In this paper we have theoretically investigated the time telescope that offers a function of pulse expanding. Time telescope, with features of high efficiency and near Fourier-transform limit, is constructed mainly by using nonlinear parametric conversions of chirped pulses. Based on both analytical and numerical studies, it is found that group-velocity mismatch of the nonlinear crystal plays an important role in temporal aberration and conversion efficiency, and the criteria for designing a near aberration-free telescope are summarized. Finally, it is experimentally demonstrated that a 70fs short pulse is expanded to a 60ps long pulse near Fourier-transform limit.