An astronomical refracting telescope will have large angular magnification and high angular resolution, when it has an objective lens of:

1. Angular magnification ($M$) of a telescope is:
$$M=\frac{f_o}{f_e}$$
To have large magnification, the focal length of the objective ($f_o$) should be large.2. Angular resolution is proportional to the diameter ($D$) of the objective lens:
$$\text{Resolution}\propto \frac{D}{\lambda }$$
To have high resolution, the diameter ($D$) should be large.Therefore, the objective lens should have a large focal length and a large diameter.