Experiments on the effect of the Geometric Distortion on Phoebe´s positional measurement

Geometric Distortion (GD) is an important factor in high-precision positional measurement. In this paper, some experiments have been conducted in order to analyze the GD´s effect on the positional measurement of Phoebe, the ninth satellite of Saturn. The approach we recently proposed in the paper [1] has been used to process our observed data in order to solve the GD, and then the GD effect was removed from the observed data for Phoebe to obtain better measurement positions. 279 CCD frames were taken by YFOSC (the Yunnan Faint Object Spectrograph and Camera) attached to the 2.4m telescope at Yunnan Observatory on January 3-4, 2011 and February 18-19, 2012. Among all of the CCD frames, 139 frames are for the open cluster NGC 2324 and the other 140 are for Phoebe. Our reduction shows that the precision of Phoebe´s positional measurement is significantly improved after correcting the GD. Besides, our data reduction is also made by the astrometric software ASTROMETRICA for comparison. Obvious improvement is also shown by our own reduction techniques.