The secondary spectrum is a main factor debasing image quality in telescope objectives that have long focal length and large relative aperture. Recently,the special dispersion character of the diffractive structure has been applied to correct secondary spectrum with ordinary material and simple construction. In this paper,a diffractive optical element is equivalent to a thin lens with infinite refractive index. Based on this ultra-high index model,the PWC method has been investigated to design hybrid diffractiverefractive (HDR) apochromatic telescope objectives. A HDR doublet apochromatic telescope objective with a focal length of 2000 mm and a relative aperture of 1/15 has been designed. Compared with conventional triplet design,it is shown that hybrid objective is better to correct secondary spectrum and spherochromatic aberration,so has better image quality.