State the condition under which a large magnification can be achieved in an astronomical telescope.

In an astronomical telescope, light is incident from infinity on the objective lens. It forms an image at the focus of the objective lens. For large magnification, the length between the lenses is adjusted so that the final image is formed at the least distance of distinct vision from the eye piece. Magnification in this lens system is given by: M = fo/fe * (1 + fe/D), where fo is the focal length of the objective lens, fe is the focal length of the eyepiece, and D is the least distance of distinct vision.