A Study Of High Precision CCD Positioning Of Himalia

Irregular satellites are one kind of the small celestial bodies in our Solar system.High precision positional measurements obtained from CCD observations of these objects can improve their orbital theory and ephemerides,and also promote studies in planetary physics.It is also important to further understanding the origin,formation and evolution of our Solar system by these high precision positional measurements.Moreover,future deep space exploration plans can benefit from the high precision positional measurements.In this paper,the biggest member of Jovian irregular satellites,Himalia,is studied to obtain high precision positional measurement results.A total of 598 CCD observations of Himalia were obtained at Yunnan Observatories over 27 nights during the years 2015–2016,together with 599 frames of CCD calibration images.Specifically,366 CCD observations of Himalia were obtained with Lijiang2.4-m telescope,and 232 CCD observations of Himalia were obtained with Kunming 1-m telescope.The main works are,1)The newest Gaia DR1 catalogue was selected in the astrometric reduction of Himalia.The UCAC4 catalogue was also used for comparison.The resulting positional precisions of Himalia by using UCAC4 catalogue are 0.058 and 0.055 in right ascension and declination,respectively.The corresponding precisions by using Gaia DR1 catalogue are 0.029 and0.030.The positional precisions of Himalia are significantly improved by a factor of two.2)An orthonormal method was applied in the previous solution of geometric distortion.A total of 20 orthonormal basis vector fields were derived from Zernike Polynomials.The improved solution of geometric distortion was applied for correcting geometric distortions.Before the corrections,the positional precisions of Himalia are 0.066 and 0.056 in right ascension and declination,respectively.After the corrections,the corresponding precisions are0.029 and 0.030.The positional precisions of Himalia are significantly improved by a factor of two.3)The IAU-SOFA library was used to calculate the topocentric apparent positions of reference stars.Both the Gaia DR1 and UCAC4 catalogues were used.The topocentric apparent positions which are affected by atmosphere refraction were computed from the ICRS astrometric positions.4)Both the JPL and IMCCE ephemerides were used for comparisons with the observed positions of Himalia.The positional precisions of Himalia in each year between JPL and IMCCE ephemerides are in good agreement,both in right ascension and declination.The means of(O-C)residuals of Himalia in right ascension by using JPL ephemeris is 0.017 for the CCD observations in the year 2015.However,the corresponding value by using IMCCE ephemeris is 0.056.The difference is 0.039.Furthermore,the difference between the means of(O-C)residuals in declination by using JPL and IMCCE ephemerides for the CCD observations in the year 2015 is only ? 0.010.The corresponding difference in each direction for the CCD observations in the year 2016 is also ? 0.010.In consideration of the same planetary ephemeris which is DE431,differences between the means of(O-C)residuals by using JPL and IMCCE ephemerides should come from the different satellite ephemerides which are JUP300 and Emelyanov(2005).Finally,after the Gaia DR1 catalogue is selected,the geometric distortion is corrected and the JPL ephemeris which includes planetary ephemeris DE431 and satellite ephemeris JUP300 is used,the positional precision of Himalia is estimated at about 0.03 in each direction.This precision is significantly improved than other results of the same kind of CCD observations for Himalia.