Preliminary results of CCD observations targeting Himalia acquired at Yunnan Observatories in 2015

In order to study the potential associated with high precision CCD astrometry of irregular satel- lites, we have acquired experimental observations of Himalia, the sixth and irregular satellite of Jupiter. A total of 185 CCD observations were obtained by using the 2.4m and 1 m telescopes administered by Yunnan Observatories over ten nights. Preliminary analysis of the observations were made, including geo- metric distortion, atmospheric refraction, and also the phase effect. All positions of Himalia are measured relative to the reference stars from the catalog UCAC4 in each CCD field of view. The theoretical positions of Himalia were retrieved from the Institute de M6chanique C61este et de Calcul des t^ph6m6rides, while the positions of Jupiter were obtained based on the planetary ephemeris INPOP13c. The results show that the means of observed minus computed （O - C） residuals are -0.004＂ and -0.002＂ in right ascension and declination, respectively. The standard deviations of （O - C） residuals are estimated to be about 0.04＂ in each direction.

应用Gaia DR1的木卫六天体测量试验观测

木卫六是木星最大的一颗不规则天然卫星,为了获得它的高精度观测位置,天体测量归算过程中各方面都需要考虑,其中特别重要的是参考星表的选择。应用最新公布的星表Gaia Data Release 1(Gaia DR1),同时采用UCAC4星表进行比较,对2015年云南天文台获得的185个木卫六的观测资料进行处理和归算,结果显示,采用Gaia DR1星表的木卫六的位置测量精度在赤经和赤纬两个方向都约为0.02″,比采用UCAC4的结果有显著提升。

An improved solution to geometric distortion using an orthogonal method

The geometric distortion of a CCD field of view has a direct influence on the positional measurements of CCD observations. In order to obtain high precision astrometric results, the geometric distortion should be derived and corrected precisely. As presented in our previous work, a convenient solution has been carried out and has also been applied to observations of Phoebe. In order to fur- ther improve the solution, an orthogonal method based on Zernike polynomials is used in this work. Four nights of CCD observations including Himalia, the sixth satellite of Jupiter, and open clusters （NGC 1664 or NGC 2324） on each night have been processed as an application. The observations were obtained from the 2.4 m telescope administered by Yunnan Observatories. The catalog UCAC4 was used to match reference stars in all of the CCD frames. The ephemeris of Himalia is retrieved from the Institut de M6canique C61este et de Calcul des Ephemerides （IMCCE）. Our results show that the means of observed minus calculated （O - C） positional residuals are -0.034 and -0.026 arcsec in right ascension and declination, respectively. The corresponding standard deviations are 0.031＂ and 0.028＂. The measurement dispersion is significantly improved compared to that by using our previous solution.

NTSC 50cm光学望远镜的天体测量精度评估

通过6个不同天区梅西耶天体的CCD观测图像,对中国科学院国家授时中心(NTSC)新建的50 cm光学望远镜进行了天体测量精度评估,确定本系统数字图像数据归算的最佳底片常数模型为3阶(20常数)模型。在此基础上,对地球同步卫星和不规则天然卫星(木卫六)进行试验观测和数据分析。结果表明:对于地球同步卫星,在赤经和赤纬方向的内符合精度分别为0.320″和0.280″,而木卫六在赤经方向和赤纬方向的观测精度分别为0.100″和0.105″。较好的天体测量精度说明了本套光学观测系统完全可以满足地球同步卫星和较亮天然卫星精密定位观测的需求。