土星主要卫星天体测量定标试验

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

在地面望远镜观测土星主要卫星的精确定位中,目前普遍采用CCD成像技术。为了测量视场中暗卫星的位置,常采用4颗主要卫星(土卫三-土卫六)进行定标。鉴于技术的进步,目前CCD视场正在逐步增加,如何精确定标图像值得研究。利用2010年1月20日在云南天文台1 m望远镜上观测的不同取向的105幅CCD图像进行了试验研究。采用了美国喷气推进实验室土星主要卫星历表和美国海军天文台的UCAC4恒星星表进行计算,相对于不同的参考对象进行定标。结果表明,采用4颗主要卫星进行定标并测量离卫星较近的观测对象时具有较好的测量准确度和内部精度。测量离定标星较远的卫星时,这种定标方法具有较大的不确定性。采用6颗主要卫星(土卫三-土卫八)进行定标,所有卫星的测量结果具有更好的外部和内部符合。即便如此,采用四常数模型和六常数模型定标的结果都不是令人满意,这揭示CCD视场存在明显的扭曲效应。如果采用视场中的恒星定标,卫星的外部符合和内部精度明显变差,这说明UCAC4恒星参考星表也不是理想的定标用参考星表。

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.