Eliminating chromatic aberration of objective lens is an important factor for improving the image quality in refracting telescopes. In this paper, the authors presented relationship between chromatic aberration and the index of refraction of the crown and flint glass in objective lens. By choosing the pair of glass for achromatic objective lens, the value of the flint to the crown focal lengths ratio (k) is approximately -1.5 to -1.4, would be changing from overco"ection to a reasonable achromatic condition. Designing and machining the objective lens using optic glass materials BF6 - TF1 has a value of k = -1.46 in Z23 company, Vietnam. Estimating objective lens's chromatic aberration measured by Foucault system, where the results appropriated with the calculation.