A lens at the left (out of picture) produces parallel light, as from a star. The second lens (at left in photograph) is the telescope objective, and the lens at the right is the eyepiece. 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 (photograph at right) by moving the source up (rays entering the telescope at a downward angle), the image is moved down and the angle at which it is viewed (inverted) is magnified relative to the incoming light, indicating the ability of the telescope to increase the 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.