- Optical telescopes use a series of lenses or a curved mirror to focus visible light. Optical telescopes that only use lenses are called refracting telescopes. Optical telescopes that use a mirror are known as reflecting telescopes. They are usually more powerful than a simple lens telescope.
- Since light passes through the lenses of a telescope, the glass has to be free of impurities. This quality of glass is known as optical glass. Optical glass is coated to improve its performance and to make it less reflective. Mirrors that refract the glass don't have to be as pure.
- Once the glass and mirrors are created, the lenses are also tested to ensure that the glass has been ground correctly. To do this, telescope makers measure the lens against a lens that is known to be perfect. Once the lenses pass inspection, they are assembled in a tube-like telescope housing. Then they are calibrated for clarity.
