A Galilean telescope is formed by a long positive focal length objective lens and a short negative focal length eyepiece, so it produces an erect image. The lens at the left is used to produce parallel rays of light, as if from a distant star. The parallel beam of rays entering the objective lens is wider than that emerging from the eyepiece, indicating the ability of the telescope to "gather" light. When the position of the "star" is changed by moving the source up, the image is moved up and the angle at which it is viewed is magnified relative to the incoming light, indicating the ability of the telescope to produce angular separation of the stars it is viewing.
Choice of slit baffle and distance of baffle from source determine the number of rays and their spacing.