For large magnifying power of a telescope

The magnifying power of an astronomical telescope in normal adjustment is given by $M = \dfrac{f_o}{f_e}$, where $f_o$ is the focal length of the objective lens and $f_e$ is the focal length of the eyepiece.

For a large magnifying power, the focal length of the objective ($f_o$) must be large and the focal length of the eyepiece ($f_e$) must be small.

Answer: focal length of objective is large