The first telescopes were refractors. They used lenses to gather and focus light. Galileo built his own in 1609 and pointed it at the sky, discovering Jupiter's moons, the phases of Venus, and the mountains on the Moon. His observations helped overturn the geocentric model and establish that Earth orbits the Sun.

A refracting telescope uses a convex objective lens to collect light and bring it to a focus. An eyepiece magnifies the image. The longer the focal length of the objective, the greater the magnification. Early refractors were very long, sometimes over 40 meters, to reduce chromatic aberration. That made them unwieldy.

Chromatic aberration remains the main weakness of refractors. Different colors of light focus at different points, producing color fringes. Achromatic lenses, invented in the 18th century, use two types of glass to reduce the effect. Apochromatic lenses reduce it further, but they are expensive.

Refractors are still popular for amateur astronomy. They are simple, rugged, and require little maintenance. They excel at observing the Moon and planets, where sharp contrast matters more than light-gathering power. But for deep-sky objects, reflectors are usually the better choice.