Images can be formed through reflection or refraction. Reflecting mirror (incoming rays hit the concave mirror, which focus the light at a certain focal length at a focal point, on the mirror axis) Image is inverted due to the multiple flipped focal points. Refracting lens (incoming light rays pass through a prism type object, bending the waves focusing it into a focal point along the lens axis at a certain focus length) Light traveling through lens is refracted differently depends on wavelength. Some light traveling through lens is absorbed. Large lens can be very heavy, and can only be supported at edge (refractor) Lens needs two optically acceptable surfaces, mirror only need one. Prime focus (rare) focuses at the top of the telescope (need to catch the signal in the telescope. Newtonian focus amateur astronomy , uses a very small reflecting mirror (secondary mirror); focuses it into the eye piece.