New evidence of precision premium for Galilean satellites from CCD imaging

After a CCD image of the four Galilean satellites of Jupiter is obtained by a long focal length telescope, we can compare the theoretical positions of these satellites with their pixel positions so as to obtain the calibration parameters of the CCD field of view. In theory, when two of the four satellites have small enough separation, their relative positional measurement will have a good accuracy since the error existing in the solved calibration parameters has a direct proportional effect on the separation of the two satellites. The 347 CCD images taken by 1-m long focal length telescope at Yunnan Observatory in 2002-2005 are used to perform the experimental test. After we improve the centroid algorithm for the satellites and our former halo-removal technique, the results show that the positional measurement of two small-separation satellites has an external precision as good as 0.01-0.03 arcsec. This precision has comparability as that from rarely occurring mutual events of the Galilean satellites. This experiment confirms the finding of the "precision premium" firstly presented by Pascu [1994. An appraisal of the USNO program of photographic astrometry of bright planetary satellites. In: Morrison, L.V., Gilmore, G.F., (Eds.), Galatic and Solar System Optical Astrometry, pp. 304-311] using photographic observations. We believe that this type of observations, besides mutual event observations, might also be used to improve our knowledge of the orbital motions of the Galilean satellites because of its much more opportunities.