Astrometric Calibration of CCD Field of View for Some Major Satellites of Saturn

CCD imaging has been used to derive their precise positions of Saturnian major satellites during past two decades. In order to obtain the positions of some faint satellites near to their primary planet, the four bright satellites Tethys, Dione, Rhea and Titan are often used to calibrate their CCD field of view. Due to continuous progress in technology, recent CCD field of view becomes greater and greater, thus precise calibration of the CCD field of views deserves to study deeply. Experimental tests have been done based on 105 CCD frames with different orientations for Saturnian major satellites taken by the 1-m telescope at Yunnan Observatory on Jan. 20, 2010. The JPL ephemeris of Saturnian satellites is used for the major satellites and the star catalogue UCAC4 is used for the stars appearing in the CCD field. Different calibrated objects are chosen to calibrate the CCD field of view. Our tests show that when the four bright satellites (Tethys, Dione, Rhea and Titan) are used, a satellite near to the four satellites can be determined precisely, this confirms the former practice. However, for a wide separated satellite it will have great uncertainty. If the six major satellites (Tethys, Dione, Rhea, Titan, Hyperion and Iapetus) appearing in the same field are used to calibrate, all satellites will have better outer and inner agreements. Anyway, the distortion has shown its existance in the CCD field of view. Besides, if only the faint stars (in UCAC4) appearing in the same field are used to calibrate, the agreement in both inner and outer will become worse obviously. This experiment has shown that the UCAC4 catalogue is not precise enough to calibrate the CCD field of view.