Determination of Optical Positions of Radio Sources 0851+202,1228+126 and 1749+701

Using the plate-constant-method and taking the NPM as reference catalogue, optical positions of radio sources 0851+202, 1228+126 and 1749+701 are determined with 15 observations which were choosed from the CCD quasar-multi-colour photometry survey of Schmidt telescope in Beijing Observatory. In this paper, three methods to determine the centers of star images are analysed and compared. In addition, It is presented that it should be better to choose the appropriate sampling range to compute the centers of star images, which is based on the magnitude(or half-peak width). The comparison between our results and radio positions are also given. 