A telescope makes use of two positive (convex) lenses:

Objective, which forms the image of a distant object at its focal length (f_o), and

Eyepiece, which acts as a simple magnifier (f_e), which is used to view the image formed by the objective.

Its length is equal to the sum of the focal lengths of the objective and eyepiece.

Angular magnification, M=-f_o/f_e, giving an inverted image.