of Saturn.For accurate measurement of Phoebe’s position, the approach recently proposedby us in the paper [2] has been used to process our observed data in order to solve theGD, and then the GD effect was removed from the observed data for Phoebe to obtainbetter measurement positions for it. Besides, our data reduction is also made by theastrometric software Astrometrica for comparison.The approach is applied to two groups of observations which were taken at theKunming and Lijiang observational sites of the Yunnan Observatory. During theobservations, we also took lots of CCD frames by image-overlapping some densestellar field (open clusters) in order to accurately derive the GD in the field of view.Then we took some CCD frames of Phoebe to measure its pixel positions.Furthermore, the GD correction (hereafter called GDC) for Phoebe and its referencestars was also made based on the GD solution from the dense stellar fields. Finally,we derive the results of positional measurement of Phoebe. Our reduction shows thatthe precision of Phoebe’s positional measurement is significantly improved aftercorrecting the GD. Especially, for the observations taken by2.4m telescope, theprecision of Phoebe’s positional measurement is also improved by a factor of3~4incomparison with the precision by the widely-used software Astrometrica.The same method can be also applied to the positional measurement of othermoving objects, such as asteroids or other natural satellites of planets.