The "objective" (lens or mirror) is the major major major part of the optical telescope. It's really the only part you need in order to make a telescope (besides something to hold the objective). You can put a piece of film or a CCD right at the focal point of the objective lens or mirror and capture 'images' (pictures) there.
If you want to use the telescope for looking through and seeing stuff with your eye, then you need the other major part ... the eyepiece lens.
