To Build Up the Inertial Coordinate System With A CCD (Ⅰ) Measuring the Relative Position of An Extragalactic System with Respect to the Reference Stars

In this present article, the method is described for measuring the relative position of an extragalactic system with respect to the reference stars by using a CCD. since each exposure time of a CCD is shorter, the successive overlap exposure method can be used in order to expand the observable range of the. sky and to overcome the shortage that the measurable field of view is too small when a CCD is used. Using one of its characteristics that the aberration is negligible and the "Four-Constant Model" can be used, only two reference stars are needed to determine the relative position of the extragalactic system with respect to the reference stars in the whole range to be measured by the overlap exposure. In addition, the formulae of determining the relative position of an extragalactic system with respect to the two reference stars are derived, and the estimate of the determining error of every plate constant is also given.