SIMBAD references

A simple but effective approach is proposed for measuring the geometric distortion of a CCD field of view of a ground-based telescope. For three open clusters (M35, M67, and NGC 2324), 425 CCD frames taken by a 1 m telescope at the Yunnan Observatory are used to test this approach. It is found that the geometric distortion pattern depends strongly on the corresponding filter used. The geometric distortion is then used to correct the pixel positions for Phoebe, the ninth satellite of Saturn, and its reference stars imaged in 220 CCD frames taken by the same telescope. The standard deviation of the (O - C; observed minus computed) residuals of Phoebe is significantly improved after correcting the geometric distortions.