The History of Telescopes

A telescope works like a huge, extra eye that funnels much more light into your own eye than your eye can gather by itself. There have been many different designs of telescopes, but they all fall into two categories. A refractor is a telescope that collects light with a lens at the front and sends the light straight through to other lenses at the back. This is the design that Galileo used almost 400 years ago. With it, he discovered that the Moon has craters, the Sun has spots, and Jupiter has four large moons of its own.
The biggest refractor in the world today has a lens 40 inches (102 cm) across. It was built at the Yerkes Observatory in Wiscon- sin over a century ago. Large refractors are difficult to build because a large lens will sag under its own weight, spoiling the view. Still, small refractors—not much bigger than Galileo’s but with much better quality lenses—are popular for backyard observing today.

The other category of telescope is called a reflector. It has a mirror at the back of the telescope to collect the light and send it to other lenses and mirrors either on the back or the side of the telescope. Some reflectors also have an extra lens on the front, but it’s still the mirror that collects the light. The big advantage of a reflector is that the mirror can be supported from the back. All of the big telescopes in the world today are reflectors, and they are up to 10 times as wide as the largest refractor.