The Cassegrain has a parabolic primary mirror and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the “star-shaped” diffraction effects caused by a straight-vanned support spider. The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power.